INSECT. - EMBRYOLOGY.

AN INAUGURAL DISSERTATION
FOR THE
Re okie SOF “DOCTOR: “OF PHILOSOPHY,
| PRESENTED TO THE
FACULTY OF ‘CLARK UNIVERSITY,

May 10, 1892.

WILLIAM MORTON WHEELER.

Reprinted from. JOURNAL OF MORPHOLOGY, Vol. VIII., No. 14.

BOSTON :
GINN & COMPANY.
1893.

A CONTRIBUTION

TO

INSECT EMBRYOLOGY.

AN INAUGURAL DISSERTATION
f DEGREE OR DOCTOR OF PHILOSOPHag

PACULEY OF | 'CLARK UNIVERSITY

May ro, 1892.

?
a)
b . BY
H WILLIAM MORTON WHEELER.
” " ea
ae
| Reprinted from JOURNAL OF MORPHOLOGY, Vol. VIII, No. 1.
ie i
fhe
An BOSTON:
‘ GINN & COMPANY.
1 pare 1893.
ok

Volume VITJ. April, 1593. Number 1.

JOURNAL

OF

MORPHOL OG

A CONTRIBUTION TO INSECT EMBRYOLOGY.

WILLIAM MORTON WHEELER.

TABLE OF CONTENTS.

PaGE
ETO RMSE WN oer tae Sn iclaenc a oncns, denvesacensinardseenenciez fans <eunaat eae nee ae 2
I Theiembryonie development of the: Locustide .....-0-2.cc.-eeeee eee. 3
1. The oviposition of Xiphidium ensifer int, Scud s....ccccceecececenteceeeveeeee 3
2. The formation of the Xiphidium embryo and its backward passage
LIT OUR [ERLE MOLI aitccosecee Sescjenl cosets tease eee ee eye toe He ee RS 5
(a) Description of surface changes beginning with the completed
DLASTO LET IPR eee ea ara dig Sop Sanson testes ae ee 5
(b) Zhe indusium ( pre@oral organ) 11 S€CtLOM .......2ecececeeceeeeeeee 12
3. The development of the embryo from the time of its reaching the
OVS SU fILECNOI RUMEN OL EATO! FEU OLULL OTE tt tenant ne 18
4. Variations in the development of the tndustUme .........0-ceeveeeecereeeeeeeene 2
al AILENGEDIOL UELOPEN OF ALIECNCTIOUT POW eee ee one estate Meee ee ete acai 27
6. The stages intervening between revolution and hatcht1g......-0- 29
Fist Le MEULLODILEMENO fa OFGICLUULUTIE® VLC OME sete rama aaetenes tedster sates stores 35
Des Gastrulationsinithet@ntho pte rae cee cere eet er ence ne sce seeds cn saetascatasscaze 36
III. The indusium and its homologues in the Arthropoda.............0...::10-+0-: 55
IV. General consideration of the embryonic envelopes and revolution of
BENG MINS CE TRAN Yea es cc ceee ee epee aden ented x ne erences eect ernganinnsee enemas 59
Tey DAB CALNE EOD AAI? et coe oe eee CEEOL CEEOL ECOL EEE FED EECESCEP CCE 59
DQ. The YOU Ren. caccscen-cesnevesesvsnsnerceneneacancesensnseeroeesscenssceeneseeenceacenssensaeases sueaeuensae 64
3. BlastoRtmests cece sence enceeneeesseenneeereenensenneeecnnnnscnascenens 67
4. The elimination of the embryonic CnVElOes -....ecsesereeeriereereriereceees 178

2 WHEELER: Vou. VIIL.

W.2 Newropenesis inthe Insecta 2.5: cocccccsmesrenpense teeter teeueecre sateeneeine ces acae iene 82
Tn LUE MEK LOC OT Pet Gea ae ten ara anata t eee ee eed tao Oe 82

Cy SN OMUALLE corbosrnmacre Eisen coubsae Cana Mans teinet Mae deus ces eats Seca creer ste foie SL nsnees 99

3. General remarks on the WEVUOUS SYSLEMD oo peorccnsencecenenncconstieeecescntncaranee 108

VI. The development of the reproductive organs in the Insecta.................... 113
DosR LE LEO FELL Sie ne ee eR eee Oe cece aoe a Se ne een aoe eee 113

Be DDE SUILALERLILCES at RLU gh eee Sa Seat Nese ray TE OME ste te eee eS 116

Se LN EN ECTLCL UCM CLA LA ete a ee eee See 119

Ae NGCHETAUIEONSLMET AULOMS Masean nent cece oa cat ara nt oon hater Nee eons eased cere cee 126

VII. The subeesophageal body in Xzphidium and Blatta ........2..2000.ce0ceveeeeeeees 136
SWAIN ee hari cyt essence Sos esac ee cc h hae aes te eee eee ee eee a eer naman EL sate tce eee 139
OSG al Bho bheyea gay olay cee ee Se eae ie tales Aaaer st Ee caee eee eat NB apes eas ae ea hae 143
Desemption of the plates... p22 oi once s2vansnsnsapanconnreetenetentrcaay 150-160

THE very primitive and synthetic character of the Orthoptera
has long been recognized by systematists and comparative
anatomists, but the full importance of the group from an
embryological standpoint has been but little appreciated, owing
to the meagre and fragmentary nature of the observations
hitherto published. For this reason I have made the Orthop-
tera the starting point of my studies, with a view to determining
their relations, on the one hand to the Apterygota and on the
other to the higher Pterygote orders. Only a portion of the
evidence bearing on these relationships is presented in the
following paper ; a number of observations on the Malpighian
vessels, corpus adiposum, cenocyte-clusters and abdominal ap-
pendages will be published as separate papers.

I have devoted more attention to X7zphzdium than to other
Orthoptera, partly because the Locustidze occupy a somewhat
central position in the order, and partly because this curious
form exhibits in its embryogeny better than any other insect
hitherto studied, the co-existence of certain very ancient with
very modern characters.

My German co-workers in the field of insect development
will probably regard my treatment of the literature as rather
perfunctory ; but Prof. Graber, Dr. Heider and others have
given from time to time such complete résumés of past and
current literature that I feel justified in departing from the
general custom. If I have failed to give credit where it is due,

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 3

I beg that this may be regarded as a fault of omission and not
as a fault of commission.

I would express my sincere gratitude to Prof. C. O. Whit-
man for his kindly guidance and friendly counsel throughout
the progress of my work in his laboratory at Clark University
during the autumn and winter, and at the Marine Biological
Laboratory during the summer months, of 1891 and 1892. I
am also indebted to Mr. S. H. Scudder for the identification
of several Orthoptera.

I. THE Empryontc DEVELOPMENT OF THE LOoOcuUSTID&.

1. Lhe Oviposition of Xiphidium ensiferum, Scud.

Aiphidium ensiferum, Scudder, a very common Locustid in
Wisconsin and the neighboring states, deposits its eggs in the
silvery napiform galls produced by Cecidomyia gnaphaloides
(and perhaps allied species) on the low willows that abound in
the marshy lands and along small water courses. I have found
the insect ovipositing from the middle of August to the middle
of September. It thrusts its ensate ovipositor in between the
imbricated scales of the gall and places its eggs singly or in a
more or less even row with their long axes directed like the
long axis of the gall. The eggs are completely concealed by
the scales, the overlapping edges of which spring back to their
original positions as soon as the ovipositor is withdrawn.
The number of eggs deposited in a gall varies greatly: some-
times but two or three will be found; more frequently from
fifty to one hundred; in one small gall I counted 170 and I
have opened a few which contained more. Sometimes as
many as ten eggs will be found under a single scale; when
this is the case, the eggs adhere to one another and are
more or less irregularly arranged, as if two or three insects
had in succession oviposited in the same place. _

The Cectdomyia galls vary considerably in shape: some are
long and more or less fusiform, others are spheroidal. In the
former variety the scales are pointed and flat, while in the
latter they are rounded and have their median concave por-
tions less closely applied to the convex surfaces of the scales

4 WHEELER. [MOL Nach,

which they overlap. These differences materially affect the
eggs, for many of those thrust in between the closely
appressed scales of the spindle-shaped galls are so much flat-
tened as to be incapable of developing; on the other hand the
eggs deposited in the more spacious interstices of the globular
galls are usually in no wise injured. The twoforms of gall do
not always occur in the same locality and may be the produc.
tions of two distinct species of Ceczdomyza or of one species
on different willows. The Locustids, however, seem to show
no preference for the globular galls.

The galls of Cecidomyia, being essentially stem-galls, do
not drop to the ground in the autumn like the various leaf-
galls on the willows, but persist through several seasons. A\l-
though the insects are not averse to ovipositing in the fresh
galls, they nevertheless seem to prefer these blackened and
weather-beaten specimens, probably because their scales are
more easily forced apart.

I have called attention to the fact (90°) that X. exszferum
departs widely in its habits of oviposition from its congeners,
several of which are known to lay their eggs in the pith of easily
penetrated twigs, like the species of the allied genus Orchedt-
mum. X. ensiferum has evidently found it of great advantage
to make use of the galls so abundant in its native haunts. So
recent may be the acquisition of this habit, that on further
investigation some females may, perhaps, even now be found
to have a tendency to oviposit, like Conocephalus ensiger, be-
tween the root-leaves and stems of plants, or even in the plant
tissues. It still occasionally happens that the eggs are run
through or into the tissues of the gall-scales, and not loosely
deposited. The fact that the insects have not yet learned to
distinguish the kind of gall best adapted to their purposes,
lends some support to the view that it is not so very long
since X. ensiferum agreed with its congeners in habits of ovi-
position.!

1In the vicinity of Worcester, Mass., I found galls very similar to those formed
on the Wisconsin willows. They contained a few slender yellow eggs, smaller
than those of X. exsiferum. As this species does not occur in New England I

conclude that these eggs were probably deposited by the very common XX, /ascza-
tum, De Geer.

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 5

2. The Formation of the Embryo and its Backward Passage
Through the Yolk.

a. SURFACE CHANGES:

The sub-opaque, cream-colored egg of X7phidium is elongate
oval, 3—5 mm. long and 1 mm. broad through its middle. One
of its poles is distinctly more attenuate than the other, and
there is a faint curvature in the polar axis which causes one
side of the egg to be distinctly convex and the other distinctly
concave. The broader pole is the posterior, and is the first
to leave the vagina during oviposition; the attenuate pole is,
therefore, the anterior. In the galls the eggs stand with their
attenuate poles pointing upwards. The convex face of the egg
is the ventral, the concave face the dorsal region. Inasmuch
as the egg undergoes no change in shape during development,
it is easy to orient the embryo in its different stages. This is
of considerable importance, as will appear from the sequel.

The yolk is pale yellow and very similar in constitution to
the yolk of other Orthopteran eggs. It is enclosed by a thin
leathery chorion which suddenly becomes transparent on
immersion in alcohol. When dry it is white, and the creamy
color of the egg is due to the yellow yolk shining through. As
in Blatta, the chorion is the only envelope of the freshly laid
egg ; what I described in a former paper ('90») as the vitelline
membrane is in reality comparable to a “ Blastodermhaut” as I
shall point out.

The chorion varies somewhat in thickness at different points
in the egg, being I11p towards the middle and I9op at the
poles. It is quite elastic and when cut curls in at the edges.
Its inner surface is very smooth, while outwardly it is covered
with round or oval projections which measure about 3.7» in
diameter. They are flattened at their summits and are placed
so closely together that only narrow channels run between
them and give the chorion the appearance of being covered
with a fine net of nearly uniform meshes. On closer examina-
tion it is seen that the projections are arranged in hexagonal
groups. ‘These are very distinct at either pole but fade away

6 WHEELER. [Vot. VIII.

on the median portions of the egg till they become very diffi-
cult to resolve. They evidently coincide with the areas covered
by the polygonal cells of the follicular epithelium.

No traces of micropyles could be found. Their absence in
Xiphidium is of interest, since Leuckart (55) long since
described and figured them in several European Locustide
(Meconema, Decticus, Locusta, Ephippigera). In these genera
they consist of funnel-like perforations on the ventral surface
of the chorion either near the anterior pole or nearer the
middle of the egg.

The preblastodermic stages were not studied. They prob-
ably resemble the corresponding stages of Blatta, of which I
have given a detailed account in a former paper (’89).

When fully formed the X7zp/zdium blastoderm, like that of
Blatta, consists of a thin sheet of cells, that have in part
reached the surface from the interior of the egg, and are in
part derived from these centrifugal cells by tangential division
after their arrival at the surface. Numerous cells-—the future
vitellophags —are to be found at different points in the yolk.
Whether they are derived from the incompleted blastoderm
by centripetal division, or are inhibited before reaching the
surface, my limited observations will not permit me to decide.

The cells forming the blastoderm are polygonal, much flat-
tened and of uniform size and distribution. Those on the
center of the convex, or ventral face of the egg soon begin to
change their dimensions; from being broad and flat, they
become more nearly cubical, their lenticular nuclei again
assuming the spherical or oval shape which they had in
preblastodermic stages. These changes take place over a
limited and somewhat oval area and result in the formation of
the ventral plate. The few eggs that I have been able to find
in the very first stages after the completion of the blastoderm
leave me in some doubt as to the exact process whereby the
embryo is established. I am_ satisfied, however, that the
thickening and narrowing of the individual blastodermic cells
does not take place simultaneously over the whole ventral
plate area, but that there appear, as in the crustacean egg (e.g.
Astacus, Homarus), several discrete centres about which the

No. 1.] COMTRIBUTION TO INSECT EMBRYOLOGY. a

cells are at first more closely aggregated. The spaces between
these centres are subsequently filled in by tangential cell-
divisions. Of such centres I can distinguish four: two of them,
the precursors of the procephalic lobes, are paired, while the
other two form respectively the growing caudal end of the
ventral plate and what I shall call the indusium.! The indusial
centre, which does not make its appearance till a short time
after the other centres are formed, does not join the body
of the embryo till after the spaces between the procephalic
and caudal centres are filled in. This is distinctly seen in Fig.
1 (Stage A) where the somewhat T-shaped embryo is already
established and distinctly marked off, at least posteriorly, from
the undifferentiated blastoderm. The nuclei of the blastoderm
are as yet no larger than the nuclei of the ventral plate.
Numerous caryokinetic figures in all parts of the embryo bear
witness to active cell proliferation. No such figures were to
be seen in the extra-embryonal blastoderm during and after
this stage. The ventral plate including the indusium is
scarcely a fifth as long as the egg, being much smaller in pro-
portion to the size of the yolk than in some other Orthoptera
(Blatta, Gryllotalpa).

The blastopore is seen in the stage figured as a very narrow
but distinct groove extending from the oral region to the
caudal end of the embryo, where it bifurcates before its
termination. The infolded cells give rise to the mesoderm
and also, I believe, to the entoderm.

In AXwphidium the three folds that form the amnion and
serosa arise like their homologues in Blatta. The first appears
as a crescentic duplication surrounding the caudal end; thence
it grows forward and after enveloping the whole postoral
portion of the embryo coalesces with the two head-folds, each
of which arises from the edge of a procephalic lobe. The pro-
gress of the anal fold is shown in Fig. 2 (Stage B) PI. I.
Although agreeing in its main features with what has been
described for most insect embryos, the process of envelope-

1JIn a preliminary note (90°) this structure was called the przoral plate
(Praoralplatte). Many reasons have led me to abandon this term together with
others referring to the parts of the organ in its subsequent development.

8 WHEELER. [Mors VLLt.

formation in X7zphidium, is, nevertheless, peculiar in two
respects: first, the envelopes are so closely applied to the
germ-band that in surface view their advancing edges can
be detected only with difficulty, though they may be distinctly
seen in sections; second, the point of closure of the envelopes
is situated further forward on the head than in 4latta, Hydro-
philus, Doryphora, etc. This I infer from an embryo, which I
figure (Fig. 15. Pl. II.) Here the cells and nuclei of the
amnion and serosa have become much larger than the cells
and nuclei of the embryo. The edges of the folds are
unusually distinct and enclose a circular space through which
the oral and praoral regions are clearly visible. On the
median anterior edge of the head the amnion and serosa are
completely interrupted. In no other insects have I found the
envelopes lacking on the anterior edge of the head in so late
a stage. This fact is probably significant when taken in con-
nection with changes about to occur in front of the head.

The wide procephalic lobes are succeeded by the strap-shaped
body In this a number of segments have made their appear-
ance. These are in order from before backwards: the mandib-
ular (wd. s), the first maxillary (wx. st), the second maxillary,
(mx. s?) the three thoracic (/. s1-f. 53), and the first abdominal
(a. st). Further back lies a small segment which is incom-
pletely constricted off from the first abdominal and which I
take to be the proliferating terminal segment, or telson. The
seven segments depicted in the figure are undoubtedly de-
finitive segments. The manner of their appearance will be
clear from a glance at Fig. I. In A the ligulate part of the
germ-band is seen to be faintly constricted at its base into two
segments with indications of a third. In B, a slightly later
stage, four definitive postoral segments are present, but a
portion of the germ-band still remains unsegmented. This is,
however, soon broken up into segments and we reach the stage
in Fig. 15, Pl. II. It will be observed that the embryos in
Fig. I are in many respects older than that in Fig. 15, Pl. IT.
The antennze have made their appearance and the amniose-
rosal fold has closed completely. These embryos prove
several points:—first, that the wave of metameric segmen-

No.t.| CONTRIBUTION TO INSECT EMBRYOLOGY: 9

tation passes from before backwards dividing the germ-band
into 7 or 8 segments; second, that these segments are the
definitive segments and not macrosomites, or complexes of
definitive segments; and third, that there is considerable varia-
tion in the time when segmentation sets in. To these points
I may add a fourth: segmentation appears first in the ectoderm
and only somewhat later in the mesoderm.

ers Oe
4553: SARE 3
Beis mad.s
My
so TAG, S"

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Be!
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mY

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pais

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Mites Le

A and B. Isolated embryos of AX7phidium in successive stages of metameriza-
tion. zzd., indusium ; /c/., procephalic lobe ; s¢., stomodzum ; a¢., antenna; md.-s.,
mandibular segment; wx. 5’, first; mx. s?, second, maxillary segment; 7. 5’,
prothoracic segment.

The indusium (Fig. 15, . a.) is still only a rounded thickening
of the blastoderm. Its small deep cells are continuous through
a zone of larger cells with the relatively very large and flat
elements of the primitive cell-layer. Two broad and flat
commissures appear to connect the organ with the procephalic
lobes. Thus a small space containing a few larger cells is
enclosed between the indusium and the head of the embryo.
This space (y), seen as a clear spot in surface view, lies at the
breach in the envelopes. In many embryos the indusium is

10 WHEELER. [Vou. VIII.

united with the head of the embryo (Fig. I, A and B) before
the stage of Fig. 15 and soon after this stage is, I believe,
normally united with it. This union is probably purely
mechanical—the organ remaining at its place of origin on
the surface of the yolk, while the embryo lengthens till its
head unites with the posterior end of the organ. This union
is of brief duration as is seen in Fig. 3 (Stage C).

During this stage the caudal tip of the embryo shows a
tendency to bury itself in the yolk. The amnion and serosa,
hitherto closely applied to each other, now separate at
the caudal end, where, as I have said, they first arose as a
crescentic fold. Soon the tendency to enter the yolk becomes
more pronounced so that the tail curls back and leaves the
ventral face of the egg. Meanwhile the remainder of the
embryo moves down the ventral face a short distance, thus
pushing its tail still further into the yolk and causing the
separation of the envelopes to advance still further headwards.
The indusium does not accompany the embryo in this move-
ment, but remains nearly or quite stationary ; consequently
the head gradually separates from the organ till it is connected
only by means of a slender band of cells in the median
line. (Fig. 3 and Fig. 16.) This link soon ruptures and the
indusium is set adrift from the embryo, or, more precisely,
the embryo is set adrift from the indusium. (Fig. 4, Stage D.)
In profile the embryo now resembles the small letter j, —the
dot being supplied by the isolated indusium.

Important changes begin to affect the indusium during or
more frequently just after its separation from the embryo.
The closely packed cells at the periphery, as indicated by their
nuclei, begin to arrange themselves radially (Fig. 16). Some
of the large nuclei of the serosa may be seen encroaching on
the edges of the disk from all sides, leaving only the median
portion free. Sections show that the organ is now forming an
amnion like that of an embryo. In the middle of the disk
appear several shrunken but distinctly defined nuclei which are
proved by focusing to be confined to the surface of the organ.!

1 Only four of these peculiar bodies are represented in the figure (x7); there

were several others in the same preparation, but for the sake of clearness I have
omitted them in the drawing.

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. If

The serosal fold continues to advance from all sides till the
organ is entirely covered. Viewed from its ventral surface
the egg now has the appearance of Fig. 4 (Stage D). Here
the indusium is cordate in outline and somewhat larger than
usual. Of the abdomen only the two basal segments still
remain on the ventral face of the egg; the remaining seg-
ments curl back into the yolk.

During this and the two preceding stages the cephalic and
thoracic appendages have become distinctly established as
rounded lateral outgrowth of their respective segments. The
antennze (@/) originate as lobular outgrowth from the posterior
edges of the procephalic lobes. They are distinctly postoral
in origin. The margins of the triangular oral orifice are some-
what swollen; the anterior edge, where the labrum is about
to appear, is cleft in the median line (Fig. 16). The three
thoracic segments are very slightly or no broader than the two
maxillary segments. The appendages of these five segments
are also alike in size, shape, and position. In very early stages
of other insect embryos, even before the amnion and serosa
are fully formed, the thoracic become broader than the maxil-
lary segments, and the legs, as soon as they appear, may be
readily distinguished from the two pairs of maxille by their
greater size and prominence. The Locustid embryo, there-
fore, has even a stronger tendency to revert to annelid-like or
myriopod-like ancestors than is apparent in any of the other
insects whose ontogenies have been investigated.

The mandibular segment of Azphzdium like that of other
insects, is somewhat retarded in its development. Between
this and the antennary segment careful study of sections and
surface preparations reveals the presence of another segment,
shown very distinctly in outline in Fig. 16 (éc.s.). This is
no other than what I have called the intercalary segment in
Doryphora. It is the tritocerebrum of Viallanes (90% ’90°).

The embryo continues to move back into the yolk, fol-
lowing the curved path established by the inflexion of the
posterior segments till its tail is finally arrested by striking the
serosa on the dorsal surface. At this time the embryo has the
form of an arc subtending the dorsoventral diameter of the egg.

12 WHEELER. [Vou. VIIL

Returning to consider the indusium, we find that it begins
to increase in size before the embryo’s head leaves the
ventral face. The organ stains much less deeply, and even in
surface view its expansion may be seen to be due to a flatten-
ing of its component cells. In Fig. 5 (Stage E) is represented
an embryo merged in the yolk up to the first maxillary seg-
ment. The indusium extends around on either side nearly
to the middle of the lateral face of the egg. Either the tran-
sition of the embryo takes place rapidly or the organ changes
very gradually, for the latter is in about the same stage after
the embryo has become established on the dorsal surface.
The manner in which the expansion of the indusium is
brought about will be apparent when I come to describe its
structure in sections.

b. THE INDUSIUM IN SECTION.

As will be seen from the preceding account, the indusium
is simply a circular thickening of the blastoderm, situated
in the median line, between and a little in front of the
procephalic lobes. It does not arise as a part of the ventral
plate but as a separate centre which is at first merely a
cluster of blastoderm cells that have changed from the pave-
ment to the cubical or columnar type. This centre is further
increased in breadth and thickness by caryokinesis. In the
earliest stages examined, sections of the organ show the same
cell-structure as sections of the procephalic lobes.

Median longitudinal sections of the embryo in Stage C are
interesting as showing the relations of the indusium to the
embryo and its envelopes. I reproduce such a section in Fig.
21, Pl. III. Here the organ (f. 0.) appears as a large flattened
cell-aggregate somewhat thinner in the centre than nearer its
periphery. Owing to the shape of the mass, the median cells,
as indicated by their nuclei, are arranged with their long axes
perpendicular to the flat outer surface of the organ, while the
cells of the thickened lateral portions become gradually more
oblique till those on the extreme periphery assume the same
position as the serosa cells (s.), The nuclei are most frequently

Novt:)|) GONTRIBOTION TO INSECT EMBRVOLOGY. 13

situated at the inner ends of the cells so that masses of
enucleate protoplasm are left at the surface. Posteriorly the
organ is linked to the embryo by means of a few flattened
cells. In the section two of these cells are seen at ¢ differing
in no wise from the serosal elements (s.) in front and on either
side of the organ ; the upper cell passes directly into the serosa
covering the embryo, while the lower abuts on the cells that
form the transition from the ectoderm to the amnion. The
ectodermal layer of the embryo (ec.) is nearly as thick as the
indusium and of similar cytological structure. The begin-
ning of the stomodzeal invagination is shown at o.

The next section figured (Fig. 17 Pl. II) is from an indusium
in a somewhat younger stage than that represented in surface
view in Fig. 2. Being transverse the section shows an evenly
convex outer surface, continuous with the surface of the serosa
(s.) enveloping the yolk. The cell-contours are still visible
and show that the cells constituting the median portion of the
organ are polygonal. The nuclei of these elements are spherical
or oval and contain one, or more rarely, two nucleoli besides the
usual chromosomes. In the peripheral ring-shaped thickening
the cells (d.) are larger and pyramidal or fusiform in outline,
while their nuclei differ in no wise from the nuclei of the
median cells. The serosal cells stain more deeply than the
cells of the organ, as may be seen at s where a single cell
overlaps the edge of the disk. This depth of color is appar-
ently purely optical, being due to the greater size and flatness
of the serosal nuclei. The walls of both the small polygonal
and larger pyramidal elements fade away towards the surface,
where the bodies of the different cells become confluent
to form a homogeneous mass.

In this surface-mass of protoplasm which takes the normal
pink stain in borax carmine, are to be found several of the
peculiar nuclei, mentioned above as distinctly discernible from
the surface (Fig. 16). They differ markedly in structure and
appearance from the normal nuclei in the inner portions of the
indusium as will be seen by comparing the cells of Fig. 24
with those in Fig. 23, both of which figures were drawn with
a high power. The normal cells (Fig. 23) have spherical or

14 WHEELER. [Vov. VIII.

oval, evenly rounded nuclei with one or two nucleoli and their
chromatin is distributed in what I take to be the typical resting
reticulum. The caryolymph, or Kernsaft, is faintly stainable.
On the other hand, in the nuclei of Fig. 24 the nuclear wall is
very irregular, the caryolymph much more limpid and refrac-
tive and the chromatic reticulum has coarser meshes. The
chromatic nodes of the reticulum are larger than in Fig. 23
and seem to be applied to the indentations of the nuclear wall.
Nucleoli appear to be absent. These specialized nuclei also vary
greatly in size. Ina series of sections it is easy to find nuclei
intermediate between the two extremes here described,
being evenly rounded but with colorless caryolymph and
coarse chromatic reticulum. A cluster of four such nuclei
is shown at zz? Fig.17. These intermediate forms, occurring
as they usually do, between the normal and the modified
nuclei may be taken to indicate that the nuclei of the extreme
types are genetically connected. Some of the normal nuclei
probably leave their respective cells in the median portions of
the organ and move up into the syncytial protoplasmic layer,
undergoing the modification in structure during their emigra-
tion. When they have reached their destination they are
perhaps broken down and converted into protoplasm. Certain
it is that later no traces of them are to be found in the
indusium. I do not believe that I am here considering collapsed
and distorted caryokinetic figures, as these delicate structures
are quite faithfully preserved in eggs killed by means of heat.
The distorted nuclei are not confined to the indusium but
occur also in the ectoderm of the embryo itself.

When the organ has reached the state just described it
usually separates from the head of the embryo; it may, how-
ever, remain attached for some time longer. Like the embryo
it is now an isolated body lying on the yolk; but unlike the
embryo it is still only a part of the serosal envelope (which is
itself only the extra-embryonal portion of the blastoderm). The
serosa is a closed sack enveloping the whole yolk and the
indusium is simply a swelling at one point on its inner
face. (Fig. II, A.) The process of envelope formation which
now begins in the indusium is much less clear than the cor-

Noss) €ONTRIBOCTION TO INSECT EMBRVOLOGY, I5

responding process already completed in the embryo. From
among the numerous preparations which I have made I select
for illustration one (Fig. 18) which seems to show the process
clearly. In surface view the organ would appear as in Fig. 3.
The spreading of the serosal cells over the edges of the
disk from all sides is now seen to be due to a process of
induplication, or folding. The circular fold is, of course,
cut in two places in the median transverse section figured.
It advances in such a manner as to leave the outer face of
the indusium evenly rounded and undisturbed, the upper sur-
face of the fold usually forming a continuous line both with
the outer surface of the serosa and with the median still
uncovered portion of the organ. The fold continues to advance
from all sides till the layers of which it consists meet and
become confluent in essentially the same manner as the folds
that form the amniotic and serosal layers over the embryo
proper. We now have three layers of cells. (Fig. 19.) The
outermost layer, s, is the serosa which has everywhere the same
structure and evenly envelops the whole egg, having been
separated first from the embryo and now by a similar process
also from the indusium (Fig. IH, B). The innermost layer
consists of the unchanged greater portion of the organ. The
median layer, to judge from its component cells, seems to
be derived exclusively from cells of the original body of the
organ and not from the serosa. This layer is, therefore, like
the amnion of the embryo proper, structurally more closely
related to the body it envelops than to the serosa. Fig. 18
favors this conclusion, which presupposes that only the outer
half of the circular fold is derived from the serosa, for in this
section the lower and thicker layer of the fold on either side
certainly consists of cells derived from the body of the organ.
Even before the layers are fully formed the edges of the two-
layered organ are sharp and somewhat irregular (Fig. 18), not
rounded like the edges of the embryo when its amnion is com-
pleted. The whole organ still has essentially the same form
that it had in the stage represented in Fig. 17.

It will be convenient to name the different layers of cells,
thus far distinguished. For the amnion of the embryo proper

16 WHEELER. [Vou. VIII.

I shall retain the old name; the corresponding envelope of the
indusium and the body of the organ will be designated as the
outer and inner indusium respectively.

In by far the greater number of cases the process of

Fie. II.

Diagrams illustrating the movements and envelopes of the X7phidium embryo.
A, after the closure of the amnioserosal folds ; 2, during the embryo’s passage to
the dorsal surface; C, just after the straightening of the embryo on the dorsal
surface. zzd., indusium —afterwards forming zd’, the inner, and z7d?, the outer
indusium ; ch., chorion ; s7., serosa; am.,amnion ; 2d., germ-band ; v., yolk ; d/.c.,
Blastodermhaut.

envelope formation over the indusium is much obscured by
rapid slurring. In fact the whole process has frequently the
appearance of being due rather to a shifting and migration
of cells than to the formation of true folds. The cells of the
serosa seem to creep over the disk while the cells forming the

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. LiF |

edge of the organ itself appear to creep along under and a little
in the rear of the advancing serosal elements. I cannot here
go into greater detail without unduly increasing the number of
my figures. Nor is it necessary, since it will, I believe, be

ies

Diagrams illustratlng the movements and envelopes of the X7piidium embryo.
D, the stage of the shortened embryo on the dorsal yolk ; “, embryo returning to
the ventral surface ; #, embryo nearly ready to hatch. c., chorion ; 6/. c., Blasto-
dermhaut ; s7., serosa ; zd‘, outer indusium ; zzd?, inner indusium ; zzd* + am.,
inner indusium and amnion fused; am., amnion; 7zzd?c., cuticle of the inner
indusium ; zd? s., granular secretion of the inner indusium; am. s., amniotic
secretion ; v., yolk ; c/., columella; gé., germ-band.

acceded that the process briefly described in the above
paragraph, though now occurring in comparatively few
embryos, is very probably the more primitive process, whereas
the slurring observed in so many cases is to be attributed to an
unquestionably rudimental condition of the organ.

18 WHEELER. [Vou. VIII.

By the time the folds have closed over the indusium the
abdomen of the embryo has sunk into the yolk to a con-
siderable extent, presenting in surface view the appearance
of Fig. 4. The organ seems to undergo no further change till
the embryo has almost left the ventral face of the egg. Then,
as we have seen, it begins to increase by spreading. An early
stage in this process is shown in section in Fig. 20. No change
is perceptible in the serosa, which is now independent of the
organ; the outer indusium (az!) is much attenuated, as may
be seen by comparison with Fig. 19. Its cells have assumed
the same shape and dimensions as those of the superjacent
serosa; only along the edges of the disk, where the outer
becomes continous with the inner indusium, or body of the
organ, do the cells still retain their original shapes. In
the body of the organ the cells are arranged in two irreg-
ular rows, whereas in the previous stage (Fig. 20) there were
three. This diminution in the number of cell-rows is the
result of horizontal spreading, a process which also accounts
for the stretching of the outer indusium as indicated by the
flatness of its cells. At zz is seen one of the large modified
nuclei, which has persisted unusually late.

In Fig. 22 I give a section of the indusium seen in sur-
face view in Fig. 5. The spreading of the cells has pro-
gressed till the organ lies like a saddle on the ventral face of
the egg, covering nearly half of its circumference. The serosal
layer (s) is, of course, unaffected. The outer indusium (a7')
is stretched to such an extent that its cells are united only by
an exceedingly thin and in many places, almost imperceptible
layer of protoplasm. The inner indusium now consists of a
single row of cells, instead of two rows as in the preceding
stage. It is in about the same state of tension as the outer
layer in Fig. 19.

3. The Development of the Embryo from the Time of ts
Reaching the Dorsal Yolk to Revolution.

In the foregoing paragraphs the development of the embryo
was traced to Stage E, when the germ-band hangs festoon-like

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. I9

in the yolk with its cephalic amnion applied to the ventral
serosa and the amnion overlying its terminal abdominal seg-
ments applied to the serosa covering the dorsal yolk. No
sooner has the caudal end become fixed than the head is
detached from the ventral face of the egg and the embryo
swings back, straightens out, and becomes applied full length
to the dorsal serosa. The movements whereby this condition
is attained resemble the movements of a leech in passing from
one side of a test-tube to the opposite surface; holding fast to
the glass by means of the oral sucker, the tail is stretched out
till it reaches the opposite surface, when the anterior end is
loosened and the body drawn over. There is, however, a
marked difference between the embryo and the leech since
the body of the former is not contracted during its transition.

Fig. 5 represents a rather rare condition in that the pro-
cephalic lobes lie at the same level and are symmetrically dis-
posed with respect to the long axis of the egg. More frequently
the germ-band is twisted during its transition so that one of the
procephalic lobes reaches further forward than the other on
the surface of the yolk. Sometimes it is the left lobe which
extends further forward but more frequently it is the right.
The twist in the germ-band occurs in the thoracic or
abdominal region, more often in the former, the abdomen being
nearly straight. I take this twisting of the embryonic axis to
indicate that the germ-band executes a screw-like movement
while penetrating the yolk, and I believe it to be perfectly
normal, having observed it in the majority of embryos. Traces
of this twisting are clearly discernible even in embryos which
have almost straightened on the dorsal surface.

As a consequence of the passage of the embryo through the
yolk in the manner above described, the germ-band has shifted
its position from the median convex ventral to the median
concave dorsal surface of the yolk, so that it is now reversed:
originally its head pointed to the tapering anterior pole, now
it lies with its head directed towards the blunt posterior pole
of the egg. The amnion, of course, accompanies and remains
in close contact with the ventral surface of the embryo during
all this time.

20 WHEELER. (Vou. VIII.

During or more frequently at the close of the embryo’s migra-
tion the primary serosa secretes from its whole outer surface
a thin chitinous cuticle. In my preliminary notes (90 '90°)
I wrongly designated this cuticle as the vitelline membrane, an
error which is, to a certain extent, pardonable, inasmuch as
the layer in question is structurally exactly like the vitelline
membranes of other insects. But it certainly cannot be
homologized with these membranes since it is secreted during
a comparatively advanced stage by an embryonic cell-layer,
the serosa, and not by the surface protoplasm of the un-
segmented egg.

As soon as the embryo has taken up its position on the
dorsal surface, the yolk segments ; each vitellophag appro-
priating as many of the yolk-bodies as the radiating filaments
of its cytoplasm can hold together and fashion into a rounded
mass. Apparently the process is delayed in order that the
passage of the embryo through the yolk may be facilitated, for
obviously the embryo will move more easily over a prescribed
path through a mass of small mobile particles than between
large masses formed by the aggregation of such particles. The
yolk-masses, at first very distinctly marked, soon fuse with
one another so that their boundaries can be traced only by
reference to their centres, which coincide with the nuclei of
the vitellophags.

After leaving the ventral face of the egg the embryo in-
creases greatly in length. Just before burying its tail in
the yolk and while still completely on the ventral surface it
measured only .7 mm.; now it measures 1.7 mm. This in-
crease in length, as will be inferred from the foregoing descrip-
tion, is due to two causes: an intercalation of new segments
in front of the anal plate to complete the abdomen, and a
stretching of the segments thus arising.

A glance at Fig. 6, which represents an embryo in the stage
of its greatest elongation on the dorsal surface, shows that
many important changes have taken place since it left the
ventral surface. The cephalic and thoracic appendages have
assumed a more definite character. The labrum (/0.) has sud-
denly appeared, the first and second maxille (#1, mx?) have

NOME: | CONTRIBUTION TO INSECT EMBRVOLOGY. 21

each become trilobed, while the metathoracic leg (3) already
exhibits unmistakable traces of its characteristic thickening in
the larvaandimago. The pleuropodia (f/. (ap")) stand out clearly
from the edges of the first abdominal segment. Shining
through the stretched ectodermal layer of the abdominal seg-
ments may be seen the paired mesodermal somites (coe.), or
mesomeres. The anal plate with its pair of cerci (cc. (ap )),
and the anus are definitely established. A faint neural furrow
runs from the mouth to the anus, and in the thoracic region
faint metameric indications of the ganglia are apparent. All
these important changes have taken place within the yolk during
the transition of the embryo. This renders their study on
hardened material very difficult, for although the embryo may
be dissected away from the yolk, it is so much curved that it
can be mounted only in pieces, and the yolk is at this period
so difficult to cut that only fragmentary series of sections can
be obtained.

One of the most interesting changes undergone while the
embryo is still in the yolk is the appearance of the labrum. In
Fig. 6 (Stage F) the labrum is a distinctly unpaired circular
appendage. But that it has a paired origin I infer from a
transverse section, part of which is represented in Fig. 35.
This passes just in front of the mouth of an embryo but little
older than Stage E. The appendage (/d.) is here seen to be
distinctly bilobed although it does not yet project beyond the
general level of the head. This bilateral condition is speedily
slurred over and the organ grows into an unpaired and in
most embryos perfectly circular disk overhanging the mouth.
Very rarely, as in Fig. 7 it may show traces of its paired origin
even during later stages.

Let us return to the indusium which we left as a thin
round plate gradually spreading over the yolk just beneath the
ventral serosa. The outlines of this plate are not always cir-
cular but exhibit traces of lobulation (Fig. 5). The spreading
is at first uniform along its whole circumference so that the
organ soon assumes the shape of a circular scroll clasping the
egg. Its lateral edges approximate on the dorsal surface just
over the ventral face of the embryo but are temporarily arrested

22 WHEELER. [Vou. VIII.

in their growth before they unite. The anterior and posterior
edges, however, continue. to advance without interruption, so
that the disk if spread out on a plane surface would in its suc-
cessive stages represent a series of ellipses with constant short
axis but continually increasing longitudinal axis. In this
manner the disk grows towards either pole while envelop-
ing the egg laterally. The edges of the organ continue to
approximate on the dorsal surface but stop growing just before
they meet. Hence, when the egg is viewed from the dorsal
surface a long, narrow slit is seen extending nearly its entire
length and separating the dorsal edges of the organ. It is not
till the anterior and posterior edges have nearly or quite
reached their respective poles that this slit closes with the
fusion of the edges of the organ. The raphe is at first so
weak that the edges may be broken apart by slight pressure
with the needles, but it soon becomes permanent and the egg
is now completely enveloped by two further membranes —the
inner and outer indusia. Before the fusion of these two mem-
branes the amnion of the embryo was in contact with the
serosa but now that the edges of the indusia have worked
their way in between the serosa and amnion, the latter comes
to lie in contact with the inner indusium. Henceforth the
serosa is excluded from taking any part in the development
of the embryo; both its position and function are now usurped
by the inner indusium.

One is enabled to follow the different stages in the progress
of the indusium, from its disk-like condition on the ventral
yolk to the complete union of its dorsad-growing edges, by
means of a peculiar secretion of its inner layer. This is a
brownish or blackish granular substance, probably some urate,
which appears to be secreted by all the cells of the inner
indusium and which gives the organ the appearance of a large
brown blotch in a stage a little older than E. At first pale and
hardly perceptible, this spot gradually deepens in color till its
advancing edges become distinctly outlined on the underly-
ing yolk. A clear idea of the closure of the edges may be
obtained from Fig. III, A-C. The dark granular secretion
is shown in Fig. 6 at exv/.

No. t.] CONTRIBUTION TO INSECT EMBRYOLOGY. 23

Soon after the union of the edges of the outer and inner
indusial layers a chitinious cuticle is secreted by the outer
surface of the latter. This cuticle is. thicker and seems to be
of a deeper hue than the cuticle secreted by the serosa. It

Fic. III.

Two stages in the spreading of the indusium. 4, lateral view of egg just after
the arrival of the embryo on the dorsal yolk ; 4, lateral view of the egg with the
indusium nearly reaching the poles ; C, same egg seen from the dorsal surface.

definitely excludes the outer indusium from any further share
in the development of the embryo. Even in Stage E, this cell-
layer was reduced to an exceedingly thin membrane. (Pl.
III, Fig. 22, am.1) It seems to fuse with the serosa and to
retain a connection with the inner indusium only at the ex-

24 WHEELER. [VeL. VIII.

treme anterior pole of the egg. I confess, however, that my
observations on this envelope are rather unsatisfactory.

After the completion of the processes described in the
preceding paragraphs we may distinguish several envelopes
in a median transverse section of the egg. Passing from with-
out inwards we have (1) the chorion, (2) the Blastodermhaut-
like cuticle secreted by the serosa, (3) the serosa, (4) the
outer indusium, (5) the layer of dark, granular secretion, (6)
the cuticle secreted by the inner indusium, (7) the inner
indusium and (8) the amnion. While envelopes I—7 invest
the whole egg, layer 8, the amnion, covers only the embryo.

The general development of the embryo has been traced to
Stage F, when it lies as a straight and attenuated body on the
dorsal yolk with its head directed towards the caudal and its
tail towards the cephalic pole of the egg.

Like all other insects that have a stage during which the
body is greatly elongated (Coleoptera, Diptera, Lepidoptera)
Xiphidium passes into a series of stages during which the
germ-band is gradually shortened. The shortening is accom-
panied by a broadening of all the segments, a growth of the
appendages, and very important internal changes. The com-
pletion of this process is reached in Stage G (Fig. 7). Besides
a greater development of the appendages seen in Stage F, Fig.
7 also shows that the abdominal appendages have appeared.
Of these there are nine pairs, exclusive of the pleuropodia
and cerci, so that in X7zphidium, just as in Slatta and many
other insects, every segment of the abdomen bears a pair of
appendages. Starting with the basal segment there are eight
pairs of stigmata. These are not all seen in the figure. Just
back of each pair of tracheal invaginations appears a second
pair of ingrowths—the metastigmatic depressions—seen as
small white spots just outside the appendages, near the pos-
terior edges of their respective segments. They are in line
(homostichous) with the tracheal invaginations which occupy
corresponding positions near the anterior edges of their
respective segments.

The ventral flexure of the abdomen constitutes another very
important difference between Stages Gand F. In X¢phidinm

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 25

this flexure always takes place between the 7th and 8th seg-
ments and is brought about during the shortening of the
embryo. It is essentially the same flexure which is found in
Blatta and in Decapod Crustacea.

In Stage G the antennz have increased to nearly one-third
the length of the embryo. The procephalic lobes on which the
segmentation of the brain is plainly visible, have developed
greatly. The appendages, instead of projecting laterally, as
they do in the younger embryo, are folded over the ventral
surface of the germ-band. The nerve cord is distinctly
marked out. (See abdominal region, Fig. 7.)

It is in this stage, or one but slightly more advanced, that
the embryo passes the winter. Cleavage and the succeeding
stages up to F are passed within a month after oviposition —
during the warm days of August and September. But even
should October and November be mild and sunny, development
seems to have come to a temporary standstill on reaching
Stage G. Among the hundreds of embryos which I collected
during three succeeding autumns, I did not find one that had
passed far beyond this stage. Nevertheless if kept in a warm,
moist atmosphere during winter, a certain number of eggs
will continue their development almost to hatching.

Before passing on to later stages in the development I will
here give a brief account of some anomalies in the development
of the indusium.

4. Variations in the Development of the Indusium.

In the preceding pages I have described what I take to
be the normal development of the indusium of A7phidium.
A considerable number of embryos (about 100), being nearly
one half of the total number examined for the stages thus far
described, deviated more or less widely in so far as the in-
dusium was concerned from what I consider the normal type of
development. Unfortunately I did not discover the organ till
it was too late in the season to obtain a large supply of material
in the requisite stages, so that the variations here briefly
noticed probably represent only a small fraction of those

26 WHEELER. [Vor wai:

observable in a large number of eggs. The variations may be
tabulated thus :—

1. Variations in size. Normally the indusium is of the
same size as one of the procephalic lobes (.2 mm. in diameter)
so that the head of the embryo resembles a clover leaf as long
as the organ is attached to it. When the chorion is removed
the organ may be distinctly seen with the unaided eye as a
milk-white spot on the translucent yolk. Occasionally, how-
ever, embryos will be found in which it is less than .t mm. in
diameter, and all variations between this and the normal size
may be observed.

2. Variations from the typical circular form. These varia-
tions are very numerous and may be regarded as belonging to
two classes. In one class the indusium is rounded in outline,
while in the other it is ragged and more or less irregular.
To the first class may be assigned the oval, cordate and
multilobulate varieties not infrequently observed; to the second
belong a number of irregularly stellate and rhizopod-like forms.
In one of my preparations, midway between the two classes,
the indusium is evenly rounded anteriorly and ragged poste-
riorly along that portion of its periphery which has just broken
away from the head of the embryo.

3. There is a variation in the time at which the organ
is set free from the head. This cannot be proved directly
by observation of the organ itself, for it usually does not
begin to form the circular fold till after its isolation, but
differences in the embryo, especially in the prominence of the
segments and appendages, show that the organ remains at-
tached to the head in some cases longer than in others.

4. Variations in the development of the circular fold. These
variations, alluded to above, are characterized by a greater or
less distinctness in the folds that give rise to the inner and
outer layers. All shades in the process may be found between
the distinct and comparatively rare: method described and
figured (Fig. 3), and the more frequent and obscurer method
whereby the three layers are formed by a shifting of the
individual cells.

5. Variations in number. I have twice observed two indusia

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 27

in the same egg. In the first case the embryo itself was in
every way normal, and the first indusium of the normal size
and shape, and in the usual position. The second, somewhat
smaller, though regularly circular organ, was situated in front
of the first and a little to the right of the median line. The
distance between the two organs was about double the distance
between the first organ and the head of the embryo. The
outlines of the second or more anterior organ were less definite
than those of the first. The amnion and serosa had formed
over the embryo, but neither of the indusia showed as yet any
tendency to form envelopes. Whether these two organs were
derived from the division of one original przoral cluster of cells,
or were originally established as two separate centres on, the
blastoderm, I am unable to decide. The latter method would
seem to be the more probable.

The other case is somewhat singular. The first indusium
was normal in size and position, but irregularly heptagonal
in outline. The second, situated a short distance to the
side of the right procephalic lobe, was not more than a third
the size of the first organ and quite regularly quadrangular.
The embryo itself was normal and covered with the amnion
and serosa. The envelopes had also formed over the two
organs, which in this case also probably originated from two
discrete centres in the blastoderm. The smaller organ had
probably never been attached to the head of the embryo.

5. The Revolution of the Embryo

During the first warm days of spring the Xiphidium embryo
resumes its development. This is characterized for some time
by a growth of the germ-band in breadth and length and a
lengthening of the appendages. The body of the embryo,
which in Stages F and G was much narrower than the egg
now becomes almost as broad so that its pleural edges embrace
the yolk. This increase in size brings the head somewhat
nearer the lower pole, and there soon sets in a decided move-
ment of the whole body in this direction. When the head has
almost reached the lower pole, the amnion covering the face

28 WHEELER. [Von. VIII.

of the cephalic end fuses with the overlying inner indusium.
A rent appears in this fused portion of the envelopes and
through it the head is soon seen protruding. Gradually more
of the body is pushed through the orifice, first the mouth parts,
then the thoracic legs and finally the abdominal segments, till
the whole embryo comes to lie free on the surface of the yolk
in the space between the inner indusium and its cuticle. The
amnion and inner indusium, which during the evagination of
the embryo have remained united at the edges of the rent
are folded over the pleural region of the embryo onto the yolk.
The two envelopes now form but a single layer enclosing the
yolk like a bag. The inner indusium is united to the edges of
the amnion and these in turn are united to the pleural edges of
the embryo, with the ectoderm of which the amniotic cells are
continuous. The small size of the amniotic cells as compared
with the huge flattened elements of the inner indusium enables
one readily to distinguish the limits of the two envelopes.

During its evagination from the cavity of the amnion the
embryo gradually passes around the lower pole of the egg
head first and begins to ascend the convex ventral surface. An
embryo freed from all its envelopes except the two that take
part in revolution is represented in Fig. 8, in the very act of
turning the lower pole. The amnion and inner indusium are
folded back over the yolk, the former (am) characterized by its
small rounded nuclei, the latter (sv.) by its large flat elements.
The line of juncture of the amnion with the body of the
embryo is marked by a denser aggregation of nuclei. The
ventral flexure still persists on the dorsal surface.

The cavity of the amnion contains a quantity of serum-like
liquid, which during the evagination of the embryo is poured
into the space separating the inner indusium from its cuticle.
This liquid collecting at the lower pole, may function as a
lubricant and cushion, and thus facilitate the movements of the
germ-band. In hardened specimens it is found as a gran-
ular magma enveloping the appendages. It is not shown in
Fig. 8.

In many respects the embryo in Stage H has advanced con-
siderably beyond that represented in Fig. 7. In the head, the

Noord CONTRIBUTION TO INSECT EMERYOLOGY, 29

eye is distinctly marked out and its cells are arranging them-
selves to form the ommatidia, as is evident from the regular
series of pale dots. The labrum, now considerably enlarged,
is spade-shaped in ventral aspect. The antennze have grown
in length, and the saltatory legs (#3) are assuming their defin-
itive characters. The large tapering pleuropodia stand out
prominently on the first abdominal segment. Near the bases
of the legs the thoracic stigmata are distinctly seen. They
had made their appearance in Stage G, but for obvious reasons
could not be shown in the figure.

The anterior end of the embryo continues to move up the
ventral surface of the egg, straightening out as it rises.
Finally the flexed terminal segments of the abdomen are
again bent back to their original position in line with the
rest of the body. Since their flexure these segments (the 8th—
11th) have been the only portion of the body provided with a
completed dorsal wall (vzde Fig. 7). After the bending back
of the abdominal tip its segments still retain a certain inde-
pendence and make no attempt to embrace the yolk of the
posterior pole as do the segments in front of them. It is for
this reason that the abdomen presents a constriction just in
front of the eighth segment. This constriction is especially
noticeable in profile view.

The turning of the lower pole of the egg seems to take place
very rapidly compared with other equally important processes
of development, such as the passage of the embryo through
the yolk. I infer this from the relative scarcity of embryos in
the act of returning to the ventral surface. I have, however,
succeeded in finding all the stages in the process of revolution,
and feel quite as confident of having correctly interpreted my
preparations as if I had studied the living egg.

6. The Stages Intervening between Revolution and
Flatching.

Fig. 9 represents an embryo that has just straightened out
on the ventral surface of the yolk, which the reader may
imagine as extending up beyond the head to nearly twice the

30 WHEELER. [Vor VINI.

length of the embryo and terminating in the pointed anterior
pole. A comparison of Figs. 8 and 9 shows that, although
the former embryo has completed its revolution, it is neverthe-
less in an earlier stage so far as the development of its organs
is concerned. This is particularly noticeable in the labrum,
antennz and mouth parts, the eyes and the saltatory legs.
Hence we may infer that the time for turning the lower pole
is subject to considerable variation.

In Fig. 9 it will be observed that many of the abdominal
appendages have disappeared. Pairs are, however, retained on
the 8th to 11th segments (ap8—cc, (ap'1)). The pleuropodia
are also still present though concealed behind the bases of the
metathoracic legs. The disappearance of the appendages on
the 2d—8th segments probably has its immediate mechanical
cause in the lateral stretching which characterizes these seg-
ments in their attempts to embrace the yolk.

The embryo continues its growth as before in two directions
—the body constantly lengthening and thus bringing the head
nearer the pointed anterior pole, while its lateral walls, envelop-
ing more and more of the yolk, gradually grow towards each
other and finally unite in the median dorsal line. The union
begins with the 7th abdominal segment, just in front of the seg-
ments which have for some time been provided with a dorsal
wall, and continues headward. I am not certain as to what
becomes of the amnion during this process. Its cells appear
to take no part in the formation of the dorsal wall, but very
probably degenerate and become supplanted by the cells of the
advancing ectoderm. It must be remembered that a hard and
fast line cannot be drawn between the amnion and the pleural
ectoderm ; the cells of both structures passing into one another
by insensible gradations. My reasons for supposing that the
amnion proper takes no part in building up the embryo are
mainly of a theoretical nature and will be given in the latter
part of this paper.

Concerning the fate of the inner indusium there can be little
doubt. While the embryo is continually advancing towards
the cephalic pole and enclosing more and more of the yolk —
this envelope, which, as above stated, is characterized by

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 31

huge flat cells and nuclei, is being as gradually restricted to a
more and more limited yolk surface. In consequence of this
restriction its component cells become broader radially and
narrower tangentially. In this stage the envelope functionally
of other insects. It cannot,

’

corresponds to the “dorsal organ’
however, be thus designated without still further increasing
the number of heterogeneous structures included under that
unfortunate term, since the “dorsal organ”’ of other insects is
a thickening of an envelope represented in A7phedium by the
serosa.

The thickened inner indusium is soon reduced to a cap of
cells on the anterior pointed pole of the egg. As the head of
the embryo advances to cover more of this pole, the envelope
is pushed further forward and finally stripped from the yolk
altogether. The anterior cranial walls then close over the
pole and thus effectually separate the yolk from the inner
indusium. The latter is reduced to a small conical mass, the
cells of which soon show unmistakable signs of degeneration.

Soon after the embryo has thus rid itself of its envelopes
and has taken into its mesenteron the whole mass of yolk not
utilized in the processes of development hitherto undergone, a
chitinous cuticle is shed from its entire surface. This may be
designated as the first larval cuticle. It appears first on the
ventral abdominal surface and spreads thence headward and
dorsad. The progress of cuticularization is readily traceable
by staining embryos in this stage, for the parts over which the
cuticle is formed will not take the color; where it is being
deposited the stain takes faintly and where it has not yet
appeared, the stain, of course, penetrates easily. Ayers (84)
observed in Cécanthus that the secretion of the cuticle
began on the ventral surface of the embryo and extended
dorsad. ‘This is just what we should expect from the fact that
the dorsal hypodermis is ontogenetically a more recent forma-
tion than that of the ventral surface.

The first larval cuticle is about 5» thick and consists of
three layers. The innermost is apparently homogencous and
stains deeply in Orth’s lithium carmine while the middle layer
remains clear and vitreous. The outer layer is radially striated

20 WHEELER. [VoL. VIII.

and has the distinctly yellow tint of old chitin. Its outer sur-
face is minutely papillate. On the appendages the cuticle is
much thinner than it is on the trunk and though it stains it
does not show a differentiation into three layers.

Before shedding the first cuticle the hypodermis secretes a
second larval skin which persists till after hatching.

In Fig. IV, I have attempted to represent semi-diagrammatic-
ally the condition of the envelopes at a time when the eyes
begin to acquire pigment. The chorion (ch.) is much distended
and the egg larger and more resistent to the touch then it was
during the autumn. Passing from without inward we first
meet with the cuticle secreted by the serosa (s7.c). Then
follows the serosa itself (sv.) to the inner face of which the
remains of the outer indusium (zzd.!) are applied. At the ex-
treme anterior end of the egg both these cellular envelopes
appear to be much thickened and pass into a cylindrical pedicel
of granular plasma which I shall call the columella (c/.). This in
turn is continuous with a conical mass of cells (zzd.2), the re-
mains of the inner indusium which was stripped from the head
in a preceding stage. Its cells, as shown in the figure, are in an
advanced stage of disintegration. The cytoplasm of the different
elements is reduced to a mass of granules and the chromosomes
have become agglomerated into little spheres floating in the
clear nuclear plasma. The process of degeneration is similar to
that which I have described as occurring in the “dorsal organ”
of Blatta. Between the mass of degenerating cells and the
head of the embryo lies a granular coagulum (am.s). This I
take to be the amniotic serum which is forced up into
the anterior pole by the enlarging of the embryo and the
consequent decrease in the space between the body walls and
the chorion. The columella and the remains of the inner indu-
sium are held together and thus temporarily prevented from
complete disintegration by the thick cuticle of the latter.
This cuticle still envelops the embryo and extends forward to
the anterior pole where it seems to be attached to the inner face
of the outer indusium. Passing further inward we next
meet with the first larval cuticle (/v. c!), which has been shed,
and the second larval cuticle (/v. c?), which is still in organic

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 22
connection with the hypodermis. In a little later stage than
the one here described the columella and the conical lump of
inner indusial elements have disintegrated, and can no longer
be distinguished from the granular amniotic serum.

The changes in the configuration of the embryo since its
arrival on the ventral yolk, relate mostly to the appendages.
At first the antennz are of about the same thickness as the

~WE- ee 222 ON,

a\ Rr ae

Se

Brey TV.

Sagittal section through the anterior pole of a Xiphidium embryo, with pig-
mented eyes. ch., chorion; c¢/., columella; s7.c., Blastodermhaut ; sv., serosa ;
ind* + am., remains of the inner indusium and amnion; zzd@', outer indusium ;
imd*s.. secretion of the inner indusium ; am.s., amniotic secretion ; Jy. c', first
larval cuticle ; Zv. c?, second larval cuticle ; ér., brain ; ¢., eye.

legs. The dark line running parallel with their inner edges,
and distinctly marked in Fig. 9, is in section seen to be a meso-
dermal partition dividing the cavity of the appendage into two
tubular sinuses. The antennz grow directly tailward till their
tips reach the femorotibial joint of the hind legs, when they di-
verge laterally, describe an arc, and then grow forward. When
the tips have reached the head further progress is arrested

34 WHEELER. [Won Vv Tir.

by the envelopes, but as the growth of the appendages does
not cease, the arcs surrounding the hind legs gradually move
tailward. This movement is arrested just before the time for
hatching, when the antennz have grown to nearly twice the
length of the embryo.

The mouth-parts and thoracic appendages have been gradu-
ally assuming their adult characters in the meantime.

The pleuropodia, as described ina former paper (90%), are
shed during hatching and just previous to that event may be
found attached to the pleural cuticle by means of very slender
pedicels.

In the male the appendages of the 9th and 11th abdom-
inal segments persist, the former as the stylets, the latter as
the cerci. In the female the cerci also persist but together
with them also the pairs on the 8th, 9th and roth segments
(Figs. 9 and 10— af! (ap) —op3 (ap'°). These are converted
into the gonapophyses.

Apart from the eyes little pigment is developed in the hypo-
dermis during embryonic life, unless we regard as such the
brown granular secretion of the inner indusium.

A number of eggs kept in the house the greater part of the
winter hatched May 15th—18th, but I am inclined to believe
that out of doors the regular time for hatching is later, prob-
ably not till the end of May. X7iphidium fasciatum apparently
does not hatch till early in June, since I found larvee of this
species on Naushon Island June 9, which could not have been
more than a few days old. Inasmuch as the imagines of A7p/z-
dium ensiferum oviposit on the average about Sept. Ist, the
whole postembryonic development cannot occupy more than
three months. As this Locustid is monogoneutic, nine
months is therefore required for embryonic development.
Even if we deduct the period of quiescence due to cold
weather, it will still be apparent that the embryonic stages
must succeed one another very slowly in X7phidium as com-
pared with those of other Ametabola (eg. Bélatia), not to
mention the Metabola.

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 35

7. The Development of Orchelimum vulgare.

This Locustid oviposits like many of the smaller members
of the family in the pith of dead plants. I found the eggs in
Ohio during the last days of September in the stems of the
wild lettuce (Lactuca canadensis), so common along the edges
of fields and thickets, and in the petioles of the common elder
(Sambucus canadensis). Oviposition probably takes place in
the beginning or towards the middle of September. In the
case of Lactuca and a few other plants which I did not identify,
the insects had invariably selected for oviposition the main
stem of the flower-panicles. From base to apex this portion
of the stem was punctured at intervals, and a single egg
thrust into the pith a short distance above each orifice. It
is an easy matter to recognize the punctures by the little tufts
which the insect evidently gnaws from the woody fibre, before
inserting its scimeter-shaped ovipositor. Great care*must be
taken in splitting the stem, so as not to tear or cut the eggs
which adhere very firmly to the pith.

The eggs are larger than those of X7phidium ensiferum,
being fully 6.-6.25 mm. long. In shape they are very similar
to Xiphidium eggs except that the sides are compressed.
In the fresh state they are smooth and opaque, and of a
pale drab or bluish tint. In this respect, as also in the
flattening of their lateral faces, they forma transition to the
eggs of our larger Locustide, e.g. Cyrtophyllus concavus,
Amblycorypha uhlerit and Muicrocentrum retinervis1 The
chorion is not readily wetted with water, but like that of
the AX7zphidium egg, immediately becomes transparent when
immersed in alcohol. The outer envelope is then seen to have
a yellow tint, deepening into brown at the poles.

As would be expected from its close systematic affinity the
embryonic development of Ovchelimum does not differ much
from that of Xzphzdium. Ihave not seen all the stages, nor
have I, as yet, sectioned any of my material, but the stages
which I have examined are essentially the same as those

1 For a description of the eggs of these species see an article on Orthoptera, by
Prof. C. V. Riley, in the Standard Natural History, Vol. Il. pp. 188-189.

36 WHEELER. [Vot. VIII.

described in X7phidium. The embryo of Ovchelimum passes
through the yolk in the same manner as the <Azphidium
embryo, shortens on the dorsal yolk, then grows apace, moves
around the lower pole and finally begins the yolk-enveloping
process on the ventral surface of the egg in the same way
as the Azphidinm embryo. It also develops an indusium
which is set free from the head and spreads over the yolk
while the embryo is passing through it backwards. In Ovchel-
zmum the inner indusial layer also secretes a brownish pigment-
like substance which enables one to follow its movements as it
gradually covers more and more of the yolk. A clear slit is
likewise left on the dorsal surface between the folds of the
organ. But in the time of closure of this slit Orchelimum
differs from X7zphidium. In the latter insect we found that
the slit closed soon after the embryo had straightened on the
dorsal yolk, before it had shortened very decidedly. In
Orchelimum the closure is considerably delayed. The embryo
shortens, then grows in length and breadth, passing beyond
Stage G of AXzphidium and its head nearly reaches the lower
pole before the two folds of the indusium meet and fuse.
Frequently in this stage, when the embryo is about to revolve,
the polar ends of the slit are still open, the membranes having
fused over the embryo. In a little later stage, however, the
indusium has completely enveloped the yolk.

II. REMARKS ON GASTRULATION IN THE ORTHOPTERA.

Although many important observations have of late been
contributed to the embryology of the Insecta, our knowledge of
the formation of the germ-layers in the Orthoptera cannot be
said to have made any signal advance. As late as 1889 so few
forms of this order had been studied that I felt justified in
expressing some doubt as to whether their mesentoderm was
formed in the same manner as in the higher Metabola (Cole-
optera, Diptera, Lepidoptera). My doubts were confirmed by a
study of B/atta, when I failed to find the oral formative centre
of the entoderm ('89°).1

1 We need not go far to seek the reasons for this gap in our comparative
studies. The eggs of the Orthoptera are almost without exception extremely

No.1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 37

Bruce ('86) appears to have been the first to describe the
origin of the mesentoderm from a median ingrowth of the
germ-band in the Orthoptera. The species which he studied,
is, I have every reason to believe, Stagmomantis carolina.
His description is very meagre and his figures are unsatis-
factory.

More convincing are Graber’s figures and description of
mesentoderm formation in Stenobothrus variabilis (88, Pl.
MIV, Pig. 71; Pl. XV, Fig. 13).0) His: Fig. 1m shows \hac
there is in the median line a distinct infolding of the ventral
plate cells —a true invagination. In a more recent paper (90),
the account is briefly repeated without any important additions.

In his recent study of the embryogeny of Blatta germanica,
Cholodkowsky (91%) gives an account of the formation of the
germ-layers more in harmony with what we know of the process
in the Coleoptera than the account which I gave. But he has
not come to any definite conclusion respecting the formation of
the entoderm, and although he maintains that there is a distinct
blastoporic groove running the length of the germ-band, he
does not figure it in surface view, and most of his sections
betray such an amount of distortion in his preparations that
one may hesitate to regard the slight depressions in his figures
(Figs. 7, 8, 10, etc.) as indicating invagination. Nevertheless
I believe from renewed study of the Orthoptera, that Cholod-
kowsky is correct in deriving the mesoderm from a median
proliferation of the primitively one-layered germ-band, and the
entoderm from two formative centres—one in the oral and
one in the anal region.

In Avphidium, soon after its first appearance, the blastoporic
depression, when seen from the surface (Fig. 1), is a straight
refractory from a technical point of view. The cells of the embryo are often
smaller and less distinct than they are in the Metabola. Moreover, the great
quantity of yolk and its singular brittleness in hardened specimens renders
paraffin sectioning most unsatisfactory, and rather than incur the great expend-
iture of time which working with celloidin involves, the student gladly selects
some Coleopteran or Dipteran egg which is all that can be demanded from a
purely technical point of view. Nevertheless the Orthoptera constitute, by com-
mon consent, one of the most primitive orders of the Insecta; their eggs are

large and may be readily procured in great numbers; their development is so
gradual that all the requisite stages may be obtained without the least difficulty.

38 WHEELER. [Vor. VIII.

groove extending nearly the entire length of the germ-band
and dividing it into two symmetrical halves. Anteriorly the
groove is rounded and seems to end rather abruptly, but
posteriorly it bifurcates, each of the two grooves thus arising
being continued for a short distance to either side till they
gradually fade away. There can be no doubt, it seems to me,
that the bifurcated termination of the blastopore is the homo-
logue of the similar structure first figured by me in Doryphora
(a9, Pl. XVIII, Fig. 71; Pl. XIX, Fig. 73) and subsequently
seen by Graber (90) in the corresponding stages of Lzna
tremule (P\. II, Figs. 25 and 27). More recently Cholodkowsky
has observed a similar widening of the blastopore in Alatta.
* He attempts to identify it with the posterior depressions of
Graber’s “lateral gastrulation.”

In Stage B (Fig. 2) when the caudal amnio-serosal fold has
covered the ligulate portion of the germ-band, the Dlasto-
pore presents a widening of its anterior end at a point which
probably lies just in front of the definitive mouth. This widen-
ing was observed in several embryos, and I therefore take it
to be a normal occurrence. It also has its homologue in the
Doryphora embryo (see my Fig. 70, Pl. XVIII, ’s9). In the
stage under consideration (Fig. 2) the anal bifurcation has
grown more indistinct and is apparently about to disappear.

The closure of the blastopore proceeds simultaneously in
two directions: from its anterior end backwards, and from its
posterior end forwards, so that the last portion of the groove
to disappear lies in that part of the germ-band which is to
become the thoracic or baso-abdominal region.

In sections the groove is seen to be much shallower than it
appears in surface view. Along its whole extent its floor is
somewhat thickened and in this portion — destined to form the
mesentoderm — the cells soon lose their columnar shapes and
become more polygonal in outline and more irregular in their
arrangement. The groove closes in such a way that no
tubular cavity results as in the Coleoptera and Diptera; the
cells at the edges of the depression appear to glide over the
median elements, so that after the fusion of the edges in the
median line the mesentoderm has the form of a solid cord

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 39

applied to the inner surface of the germ-band. The process
whereby the inner layers are formed is, therefore, a slurred
invagination. In this respect X7phzdium resembles Llatia.

The further differentiation of the mesentoderm is quite as
difficult to follow in Xzphidiuwm as in other Orthoptera. In
these stages the embryo cannot be satisfactorily isolated from
the yolk and sectioned by itself, and so friable is the yolk that it
is almost impossible to obtain thin sections through the entire
egg by the ordinary methods. After studying a few series of
sections obtained by means of the celloidin method I can, how-
ever, affirm that the invaginated cells give rise to both
entoderm and mesoderm. The former has a bipolar origin, as
has been made out in the higher forms; in Agzs by Grassi (84) ;
in Hydrophilus by Heider ('89); in Doryphora by myself (89);
in Musca by Voeltzkow (89) and by Graber (89); and in
Chalicodoma by Carriére (90). The anal is considerably larger
than the oral formative centre and its elements seem to arise
in part from the bifurcation and in part from the deeper
portion of the blastopore just in front of the bifurcation.

In Xiphidium, just as in the higher Metabola, a pair of
entoderm-bands grows towards the baso-abdominal region from
either entoderm-pole. Each band, consisting of only one
layer of much-flattened cells, meets that of its respective side
and then begins to envelop the yolk by proliferation at its
ventral and dorsal edges. Transverse sections show that
at first the bands are only two or three cells in breadth and that
these are closely applied to the dorsal faces of the mesomeres
which are formed by this time.

I have made no observations on the relations of the procto-
dzeum to the posterior end of the blastopore, but in regard to
the anterior end and its relation to the stomodzum my results
are more definite. Figs. 32-34 represent three successive
sections through the head of an embryo in Stage D. The last
section (Fig. 34) passes through the stomodzum (sz.) which is
just forming as a rounded depression in the cephalic ectoderm.
Its large columnar cells are regularly arranged and have their
nuclei in the inner ends. The next section (Fig. 33) passes
just in front of the stomodaeum and cuts two masses of cells in

40 WHEELER. [Voi. VIII.

the median line. The upper of these masses is a thickening
of the ectoderm distinctly separated on either side from the
elements of the same layer by the peculiar character of its
cells. These are much smaller than those of the remaining
ectoderm and stain more deeply, especially in the inner portions
of the layer. The lower mass of cells is entirely cut off from
the ectodermal thickening, though its elements are very similar
in size and staining qualities. The ectodermal thickening
marks the point where the paired labrum is about to appear
(cf. Fig. 35). In the next section (Fig. 32), which also
passes through the labral region, we again meet with the
thickening of the ectoderm. Unlike its portion in the pre-
ceding section, it is not bounded below by a curved line,
but juts in as a ragged mass of cells, in which it is possible to
distinguish a pair of lateral wings and a median projection.
The median portion thus proliferated beyond the limits of the
ectoderm, is the anterior or oral entoderm centre — the lateral
wings I regard as mesodermal. By combining Figs. 32 and
33 the flattened mass of cells underlying the ectoderm in the
latter section is seen to be the backward continuation of the
mesentoderm. Section Fig. 34 shows that this median unpaired
mass splits into two masses, one on either side of the mouth.
In this paired condition the bands run backwards through the
trunk of the embryo.

Essentially the same condition of the germ-layers in front of
the mouth persists till the labrum is definitely formed, as I have
observed in a few series of sections. It is difficult to account
for the late and intimate union of the mesentoderm with the
ectoderm in the labral region, unless we suppose that the
blastopore originally extended as far forward as this region and
here closed in such a manner that the three layers were not at
once separated into ectoderm on the one hand and mesentoderm
on the other. It is mainly on this supposition that I take the
labral region to coincide with the anterior widening of the
blastopore seen in Fig. 2. This widening probably does not
coincide with the stomodzum, but lies in front of it, and the
definitive mouth is a later formation arising independently from
the ectoderm alone.

Novi CONLRIBUTTON. LO. INSECT EMBRVOLOGY, 4I

I would here insert a few observations on gastrulation in
Stagmomantis carolina, Gryllus luctuosus, and Cicanthus
niveus.

In Fig. 12 the egg of Stagmomantis is represented in out-
line for the purpose of showing the relatively small size of the
germ-band which arises as in other forms from a thickening of
the blastoderm on the ventral face of the yolk. It is seen to
lie somewhat nearer the broad posterior than the pointed
anterior pole. It is but slightly longer than broad, and its
wider anterior end, which is directed towards the upper pole
of the egg, foreshadows the procephalic lobes. Fig. 11 shows
that the germ-band of the Mantid, unlike that of X7zphzdium,
is raised above the niveau of the yolk and has its marginal cells
sharply separated from the extra-embryonal blastoderm — or
serosa—as it is nowcalled. This much flattened layer is,
nevertheless, encroaching on the edges of the germ-band to
form the amnio-serosal fold (ams.). At theanterior edge lies a
small cluster of cells (f.0.) but little larger than those of the
germ-band. I believe that these may represent all that remains
of an indusium in Stagmomantis.

The narrowly pear-shaped blastopore is very short. Sections
show it to be a deep groove, which like the median infolding
of other forms (Doryphora, Hydrophilus, Musca, etc.) is deep-
est posteriorly and grows shallower headward. As I failed to
find any of the stages immediately following the one figured, I
could not trace out the formation of the germ-layers.

According to Bruce (86, p. 17), who studied the same species
of Stagmomantis, “When the union of the folds (of the amnion
and serosa) is effected and the embryo is separated from the sur-
face and covered ventrally by the amnion, the under layer is
formed, as in Meloéand Thyridopteryx as an ingrowth from the
median line of the embryo.”’ This remark, together with his
Figs. XLII-XLIV, Pl. IV, shows that he could not have ob-
served the formation of the layers from a groove and that he
must have based his inference on a stage later than the one I
have figured.

In Gryllus luctuosus the blastopore is more like that of
Aiphidium. The outline of the egg is shown in Fig. 14. The

42 WHEELER. [Vou. VIII.

germ-band is relatively much larger when compared with the
yolk-mass than the germ-band of Stagmomantis. It arises on
the ventral surface very near the lower pole. That such is the
correct position of the embryo may be easily ascertained, since
the mother-insect thrusts her eggs into the ground with their
long axes perpendicular to the surface. In a glass jar con-
taining a few inches of earth, many eggs were deposited
between the surface of the glass and the earth, so that the
exact position of the apical pole could be noted, and the egg
removed and hardened with this pole constantly in sight. Thus
it was possible to determine the exact topographical relations
of the embryo to the yolk throughout the important stages of
early development.

During gastrulation the germ-band of Gryllus (Fig. 13) is
more elliptical and somewhat narrower than the germ-band of
Stagmomantis. Its edges are also distinctly marked off from
the blastoderm and here, too, the amnio-serosal fold (ams.)
arises along the entire periphery. The blastopore (6/.) is much
narrower than the corresponding depression in Stagmomantis.
It is deepest posteriorly.

The discovery of an invaginate gastrula in Gryl/us made it
extremely probable that this stage had been overlooked in the
other members of the same family which have been studied
from an embryological standpoint. Neither Korotneff in his
study of Gryllotalpa (85), nor Ayers in his study of @canthus
('84), succeeded in finding an invagination. I was unable to
secure the eggs of any of our native Gryllotalpa, but I col-
lected a great number of @canthus eggs in Ohio during the last
days of September. An examination of these soon convinced
me that Ayers had not seen the youngest stages in the develop-
ment of the germ-band. The youngest germ-band that he
figures (Figs. 1-18) lies near the posterior end of the egg
with its tail pointing towards the micropylar pole. According
to Ayers “A tract of the blastoderm along the median line of
the ventral (concave) side, lying nearest the deep or primitively
head-end of the egg, becomes thickened into a germinal band,
which is the first trace of the Jody of the embryo.” But this is
not the first trace of the body of the embryo, nor does it

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 43

arise on the concave face of the egg. The germ-band of
Gicanthus, like that of Gry/lus, first makes its appearance as a
thickening of the blastoderm on the convex surface near the
lower pole of the egg. This convex surface is, therefore, the
ventral surface and the micropyle marks the “ primitively head-
end”’ of the egg as is shown by the fact that the procephaleum
is originally directed towards this and not towards the opposite
pole, which Ayers incorrectly calls the “ primitively head-end.”
The germ-band, however, soon leaves its position on the convex
ventral surface and, moving around the lower pole tail first,
comes to lie on the concave dorsal yolk. It is clear that Ayers
could not have seen the stages preceding the arrival of the
germ-band on the dorsal surface, and it is during these very
stages that the blastopore forms and closes.

Before turning the lower pole the germ-band of @canthus
resembles that of Stagmomantis. Its anterior is distinctly
wider than its posterior end and represents the future pro-
cephalic region. A narrow, but distinct groove runs from the
oral to the anal end as in the forms we have been considering.
At the posterior end the groove bifurcates much as in
Atphidium. That this median groove gives rise to the mesen-
toderm admits of little doubt after what has been said of other
Orthoptera. The amnio-serosal fold appears to arise simul-
taneously along the entire margin of the germ-band as in
Gryllus.

It follows from the observations here recorded, fragmentary
as they are in many respects, together with Graber’s observa-
tions on Stexobothrus, that the Orthoptera can no longer be
regarded as hors de ligne so far as the formation of their
germ-layers is concerned. In all the families of the order,
save the Phasmidz, an invaginate gastrula has been found,
and there can be little doubt that the investigator who
is so fortunate as to study embryos of this family will find in
them essentially the same process of germ-layer formation.

44 WHEELER. [VoL. VIIi.

The view is now pretty generally held that in the Insecta
both mesoderm and entoderm arise from a median longitudinal
furrow — the former layer throughout nearly the entire length,
the latter only in the oral and anal regions of the germ-band —
and that the vitellophags, or cells left in the yolk at a time
when the remaining cleavage products are traveling to the sur-
face to form the blastoderm, take no part whatsoever in the
formation of the mesenteron, but degenerate zz sztu and
finally undergo dissolution. Discussions of the literature on
this subject are to be found in the papers of Heider (89) and
Graber ('89,’90), and so few are the facts accumulated since
these résumés were penned that I may dispense with an
historical consideration of the insect germ-layers in the present
paper.

In the interpretation of the insect gastrula the entoderm has
always played an important réle. The origin of the mesoderm
has long been known and has been duly provided for in the
various germ-layer hypotheses which have from time to time
been advanced. But the true origin of the lining of the mid-gut
has been ascertained only within the last few years, so that we
cannot expect to find an adequate treatment of this layer in
the older theories. Led astray by what had been observed in
Crustacea and Arachnida, some writers chose to regard the
vitellophags as forming the mesenteron and shaped their
theories accordingly (Oscar and Richard Hertwig, ’81). But
now that it has been shown that the vitellophags take no
part in forming the lining of the mid-gut, their morphological
position is rendered even more obscure, and we are brought
face to face with the question: Are the vitellophags a portion
of the entoderm which has been set apart very early in develop-
ment for the purpose of yolk-liquefaction or are they an entirely
new segregation of cells belonging to none of the conventional
germ-layers? Those who defend the former alternative main-
tain that the vitellophags of insects are entodermal in function
inasmuch as they digest yolk and closely resemble the amceboid
Crustacean yolk-cells which are actually converted into the lin-
ing of the mesenteron. On the other hand it is urged, that as
the yolk-cells arise and function before the blastoderm is com-

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 45

pleted and hence some time before the germ-layers are formed,
they cannot properly be assigned to the entoderm.!

It is probably best to await the results of further investiga-
tion before deciding on the phylogenetic relations of the vitel-
lophags to the entoderm. Heider (89) has also expressed
himself to this effect and I fully endorse his opinion when he
says: ‘“Immerhin wird man vorlaufig iiber vage Vermuthungen
nach dieser Richtung nicht hinauskommen, und ist die Frage
nach der Auffassung der Dotterzellen bei dem Nachweise, dass
sie an dem Aufbau des Embryos keinen Antheil nehmen,
meiner Ansicht nach von geringerer Wichtigkeit.”

1 Besides these vitellophags which with Cholodkowsky (’91%) we may call the
primary yolk-cells —there are other cells which detach themselves from the blas-
toderm or embryo and enter the yolk. These Cholodkowsky calls secondary
yolk-cells. While the origin of the primary yolk-cells has been quite satisfactorily
demonstrated, this cannot be said of those of the second class. They appear to
descend into the yolk at different times in different species. Thus, according to
Patten (’84), all the cleavage products in Meophylax ascend to the surface, the
yolk-cells subsequently descending from the blastoderm. I claimed a similar total
migration of the cleavage products to the surface in atta (’89); Cholodkowsky,
however, claims that some of the cells never reach the surface, but remain in the
yolk. Be this as it may, in later stages I believe it can be shown that cells do
migrate into the yolk from the embryo and especially from the entoderm-centres.
This was shown by me to be the case in Doryphora, where many cells pass into the
yolk from either entoderm pole (Pl. XIX, Fig. 82; Pl. XX, Fig. 88). I have since
observed an exactly similar phenomenon in Ze/ea polyphemus in a correspond-
ing stage of development. Graber, (’89, p. 11) too, has made a similar observa-
tion on Jelolontha, where he saw “vom invaginirten Blastodermwulst aus unter
lebhaften Theilungserscheinungen ganze Stréme von Zellen in den Dotter hinein-
wandern, Zellen die freilich von den primaren, gleichzeitig vorkommenden und
auffallend grosskernigen Centroblastelementen ganz enorm verschieden sind, und
die sich tiberhaupt durch ihre ganze Beschaffenheit als unzweideutige Abkommlinge,
man kénnte sagen A uswiirflinge eines wahren Keimblattes, erweisen.” So far as the
migrant cells described in Doryphora are concerned, I am sure they come from
the entoderm. They occur only at or near the entodermal Anlagen and may be
traced from this germ-layer into the yolk. These cells are not actively dividing
like those described by Graber, but actively disintegrating. (May not Graber
have mistaken disintegration-figures for caryokinetic figures?) In somewhat
later stages no traces of these migrant cells are to be found. The yolk is
segmented at the time of their leaving the entoderm and their invasion appears
not to disturb in the least the activities of the vitellophags. Whether there
is any relation between these evanescent entoderm cells and the “secondary
mesoderm” of Reichenbach (’86), the “spores” of F. H. Herrick (86), or the
“chromatin nebule” of Bumpus (91) is a question which cannot be answered at
' present.

46 WHEELER. [Vot. VIII.

Among those who take a decided stand on the relations of
the vitellophags to the definitive entoderm, Graber and Cholod-
kowsky may be mentioned. Graber (89, p. 10), after intro-
ducing the superfluous and inapplicable term “centroblast,”?
says: Dabei nehme ich zugleich, was indessen kaum misbilligt
werden diirfte, stillschweigend auch an, dass dieses gegenwartig,
wie es scheint, von der Darm- and Gewebsbildung ausgeschlos-
sene Zellenlager auch friiher niemals eine dem echten Ento-
derm anderer Thiere entsprechende Rolle inne gehabt habe,
sondern vielmehr dem letzteren gegeniiber ein neues, wahr-
scheinlich mit der starkeren Entwicklung des Dotters im
Zusammenhang stehendes Differenzirungsproduct ist.”

Cholodkowsky (912) does not dismiss the matter so briefly.
Like Graber he draws a hard and fast line between primary
and secondary yolk-cells, and admits no phylogenetic continuity
between the vitellophags and the definitive entoderm. The
vitellophags belong to none of the germ-layers. His reasons
for not regarding them as a precociously segregated portion of
the entoderm are neither new nor conclusive. Like other
recent investigators he admits that the vitellophags are in part
digested or discharged from the alimentary tract along with
the remains of the yolk after hatching. But he is not satisfied
that the yolk-cells should play a humble rdle in the insect
economy. Some of them were predestined to a higher function
than yolk-liquefaction— viz: to give rise to the blood, the fat-
body and even to the germ-cells. He therefore supposes that the
vitellophags are undifferentiated cells. But this supposition is
not supported by the facts. That they are on the contrary,
considerably specialized is shown by their limited function and
mobility, their gradual and prolonged growth (especially in
some Orthoptera), their inability to undergo caryokinesis or
even akinesis, and their suspicious relations to the bacteria-
like corpuscles of Blochmann. On a priori grounds we should
not expect to derive whole sets of tissues from such specialized
elements.

1 Superfluous because we have enough names for these cells already, inap-
plicable because the termination “blast” is properly applied only to cells or
tissues of a germinal character —not to decrepit elements like the yolk-cells.

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 47

But a more weighty objection may be adduced. It has been
shown by Heider ('89) and Heymons ('90), not to mention many
previous investigators, that the fat-body and sexual-cells arise
from the mesoderm, and my own studies fully confirm this view.
Concerning the origin of the blood there is some doubt, but it
should be stated that Cholodkowsky has made no satisfactory
observations of his own on this point and that, although some
facts point to a derivation of the blood from the yolk-cells,
others as definitely point to its origin in the mesodermal layer.

After taking for granted that the vitellophags are undiffer-
entiated cells, that they have nothing and, what is more, never
have had anything to do with the entoderm, and that they give
rise to blood-corpuscles, adipose-tissue and germ-cells, Cholod-
kowsky ushers in the parablast theory. It was to have been
hoped that this theory might have been permitted to end its
days in peace within the confines of vertebrate embryology
where it originated. Fortunately, however, it has grown too
old and decrepit, even under the skillful medical treatment
which it has received from time to time, to be of any service
in invertebrate morphology.

There is something almost ludicrous in Cholodkowsky’s ap-
plication of the parablast theory to the Insecta when we con-
sider the methods which he employed in preparing the yolk of
the Llatta egg. The capsules opened at both ends were sub-
jected to the action of undiluted Perenyi’s fluid for 12 hours
and the eggs after treatment with the customary grades of
alcohol, cleared in green cedar oil 24 hours. Thence they
were transferred to paraffine (5§5—-60° C.) and left 3~5 hours.
The result of this heroic method is apparent enough in the dis-
tortion of the tissues, but its effect on the yolk is quite
remarkable.

Both Blochmann (87) and myself (89) described the yolk of
the 4latta egg as consisting of a mass of homogeneous and
granular albuminoid bodies sharply polygonal from mutual
pressure and interspersed with spherical oil-globules. We also
described a peculiar distribution of the polygonal bodies; those
of a homogeneous nature constituting an oval central core in-
vested with the granular bodies. I further claimed that the

48 WHEELER. [Vo.. VIII.

Blatta egg exhibited a yolk-segmentation which though faint
and appearing late was, nevertheless, comparable to the yolk-
segmentation in such forms as Doryphora.

Cholodkowsky (91) thus describes the yolk: “So kann ich, z.
B., nicht bestatigen, dass der Dotter aus einzelnen polygonalen
Dotterkérpern bestehe, wie derselbe von Blochmann (und
Wheeler) beschrieben und abgebildet wird. Der ganze Dotter
besteht aus einer continuirlichen plasmatischen Substanz, deren
Vacuolen gréssere und kleinere Fetttropfen enthalten. Die
Continuirlichkeit der Dottermasse tritt nun um so deutlicher
hervor, je besser die Objekte conservirt sind. Das Bild (ich
mochte sagen, das Trugbild) der polygonalen Dotterkorper ent-
steht durch Bersten des Dotters nach der Bearbeitung mit
nicht ganz passenden Reactiven.” .... “Auch kann ich
die Blochmann’sche Unterscheidung des ‘inneren’ und
‘jusseren’ Dotters nicht annehmen; der ganze Unterschied
in den Farbungsverhaltnissen der beiden angeblichen Theile
des Dotters lasst sich einfach dadurch erklaren, dass die Farbe
aus den peripherischen Theilen des Eies leichter als aus den
inneren mit Saure ausgezogen wird.” And at p. 58 he remarks:
«Es ist bemerkenswerth, dass bei Blatta germanica eine
Dotterzerkliiftung vollkommen fehlt. Ich kann also mit
Wheeler nicht iibereinstimmen, wenn er sagt (p. 359), dass bei
Blatta der Dotter, wenn auch sehr spat (nach Bildung der
Extremitaten) sich furchen soll; héchst wahrscheinlich war
Wheeler zu dieser irrigen Annahme durch die ausserordentliche
Briichigkeit des Dotters verleitet.”’

On reading these criticisms I re-examined my preparations
and must emphatically re-assert what I claimed in my descrip-
tion of the yolk of the B/atta egg. Among my preparations I
find several mature ovarian eggs hardened in Perenyi’s fluid —
not, however, treated with that vigorous reagent for 12 con-
secutive hours — and these show the yolk-bodies very distinctly
as polygonal masses. There are no traces of a ‘“ Bersten des
Dotters.”’ Eggs killed in ordinary alcohol and mounted zz toto
show the polygonal yolk-bodies distinctly and in these same
specimens the distribution of the different yolk-elements may
be followed by carefully focusing. That Cholodkowsky should

Noir.) CONTRIBCTION (TO INSECT EMBRYOLOGY. 49

be unable to detect the outlines of the segments in the yolk of
eggs treated for half a day with Perenyi’s fluid is not surpris-
ing, especially as this segmentation is of very short duration in
Blatta as in other Orthoptera. It is present, however, as I
have convinced myself from eggs mounted zz foto and from
sections.

If prolonged immersion in Perenyi’s fluid can bring about a
complete fusion of the yolk-bodies and an obliteration of their
true structure, what must be its effect on the vitellophags
scattered through the yolk? And how much importance are
we to attach to Cholodkowsky’s assertion that the fat-body,
blood-corpuscles and sexual-cells arise from the vitellophags,
and to the parablast theory as applied to the B/atta-ovum ?

Let us return from this digression to the germ-layers. The
curious fact that the definitive entoderm of the Insecta arises
from two separate centres—one oral and the other anal —is
too recent to have given rise to much speculation. Since
the entoderm of other animals arises from a single centre it is
tacitly assumed that such must originally have been the case
with the Insecta, and that the present bipolar condition must be
due to secondary modification. Starting with this postulate,
there are, of course, many ways in which the bipartition of the
original unipolar entoderm May be supposed to have taken
place. Two of these possibilities are worked out in the hypo-
theses of Kowalevsky ('86) and Cholodkowsky (91°).

Kowalevsky has expressed his views so clearly and con-
cisely that I cannot do better than quote his own words:
«Wenn wir jetzt versuchen, diese Bildung des Ento- und
Mesoderms bei den Musciden mit der Bildung dieser Blatter
bei anderen Thieren zu vergleichen, so sehen wir erstens, dass
hier auch eine Art sehr in die Lange ausgezogener Gastrula
entsteht, und dass aus dem eingestiilpten Teil das Ento- und
Mesoderm sich bildet. Also in diesen allgemeinen Ziigen
finden wir eine Uebereinstimmung. Es scheint mir aber, dass
die Parallele noch weiter gezogen werden kann. Namentlich
wenn wir der Bildung des Ento-Mesoderms bei Sagztta uns
erinnern, so finden wir bei derselben dass der eingestiilpte
Teil des Blastoderms in drei parallele Sacke zerfallt, von

50 WHEELER. pVeru, Vii

denen der mittlere das Entoderm liefert, die seitlichen aber das
Mesoderm. Bei den Musciden entsteht auch eine solche Ein-
stiilpung wie bei Sagz¢ta, und auch der mittlere Teil —aller-
dings nur an beiden Enden vorhanden —liefert das Entoderm,
die seitlichen Teile liefern das Mesoderm: also ahnlich dem,
was wir bei der Sagitta beobachten. Um die Aehnlichkeit
weiter zu fiihren, kann vorausgesetzt werden, dass bei der so
in die Lange gezogenen Gastrula der Insekten der mittlere, das
Entoderm liefernde Sack so ausgezogen ist, dass er in der
Mitte ganz verschwindet und nur an seinem vorderen und
hinteren Ende bestehen bleibt. Bei dieser Auffassung wird es
von selbst schon folgen, dass die sich schliessende Rinne fast
auf ihrer ganzen Lange nur das Mesoderm liefert.

Jetzt bleibt noch die Frage iibrig: wie verhalten sich die
Flachen der Gastrula zu den Flachen des sich bildenden Ento-
derms. Bei der Sagz¢ta wird die aussere Oberflache der Blastula
nach der Einstiilpung zur inneren Oberflache des Darmkanals,
d. h. die Seiten der Zellen, welche bei der Blastula nach aussen
gerichtet waren, werden im Darmkanal nach seinem Lumen
gerichtet. Bei den Insekten kann dasselbe auch vorausgesetzt
werden. Wenn wir uns die eingestiilpte Rinne vorstellen, So
sind deren Oberflachen ganz ahnlich gelagert wie bei der
Gastrula; wenn wir weiter die Bildung der beiden Entoderm-
anlagen dem mittleren Sacke der Sagzt¢a vergleichen, so bleibt
die Lagerung der Zellenflachen noch ganz dieselbe. Wenn wir
dann voraussetzen, dass der mittlere Sack durch die weite Aus-
breitung und durch das Eindringen der Masse des Dotters
gewissermassen in seinen vordern und hintern Teil zersprengt
ist, so kommt der Dotter ins innere des hypothetischen Sackes,
und die Zellen, die den Dotter bedecken, werden zu dem
Dotter in derselben Beziehung stehen, wie bei der Sagztta zu
der eingestiilpten Flache.”

A few years after these remarks were written Heider (89)
and myself (89) at about the same time published observations
on the Coleopteran germ-layers which seemed to support the
hypothesis of the celebrated Russian embryologist. As further
support to Kowalevsky’s view I believe we may point to such
gastrulas as that of Stagmomantis described above. In this

Nous) CONPAIBUTION TO INSECT BMBRYVOLOGY. I

gastrula, which is so very short and broad, we may suppose
that the oral and anal entoderm-centres are really continuous,
covering the floor of the blastopore from end to end. In sec-
tions, it is true, I failed to detect any differentiation of the
cells forming the walls of the furrow, into entodermal and
mesodermal elements, but this would also be the case in the
elongated gastrulas of other insects in a correspondingly early
stage (Xzphidium, Doryphora). As favoring a purely mechan-
ical separation of an originally single entoderm Anlage, it may
be noted that the most rapid elongation of the germ-band
occurs at a time when the entoderm is differentiating from the
mesentoderm Anlage. There is probably more than an acci-
dental correlation of these two processes. During this time
some germ-bands (Doryphora, Xiphidium) double their length.t
Inasmuch as the lengthening of the superficial layers of the
embryo is much more rapid than the differentiation of the
entoderm, this germ-layer must lag behind. In most insects
the embryo is at the time of its greatest elongation much longer
than the yolk-mass and must again shorten to the length of this
mass, so that a rapid proliferation of the entoderm may be
superfluous, since this layer would have to readjust itself to
the yolk with the contraction of the embryo. It may, therefore,
be an advantage for the entoderm to be somewhat retarded in
its growth. In the Orthoptera, where the embryo lengthens
rapidly, shortens, and then lengthens again to envelope the yolk
we may suppose, for reasons to be given in the sequel, that
yolk has been acquired. This seems also to be suggested by
the histological structure of the embryonic entoderm; this
layer consisting of large polygonal cells in the Coleoptera,
which have only a medium amount of yolk, while in the
Orthoptera the attenuate entoderm-bands consist of a very
few flat cells.?

It is probable that when more forms have been carefully

1 Blatta forms a very rare exception in this respect.

2 A very similar condition may be observed in the case of the blastoderm. In
the Coleoptera and Diptera, which have a medium or small amount of yolk, the
newly formed blastoderm is a deeply columnar epithelium ; in the Orthoptera it is
a true pavement epithelium.

52 WHEELER. [Vot. VIII.

studied a method of entoderm formation midway between the
unipolar and bipolar methods will be found to obtain in some
insects. We must admit that a contribution of elements to the
entoderm from the interpolar region of the furrow is not with
certainty precluded in several of the species which have been
studied. Thus Heider, (89) while inclined to believe that such
a contribution does not take place in the anterior portion of
the germ-band, believes that it may take place in the posterior
abdominal segments. Cholodkowsky ('91%) is inclined to accept
a still more diffuse origin for the entoderm. ‘Untersuchungen
zahlreicher Schnittserien machen es wahrscheinlich, dass an
verschiedenen Stellen des Keimstreifens sich einzelne Zellen
vom ausseren Blatte abspalten und an der Bildung des inneren
Blattes betheiligen, so dass die Entstehungsart des letzteren
sehr complicirt erscheint.”

Starting with the same postulate as Kowalevsky, viz: that
the bipolar is derivable from a unipolar condition of the ento-
derm — Cholodkowsky (91% ’91°) proceeds to account for this
phenomenon in a very different way from his compatriot. He
takes the small, round blastopore of Aszacus, stretches it till it
equals the insect blastopore in length, introduces a number of
modifications —such as the median groove and the pairs of
lateral depressions and believes that he has found an explana-
tion “sehr klar und ungezwungen”’ for all the different blasto-
pores, not only in the Insecta, but also in the meroblastic eggs
of vertebrates. It was Kleinenberg who said: “‘Gewagte Hypo-
thesen, kiihne Schliisse niitzen der Wissenschaft fast immer,
die Schemata schaden ihr, wenn sie die vorhandenen Kenntnisse
in eine leere und dazu noch schiefe Form bringen und _bean-
spruchen tiefere Einsicht zu geben.”’ The latter part of this
aphorism seems to be particularly applicable to Cholodkowsky’s
exposition. As Graber (91) has briefly pointed out, there are no
erounds for comparing the As¢acus blastopore with the entire
insect blastopore. In the Decapods this orifice is confined to
the anal region and if comparable at all to the median furrow
in insects, must be compared with the caudal entoderm pole.
This is all that is admissible, since the mesoderm of the
Decapoda arises from the anterior lip of the blastopore and

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 53

proliferates headward. That such is its origin has been shown
by Bobretzky and Reichenbach for Astacus (86), by Paul
Mayer for Expagurus (77), and by Bumpus for Homarus ('91).
To Cholodkowsky both the extent and position of the blastopore
are of little consequence as is abundantly evident from his
reply to Graber’s well-founded objection. It is this very neglect
of what are generally and, I believe, rightly considered two of
the most important matters in the discussion of the germ-
layers, which stamps Cholodkowsky’s hypothesis as superficial
and inadequate.

There is, however, one redeeming suggestion in his hypo-
thesis, viz: that the diverging grooves at the posterior end of
the blastopore in insects may correspond to the ‘ Sichelrinne”’
of vertebrates. Certainly the relations of the grooves to the
median furrow in X7zphzdium (see Fig. 1.) closely resemble in
surface view the relations of the “Sichelrinne”’ to the primitive
streak in the chick as figured by Koller (81) and in the triton
as figured by Oscar Hertwig (90, p. 99).

While most investigators probably agree with Kowalevsky
and Cholodkowsky in deriving the bipolar from a unipolar con-
dition of the entoderm, Patten does not share this view ('90).
In his opinion, which is based on Kleinenberg’s interpretation
of the gastrula, the blastopore is restricted to the oral region,
and such depressions as occur at the posterior end of the germ-
band, as well as the formation of teloblasts in that region, are
supposed by him to have no connection with the blastopore,
but to be merely the instruments of unipolar growth. “The
Arthropod body represents an outgrowth from the trochosphere,
but the trochosphere itself, the coelenterate stage, has disap-
peared) Hence’ there “is no, such (thing as; acastaula}in
Arthropods and strictly speaking, no germ-layers.” It is clear
that this view must stand or fall with Kleinenberg’s theoretical
conclusions on which it is based, and we may venture to say
that E. B. Wilson’s recent work (90) has rendered this founda-
tion very insecure, notwithstanding Patten’s rather confidant
assertion that “in Lopadorhynchus it is certain that the greater
part of the mesoderm arises from the ectoderm at the growing
tip of the tail, and has nothing to do with primitive mesoderm.”

54 WHEELER. Yor. Vili.

But it would be out of place to consider the widest bearings
of Patten’s hypothesis in this paper since I am concerned with
it only in so far as it bears on the germ-layers of insects.
Starting with the assumption that the blastopore is confined
to the mouth, he attempts to show that the median furrow is a
purely secondary structure. “That the median furrow of in-
sects is merely an ontogenetic adaptation is sufficiently evident
from the fact that it may be present or absent in closely
related forms.” This, however, is not the case. On the contrary
the furrow or a slight modification of it is, we have every rea-
son to suppose, universally present in the Insecta, at least in
the Pterygota, and this wide occurrence of the structure is one
of the surest indications of its high antiquity and phylogenetic
importance.

In the latter part of his discussion Patten admits that there
are ‘‘structures in Arthropods which may represent remnants
of gastrulas. For example, if the mouth and cesophagus of
Arthropods is primitive—and there is no reason to suppose it
is secondarily acquired——_then we must look for primitive
entoderm at its inner end. I have figured in ‘Eyes of
Acilius,’ at the very anterior end of the embryo, a great sac
of entoderm cells which probably arise by invagination,
although the process was not directly observed. The sac,
which soon opens outward by the cesophagus, afterwards
becomes solid, and finally is converted into two longitudinal
bands, one on either side, extending backwards to the middle
of the body, where they become continuous with similar bands
extending forwards from the posterior end of the embryo.”
Patten admits that true entoderm is formed at two widely
separated regions of the body, but he implies that only the
anterior centre is comparable to the entoderm of other animals,
the posterior centre being a new and purely adaptive formation.
It is just here that his theory appears to me to fail, since it
does not explain why the oral and anal centres should resemble
each other so very closely in origin, method of growth and
histological structure.

bf

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 55

III. THe INpusium Aanp irs HoMOLOGUEs.

In none of the Pterygota hitherto studied has there been
found any trace of a structure comparable to the indusium
of Xaphidium and Orchelimum. The organ appears to have
been retained by the Locustidz and completely lost by the
embryos of other winged insects. In some of the Apterygota,
however, there is an embryonic organ which gives a clue to the
possible homologues of the indusium. I allude to the so-called
“micropyle”’ of the Poduride.

During the summer of ’91 I was so fortunate as to secure the
eggs of Anurida maritima in great numbers. They are much
larger than any of the Poduran eggs hitherto studied — so large
that they may be removed from their choria by means of
dissecting-needles and partially stained for surface views. It
is also an easy matter to obtain good sections.!_ When first
deposited the eggs are provided with a thin transparent chorion
and vitelline membrane, but after cleavage, which is total, is
completed and the blastoderm formed, a yellow, peculiarly
striated chitinous membrane is secreted from the surface. The
egg then enlarges till the chorion and vitelline membrane are
burst. The striated membrane was described by Ryder (86),
but he failed to observe that it is attached to a large circular
ring —the “ micropyle.” In section (Fig. V) this organ is seen
to be a very decided thickening of the blastoderm which at
this time covers the whole yolk-mass as a single layer of
minute columnar cells. In the ‘“micropyle” the cells and
nuclei are much enlarged and often considerably vacuolated.
Surface views prepared according to the partial staining method
show that the embryo is already faintly outlined on the yolk and
that the ring-shaped organ lies just in front of its head (Fig. V1).
The egg being spherical, the embryo is curled in a semicircle
and the “micropyle” thus comes to lie on the dorsal surface
nearer the head than the tail of the germ-band. In the figure
a more advanced embryo is represented as spread out on a flat

1T mention this because the few fragmentary accounts that have been published
on the development of the Poduridz are based on the study of the embryo viewed

through the chorion and other envelopes. This has given rise to some errors
which I hope to point out in a future paper.

56 WHEELER. [Vo.. VIII.

surface. The resemblance of the “micropyle”’ to the indusium
is apparent at a glance (cf. Fig. 2, Pl. I). I have followed the
organ in Anxurida through the later stages by means of sections
and find that it persists for some time as a simple thickening
of the blastoderm, still connected with the peculiar striated
membrane which stands away from the surface of the blasto-
derm at all other points. Finally, when the embryo has
become flexed dorsoventrally and the body-walls are closed,
it sinks into the yolk and is absorbed.

Fic. V.

Median section of the egg of Anurida maritima. d.o., “micropyle”; d/d.,
blastoderm.

Although much simpler in its structure, I do not hesitate
to homologize this “micropylar”’ organ in Azurida and the
Poduridz in general with the indusium of X7phidium. A pos-
sible objection to this homology, on the ground that the indu-
sium arises on the ventral face of the egg, while the Podurid
“‘micropyle”’ is dorsal, has little weight, since the organ bears
in either case the same relation to the head of the embryo.
Provided, therefore, the egg of Anxurida were to acquire yolk

”

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 57

and become greatly elongated, like the X7phidium egg, the
micropylar organ must come to lie on the same surface of the

yolk as the germ-band.

It has been repeatedly suggested, and, I believe, on very
good grounds, that the Podurid “micropyle”’ is the homologue

Bre. Vile

Embryo of Anurida mari-
tima spread out on a flat
surface. d.o., “micropyle ” Zé.,
labrum; /c/.,procephalic lobe;
at,antenna; ¢c. af., minute ap-
pendage of the tritocerebral
segment; md., mandible; mx’,
mx’, firstand second maxillz;
7-3, first to third thoracic
appendages; af’, first abdom-
inal appendage (= left half of
collophore); a@f*, second ab-
dominal appendage; a@z., anus.

telson.

of the crustacean “dorsal organ.”’ In
both groups the organ arises soon
after the germ-band is mapped out on
the yolk, and in both groups it is a
circular or oval thickening of the blas-
toderm lying in the median dorsal line
distinctly nearer the head than the
In the Crustacea its centre
often shows a depression to the walls
of which the Blastoderm-haut is
attached, while standing away from
the surface of the egg at other points.
An exactly similar condition obtains
in the Poduridz; a slight depression
marks the centre of the organ in
Anurida, while in Anurophorus (Le-
moine, '87) there appears to be a deep
pit at the attachment of the chitinous
envelope. This depression is com-
parable to the depression seen in
Fig. 3 in X7phidiwm, where the cir-
cular fold is encroaching on the
disk. :

Up to the stage represented in
the figure just referred to, the indu-
sium will bear
with the crustacean “dorsal organ.”
In the first stages of its spread-
ing it also resembles to some extent

close comparison

the saddle-shaped ‘dorsal organ”’
of Oniscus, Porcellio, and Liga.
But beyond this point it differs

widely from its homologues, and it is difficult to see why it

58 WHEELER. [Vou. VIII.

should persist, and instead of sinking into the yolk, envelop
the whole egg, secrete a granular and thereupon a chitinous
layer, and finally, during revolution take on the function of a
true serosa. That the organ is rudimental is shown by its
tendency to vary, especially during the earlier stages of its
development; that it still performs some function is indicated
by its somewhat complicated later development and by its sur-
vival in but very few forms out of the vast group of Ptery-
gotous insects. This seeming paradox may be explained, if
we suppose that the indusium was on the verge of disappear-
ing, being the last rudiment of some very ancient structure.
As such a rudiment it no longer fell under the influence of
natural selection, and for this reason began to vary consider-
ably like other rudimental organs. Some of these fortuitous
variations may have come to be advantageous to the embryo,
and were perhaps again seized upon by natural selection; the
nearly extinct organ being thus resuscitated and again forced
to take an active part in the processes of development.

Pursuing the homologies of the indusium still further we
come to the Arachnida, where we find in the primitive cumulus
of spiders a structure comparable in many ways to the
Podurid “ micropyle,” as v. Kennel ('85, '88) and Lemoine ('87)
have suggested. There is, however, so much difference of
opinion regarding the position and signification of the primitive
cumulus that I should hardly be willing to agree with these
authors, were it not for two of Claparéde’s figures of the
Pholcus embryo (62, Figs. 6 and 7, Pl. I). These show in the
median dorsal line a thickening which forcibly recalls the
“micropyle” of Azurida. Still it must be admitted that
Claparéde has failed to prove the identity of this thickening
with the primitive cumulus.

In Pentastomids the “facette”’ or ‘cervical cross” described
by Leuckart (60) and Stiles (91) is very probably the homo-
logue of the crustacean dorsal organ and the insect indusium.

Although no homologous structure has yet been detected in
the Myriopoda, the occurrence of a dorsal-organ-like structure
in such widely separated groups as the Hexapoda, Araneina,
Pentastomidz, and Crustacea is sufficient reason for regarding

No.1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 59

it as exceedingly ancient and as well-developed before the
existing subdivisions of the Arthropoda were established. To
seek a homologue of the “dorsal organ” among existing
annelids may be regarded by some as a hopeless undertaking.
Still I would call attention to Apathy’s observation (sg) on
Clepsine bioculata. The adult of this species has long been
known to possess a chitinous plate in the median line between
the head and the preeclitellum. Apathy finds that this plate is
the remnant of an embryonic sucking-disk, the glandular cells
of which secrete a bundle of byssus-like threads that harden
on contact with the water and serve to anchor the undeveloped
young to the ventral concavity of the mother-leech. A similar
organ is also found in the young of Clepsine heteroclita. It
is certainly no great step from this embryonic sucking-disk of
the Hirudinea to the Phyllopod “cervical gland”’ which is also
used as a sucker, and which Fritz Miiller (64) and Grobben
(79) regard as homologous with the ‘dorsal organ’”’ of the
Amphipoda.

IV. THE ENVELOPES AND REVOLUTION OF THE INSECT EMBRYO.
1. The Amnion and Serosa.

The formation of two cellular envelopes, the amnion and se-
rosa, by a folding of the primitive extra-embryonal blastoderm,
is rightly considered one of the most characteristic features of
the Hexapod embryo. The envelopes are not, however, com-
mon to all insects. An amnion is completely lacking in the
Poduridz,! and consequently the extra-embryonal blastoderm in
these forms is strictly comparable to the corresponding por-
tion of the blastoderm in Crustacea, Myriopoda, and Arachnida.
This is proved by the fact that it ultimately forms the definitive
dorsal body-wall. So far as our present knowledge extends,
the Apterygota may be regarded as Hexapoda Anamniota, and

1 Lemoine (’87) describes a cellular “membrane amniotique” in Axurophorus,
but he does not represent it in his figures and did not study it in section. I
therefore incline to doubt the correctness of his observation, especially as I can
find no traces of a cellular envelope in the Axwrida egg, which on account of its
size is a far more favorable egg for study than that of Anurophorus.

60 WHEELER. [Vou. VIII.

placed over against the Pterygota, which are characterized by
the possession of an amnion (Hexapoda Amniota). There is
a gap between these two groups of insects similar to the gap
between the amniote and anamniote vertebrates. Whether it
will be filled by the future study of such orthopteroid forms as
Machilis, Lepisma and Forficula remains to be seen. For the
present I am inclined to believe that the amnion first made its
appearance in the ancestral Pterygota. Even if it be contended
that the amnion was once present in the Apterygota and subse-
quently lost, its origin could not consistently be pushed further
back than the Hexapoda, since this envelope is lacking in the
Myriopoda, which, there is reason to believe, lie in the direct
line of descent. The proof that the so-called amnions of Pe77-
patus, Scorpions and Pseudoscorpions are the homologues of
the insect amnion is not forthcoming. Judging from the few
descriptions of their formation, they appear to have arisen in-
dependently within their respective groups.

Just as many of the Pterygota develop only rudiments of
wings or have altogether ceased to develop these organs in the
adult state, so the embryos of the Pterygota in some cases
develop only rudimental envelopes or none at all. It is reported
that the amnion is lacking in the Proctotrupid Hymenoptera
(Ayers, '84) and rudimental in Muscide (Kowalevsky, '86 ;
Graber, '’89) and viviparous Cecidomyidz (Metschnikoff, ’66).
Certain ants of Madeira are incidentally mentioned by Met-
schnikoff as having the envelopes represented only by a small
mass of cells in the dorsal region. The absence or abortion
of the amnion is almost certainly a secondary condition. The
Proctotrupide are egg-parasites and undergo an extremely ab-
errant embryonic and larval development. Both these and the
other insects mentioned belong to groups characterized by
high specialization. This is notably the case with the ants
and with the Muscidze which show considerable aberration in
their embryonic and larval stages. The pzdogenesis of the
Cecidomyids studied by Metschnikoff stamps them also as ab-
errant. Moreover the embryos of other Orthorrhaphous Dip-
tera (Simulidz, Chironomide, Tabanidz) have perfectly normal
envelopes.

No.1.) CONTRIBUTION TO INSECT EMBRYOLOGY. 61

Many attempts have been made to explain the origin of the
amnion in insects. It first appears abruptly and fully devel-
oped in the Orthoptera just as the vertebrate amnion appears
abruptly in the Reptilia. One school, represented by Nus-
baum (87) and v. Kennel (’85, '88), regards the insect amnion as
a structure of high phylogenetic value and would trace it to
some organ in the lower Arthropods or in the worms. Ac-
cording to another view advocated by Will (ss) and myself
('89), the amnion has had no such remote phylogenetic history,
but has arisen more recently in response to certain purely
mechanical conditions of development.

Nusbaum advances the opinion that the cellular envelopes
of the insect embryo are homologous with the “dorsal organ”
of Crustacea. The saddle-shaped “dorsal organ”’ of Lzgza and
Oniscus is regarded as the key to this homology, the two flaps
which clasp the sides of the Isopod embryo being equivalent
to undeveloped amnioserosal folds. But I have shown in the
present paper that the indusium of X7zphzdium is very probably
the homologue of the crustacean “dorsal organ,” and as there
is besides a well developed amnion and serosa in Xzphidium,
Nusbaum’s hypothesis must fall to the ground. His assertion
was certainly premature that the “deux séries des organes aussi
charactéristiques que le sont l’organe dorsal et les enveloppes
embryonnaires, s’excluent réciproquement dans les deux
groupes des Arthropodes, c’est-a-dire chez les Trachéates et
les Crustacés.”’

So far as the insect envelopes are concerned v. Kennel’s
views do not differ essentially from Nusbaum’s. He lkewise
homologizes the crustacean “dorsal organ’ and the Poduran
“‘micropyle’’ with the Hexapod amnion and serosa. But he
goes further and includes under the same homology the
amnion of Perifatus, Scorpions and Chelifer and the chitinous
envelopes of Myriopods. He supposes all these structures to
represent remnants of the annelid trochophore. I feel con-
fident that he has jumbled together at least three categories of
organs which cannot be regarded as homologous zz/er se, viz.:
(1) the series of structures typically represented by the Crus-
tacean “dorsal organ’’; (2) the cellular envelopes of insects;

,

62 WHEELER. (VoL. Vili

(3) the chitinous cuticles. As stated above, the amnions of
Peripatus and Scorpions probably also represent structures
of independent origin and no wise homologous with the
envelopes of insects. It is perhaps unnecessary to add that
the reduction of all these structures to the annelid trochophore
is in the present state of our knowledge little more than a wild
guess.

Graber (90) has criticized the view advanced by Will and
myself, that the insect amnion arose by an invagination of the
germ-band like that of some Myriopods (Geophilus). His
contention is certainly in great measure well-founded. Still
I believe that it does not affect the essential point of the
hypothesis which implies that the amnioserosal fold is the
mechanical result of a local induplication of the blastoderm
due to rapid proliferation in a single layer of cells.

Ryder (86) has sought a mechanical explanation for the
amnion, and although his paper treats mainly of the vertebrate
amnion, he evidently implies that the homonymous envelope of
the Insecta had a similar origin. According to him ‘the
amnion in all forms has arisen in consequence of the forces
of growth resident in the embryo, encountering peripheral
and external resistance either in the form of a rigid outer
egg-shell (zona radiata) or decidua reflexa, or even the walls
of the uterine cavity itself, supposing of course that a large
vesicular blastoderm containing yolk has been formed by
epiboly.”’

This view applies with little alteration to the Insecta. There
is the vesicular one-layered blastoderm filled with yolk and the
germ-band arising by rapid proliferation at one point. The
resistance of the yolk being less than the external resistance
of the tightly fitting chorion and vitelline membrane on the
one hand combined with the peripheral resistance of the extra-
embryonal blastoderm on the other, the germ-band is forced to
invaginate. This invaginative process is favored by the dis-
placement of yolk during its liquefaction and absorption by the
growing embryo. We may suppose that this invagination
which results in the formation of the amnioserosal fold,
assumed a definite and specific character in different groups of
insects.

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 63

Conditions similar to those to which the insect germ-band is
subjected during its younger stages are often present in the
ova and young of other animals, and would be expected to lead
to the formation of structures similar to the insect amnion.
And this is found to be the case. A hasty glance through the
animal kingdom at once suggests a number of parallel instances:
the invagination from which the Cestode head develops in the
Cysticercus ; the similar invagination in the larval Gordiid; the
origin of the Nemertine in the Pz/zdiwm,; the formation of the
definitive trunk in Az/astoma, according to Bergh (85); the
development of the trunk and Scheitelplatte in Szpunculus,
according to Hatschek (84); the formation of the young
Spatangid in the Pluteus, according to Metschnikoff, and the
somewhat similar conditions in the development of the Avz-
tedon, according to Barrois (88); the formation of the trunk
in the Actinotrocha of Phoronis (E. B. Wilson '81); the de-
velopment of the Polyzoan within the statoblast (Oka ’91 ;
Davenport '91). I need hardly say that the development of
the amnion and serosa in vertebrates is a strictly analogous
case. A case still more to the point, because occurring in
the Insecta, is the formation of the imaginal disks. In this
process we have all gradations till we reach the extreme in
Musca, where the hollow disks whose inner walls bud forth
the imaginal appendages are almost completely abstricted from
the original hypodermis. The resistance of the chitinous cuti-
cle of the larva in causing the invagination of the disks admits
of easy observation. It certainly cannot be claimed that in all
the different forms here enumerated genetic relationship lies at
the bottom of the mutual agreement in the methods of form-
ing the trunk or certain organs. On the contrary, everything
goes to show that these similar methods in widely separated
groups have been independently acquired under the stress of
similar developmental conditions.

Perhaps the most difficult point to explain in the view here
advanced, is the complete abstriction of the amnion from the
serosa in nearly all insects. It is more natural to suppose
that the inner envelope would remain continuous with the
outer, so that the embryo could the more readily be everted

64 WHEELER. [Vou V.DEE.

during revolution. The only explanation I have to offer, will
be given in connection with a discussion of the movements of
the germ-band. In that connection the variations in the devel-
opment and amputation of the envelopes in the different groups
of insects may also be treated to greater advantage.

2 The Volk.

To my knowledge, the quantity of yolk in the insect egg
has not been made the subject of comparative study. It has
long been vaguely stated (vzde Brauer, '69 and '70) that the eggs
of Ametabolous insects contain relatively more yolk than the
eggs of the Metabola. In other groups of animals (Crustacea,
Annelida, Mollusca, Vertebrata), it is often observed that ab-
sence of yolk is correlated with free larval development, while
in eggs provided with an abundance of yolk the larval stages are
either lacking or considerably modified. This same law obtains
also in the Hexapoda, though it can hardly be formulated so
concisely as in other groups of animals. And this is not sur-
prising when we stop to consider that, as regards complexity
of organization, the difference between the simplest insect
larvae, such as those of the Muscidz and their highly special-
ized imagines, is far from being as great as the differences
between the trochophore and the Annelid, or the Nauplius and
the crustacean.

Beginning with the Orthoptera we find that the egg is pro-
vided with an abundance of yolk,—the germ-band when first
formed in most cases covers only a very small portion of its
surface, and when it reaches its maximum length before revolu-
tion is no longer than, and usually not so long as, the egg.}
The period of embryonic development is greatly prolonged;
most of the species are monogoneutic and oviposit in the fall,
the larve not hatching till the following spring or summer.
There is practically no metamorphosis.

In the most highly metabolic insects (Muscidz) on the other
hand, the quantity of yolk is comparatively limited. The germ-
band before revolution is nearly double the length of the egg,

1 To this rule Gry//otalpa seems to be a noteworthy exception.

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 65

so that the head and tail ends nearly meet. Embryonic de-
velopment is completed in a day, and the larva must pass
through a complex metamorphosis to reach the imaginal state.

The chasm between these two extremes is bridged by the
less metabolic insects (Coleoptera, Neuropotera, Lepidoptera,
Hymenoptera, etc.). The quantity of yolk is intermediate be-
tween that of the Orthopteran and Dipteran egg. The germ-
band, like that of the Muscide, is longer than the egg when it
reaches its full length. But it is at this time much narrower
than the yolk-mass, whereas in the Muscidze it embraces nearly
half the circumference of the yolk. The larvz usually hatch
after a period of ten to thirty days in a relatively more ad-
vanced stage of organization than Dipteran larve.

It is probable that the quality of the yolk is also an impor-
tant factor in development. The yolk of the Orthoptera and
Rhynchota is dense and resembles that of the crustacean and
Arachnid egg, while the yolk of the Metabola seems to have a
much looser molecular structure. Hence, bulk alone is no
criterion of the amount of yolk in an insect’s egg.

The view here advocated, that the eggs of the Ametabola
contain more yolk than those of the Metabola, admits of some
exceptions. Thus the 17-year locust (Cicada septendecim)
is a large insect with incomplete metamorphosis, but it never-
theless produces a great number of very small eggs. This is,
however, seen to be a greater advantage to the insect than the
production of a few large eggs, when we consider the extremely
long period of larval life and the vicissitudes to which the
larvee may be subjected during all this time. Similarly, W/eloé
angusticollis produces a great number of very small eggs, while
the eggs of the smaller beetles (Doryphora, e.g.) are much
larger. But Je/oé is a parasite form, and probably only a few
of its many offspring ever succeed in gaining access to the eggs
of the bee. The larva, as shown by their hypermetamorpho-
sis, are subjected to very varied conditions, and this would still
further tend to reduce the number of successful individuals.
As in anemophilous plants many germs are produced, but very
few are destined ever to prosper. Many other exceptions to
the general rule, like these two, are probably due to habits

66 WHEELER. [VoL. VIII.

which necessitate the production of a great number of ova at
the expense of their size. The opposite exception occurs in
the parasitic Pupipara, where the nourishment of the single
larva within the parent is equivalent to the production of a large
yolk-laden egg.?

The question naturally arises: Were the eggs of the prim-
itive Insecta poor or rich in yolk? As all the evidence of com-
parative anatomy, embryology and paleontology goes to show
that the Metabola are the more recent, the Ametabola the
more ancient forms, we are justified in maintaining that prim-
itive insects, or at any rate the primitive Pterygota supplied
their eggs with a considerable quantity of yolk. At first sight
the Apterygota, which have holoblastic eggs, would seem to
constitute a serious obstacle to this view, but it must be
remembered that total cleavage is not necessarily a criterion of
paucity of yolk (witness Arachnida, Crustacea, and Myriopoda).
Furthermore, the eggs of some Thysanura, Azwrida, e.g. are
provided with an abundance of yolk. Holoblastic cleavage in
this group is probably a Myriopod trait, as was long ago sug-
gested by Metschnikoff (74). We might perhaps conclude
that the superficial type of cleavage, like the embryonic en-

1 The differences between the eggs of different insects with respect to the
amount of yolk is systematically disregarded by Graber (90). This is shown by
his classification of germ-bands as microblastic and macroblastic, brachyblastic and
tanyblastic. These distinctions are readily shown to be distinctions in the amount
of yolk and not in the germ-band. Thus the just-established germ-bands of the
Saltatory Orthoptera appear to be very small because the eggs contain an enor-
mous quantity of yolk; while the germ-band of the Muscidz appears correspond-
ingly large on account of the small quantity of yolk. The amount of yolk
fluctuates even within the limits of the single orders so that the newly-formed germ-
bands appear to differ in length more than they really do. In the Orthoptera we
have the following series in which the amount of yolk decreases, the germ-band in
consequence appearing to increase: MJelanoplus, Mantis, Gcanthus, Gryllus,
Xiphidium, Blatta, (?) Gryllotalpa.

Graber’s further classification of germ-bands as orthoblastic and ankyloblastic,
or straight and curved, is equally artificial In the great majority of cases the
shape of the germ-band depends upon the yolk surface on which it arises, or over
which it happens to grow. The uselessness of such a classification is also shown
in the case of Xiphidium and Orchelimum, where the just-established germ-band
is straight, but becomes curved in passing to the dorsal surface, and thereupon
again becomes straight. To which of Graber’s classes does this germ-band
belong?

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 67

velopes and the wings, originated in the ancestral Pterygota.
But Lemoine (87) claims that the segmentation of the Poduran
Anurophorus laricis approaches the superficial type, so that
this latter may have had a still more remote origin. It is,
however, hopeless to speculate on this subject till the eggs of
many more Thysanura and Myriopoda, including the Symphyla,
have been studied.

The relations of yolk-quantity to the movements of the
embryo will be considered in the following paragraphs.

3. Blastokinesis.

According to Hallez (85 and '86) “La cellule-oeuf posséde
la méme orientation que l’organisme maternel qui l’a produite:
elle a un pdle cephalique et un pédle caudal, un cété droit et
un cdéte gauche, une face dorsale et une face ventrale; et ces
différentes face de la cellule-oeuf coincident aux faces cor-
respondantes de l’embryon.”” This law was founded on a study
of the eggs of Periplaneta, Hydrophilus and Locusta, but it
finds full support in the descriptions and figures of all investi-
gators of insect development.!_ My own observations, based on
some thirty different insects, accord perfectly with those of
Hallez.

In most eggs the cephalic and caudal poles are readily dis-
tinguishable, the micropyle being usually located at or near
the former. In exceptional cases, however, it is located at the
caudal pole. There is frequently a slight flexure in the longi-
tudinal axis of the egg, foreshadowing the dorsal and ventral,
and consequently also the lateral regions of the mature em-
bryo. The more nearly the egg approaches the spherical form,
as in certain Lepidoptera and Coleoptera and in the Tricho-
ptera, the more obscure become the relations of the egg-sur-
faces to the body-surfaces of the mature embryo. There is,
however, every reason to suppose that these relations still exist.

The practical value of Hallez’ law was shown in studying
the Xzphidium egg ; all the movements of the germ-band could

1 The only exception is Ayers, who was undoubtedly mistaken in regard to the
orientation of the young @canthus embryo.

68 WHEELER. [Vou. VIII.

at once be referred to the axis of the mature embryo. When
the eggs of other insects.are oriented in the same manner, it is
seen that the germ-band invariably arises on the ventral sur-
face of the yolk with its procephaleum directed towards the
cephalic, and its tail towards the caudal pole. No matter what
positions it may subsequently assume, it always returns to its
original position before hatching. Frequently the germ-band,
when newly formed, lies nearer the lower than the upper pole
(Calopteryx, Ecanthus, Stagmomantis, Hydrophilus, etc.). The
usual movements are very simple ; from a position of rest on
the ventral surface of the egg, the germ-band moves through
an arc till its body is completely inverted. Then it rests and
again passes back through the same arc to its original position
on the ventral yolk. These movements may be compared to
the single vibration of a pendulum. The ascending movement
I shall designate as anatrepsis, the descending as katatrepsis,
the intervening resting stage as the diapause. The general
term dlastokinesis may be used to include all the oscillatory
movements of the germ-band.

Inasmuch as the germ-bands in other Arthropods (Crustacea,
Myriopoda, Arachnida, and Thysanura) exhibit no movements
comparable to those of the lower Pterygota, and since, more-
over, the insect germ-band is formed in exactly the same
manner as that of other Arthropods and ultimately returns to
its original position, no matter what oscillations may intervene,
it is safe to infer that blastokinesis has been acquired within
the Hexapod and probably even within the Pterygote group.
We may also infer from the intimate relations of envelope-
formation to blastokinesis in most forms, that both of these
processes arose at about the same time.

No attempt has been made to account for the origin of blasto-
kinesis. It has occurred to me that it may be due to causes of
a purely physiological nature. The eggs of the primitive Pter-
ygota were, as I have attempted to show, provided with a con-
siderable amount of food yolk. Like their modern descendants
they were probably also invested with dense chitinous envelopes.
These must render the respiration of the embryo difficult as
compared with embryonic respiration in annelids, mollusks and

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 69

vertebrates, or even as compared with the Crustacea, which
usually have much thinner envelopes than insect eggs. Special
provision is also made in many of the Crustacea for aerating the
eggs. Now the cells of the rapidly growing insect embryo not
only absorb and metabolize the yolk but also give off a certain
amount of waste matter. That this is not wholly of a gaseous
nature is seen in older embryos which have considerable
accumulations of uric salts in the blood corpuscles and fat-
body. Waste products are undoubtedly given off during the
stages preceding anatrepsis, and probably permeate the yolk in
the immediate neighborhood of the germ-band. As the oxida-
tion of these waste products is very probably retarded by slug-
gish transpiration, and as growth under such conditions would
be seriously impeded, we may suppose that the embryo has
acquired the habit of moving to another part of the egg where
the yolk is as yet unpolluted. Here it grows apace till the
surrounding yolk is again charged with excreta. Growth is
then temporarily suspended and the embryo moves back to the
ventral surface. The embryo reaches a considerable size be-
fore katatrepsis, so that its rotation must cause a considerable
circulation in the yolk bodies. This would also serve to aerate
the yolk and to bring fresh pabulum in contact with the
assimilating cells of the embryo. It may also be noted that
in many insects the movements set in at critical periods of
growth. Thus in AX7phzdium anatrepsis occurs during the
addition of new segments, and in many other forms it im-
mediately precedes the formation of new segments. In the
Orthoptera, katatrepsis usually occurs in the spring and is the
signal for a decisive advance in the development of the heart,
sexual organs, compound eyes, etc. During this period, also,
the abortion of such rudimental structures as the pleuropodia,
abdominal appendages and envelopes seems to be hastened. In
short, the whole process of katatrepsis, at least in X7phidium,
has the aspect of rejuvenescence. It will be remembered that
the amnion is formed just before or during anatrepsis. It is
probable that the complete abstriction of this envelope from
the serosa is a device for favoring the movements of the embryo.
The germ-band is thereby set adrift on the yolk and enabled to

70 WHEELER. [Vot. VIII.

migrate to some other surface. This, of course, necessitates a
secondary union of the envelopes previous to katatrepsis.

The hypothesis set forth in the preceding paragraphs is also
supported indirectly by the fact that in the eggs of the Meta-
bola which are less abundantly provided with yolk than the
eggs of the Ametabola, blastokinesis is either faint or wanting.
Aeration would be much less necessary in such small eggs.
The lengthening and shortening movements seen in the
embryos of the Metabola as well as in those of the Ametabola
may suffice to keep the yolk circulating. The Lepidopteran
germ-band, it is true, exhibits movements, but the eggs of
these insects are laid in exposed situations and provided with
unusually thick envelopes, so that the movements of the em-
bryo, though differing widely from the typical blastokinesis of
lower forms, have perhaps been independently acquired for a
similar purpose.

I had intended to give a comparative description of blasto-
kinesis in the different orders of insects but as the known
facts have been recently summarized in a masterly manner
by Korschelt and Heider (92) I shall confine my remarks
mainly to the Orthoptera. Although Graber, Ayers and
others have studied representatives of this very important
group, they have given but fragmentary and often inaccurate
accounts of the relations of the embryo to the yolk-mass at
different periods of development.

I may begin my account with the Saltatoria which comprise
the three families Gryllidz, Locustidz and Acridide. As
representatives of the first, Gryllus luctuosus and Cicanthus
niveus were studied. In both of these insects as was pointed
out at p. 42 the germ-band arises on the ventral surface of
the yolk near the caudal pole. During the formation of the
envelopes anatrepsis sets in and carries the germ-band to the
dorsal surface where it rests through the winter in an inverted
position with its head directed to the caudal and its tail to the
cephalic pole. In the spring the envelopes over the head end
first fuse and then rupture ; the embryo is thereupon everted
and during katatrepsis passes around the caudal pole to regain
its upright position on the ventral yolk. The envelopes during

No. 1.] CONTRIBUTION TO INSECT EMBRVOLOGY. 7E

this process are stripped back over, and finally drawn into the
yolk, where they undergo dissolution when the body walls have
met in the median dorsal line. The defects in Ayers’ descrip-
tion of @canthus ('84) were pointed out at p. 43.

Gryllotalpa, the only other Gryllid, which has been studied,
seems to differ considerably from Gryl/us and Gcanthus. Ex-
amination of Korotneff’s figures (85) shows that this difference
is probably more apparent than real. In his surface views,
there is a wide gap between his Fig. 2, representing the egg in
a preblastodermic stage, and his Fig. 3, representing quite an
advanced embryo. One is thus left without any guide to the
exact relation of the just-established germ-band to the yolk-
surfaces. Korotneff’s defective account of the formation of
the germ-layers would seem to show that he did not study these
early stages closely. It is obvious that Gvyllotalpa is blasto-
kinetic both from Korotneff’s statement that the embryo
moves during revolution and from his figures 5, 7, and 8, but
the exact nature of the process is not clear. The possibility
of the embryo’s passing to the opposite surface of the egg is
not precluded by the conditions seen in Figs. 7 and 8. Judg-
ing from Gryllus and Gicanthus I am inclined to think that the
embryo exhibits both ana- and katatrepsis, but that Korotneff
has overlooked the former and misinterpreted the latter
movement.

In the Locustidz, as represented by AX7phidiwm and Orcheli-
mum, we find a modification of the blastokinetic process ob-
served in Gryl/us. Instead, however, of arising near the caudal
pole, the germ-band is formed on the middle of the ventral
surface, and instead of passing around the caudal pole during
anatrepsis it passes through the yolk as if to reach the dorsal
surface by a shorter path. Katatrepsis is essentially the same
as in the Gryllide, the embryo passing around the caudal pole.
This lack of coincidence in the anatreptic and katatreptic paths
is one of the most striking peculiarities of Locustid develop-
ment; since it is known to occur in no other insect. It is
probable that the anatreptic embryo originally passed around
the lower pole, but that owing to the formation of the embryo
higher up on the ventral surface, and perhaps also to an acqui-

72 WHEELER. [Vou. VIII.

sition of yolk at the lower pole, this movement has been
deflected.

Melanoplus femur-rubrum was studied as a representative of
the Acrididea. The germ-band is formed very near the caudal
pole of the egg, but still on the convex ventral surface. During
the formation of the envelopes the posterior end of the body
grows around the pole onto the dorsal surface, while its head
remains fixed at the pole. It is not until the germ-band has
reached a stage corresponding to Stage F. in X7zphidium that
its head leaves the pole and the whole body moves upward on
the dorsal surface. It soon comes to a standstill and passes
the winter in this inverted position. In the spring it moves
back around the lower pole and, like the Gryllid and Locustid
embryo in a corresponding stage, proceeds to lengthen and en-
velop the yolk till its head reaches the cephalic pole.

Packard (83) seems to have been the first to study the devel-
opment of Acridians (Welanoplus spretus and M. atlanis). But
he had no conception of the true relations of the embryo to the
yolk, as is shown by his Fig. 1, Pl. XVII, where the egg is
depicted with the micropylar end uppermost. Leuckart (’55)
long ago showed that the Acridian micropyle is located at the
caudal pole. If the egg figured by Packard be inverted, it will
represent the embryo on the point of undergoing katatrepsis.

The same error is committed by Graber in his accounts
of Stenobothrus variabilis ('88,'90). Misled, like Packard, by
the position of the micropyle, he has mistaken the caudal for
the cephalic pole. To mean anything his figures must be in-
verted. As I have not yet studied the later stages of MWelan-
oplus in section, I will not attempt to describe the details of
katatrepsis. Graber claims to have observed that the pleural
ectoderm, where it passes into the amnion proliferates a thin
cell-lamella to form the dorsal wall, while the amnion remains
intact and still covers the ventral face of the embryo. This
account is not substantiated by his figures (1 and 2, Pl. I).
The two thin cell-lamellz extending over the dorsal surface
have every appearance of being the walls of the heart, and
therefore mesodermal, although it is difficult to see how this
organ could be so completely formed in so early a stage. As

See

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. ee

Graber has paid no attention to the movements of the Stezo-
bothrus embryo, and as he most assuredly has not demonstrated
from a careful study of the later stages that the lamella in
question is really converted into the dorsal wall, I cannot
attribute much value to his observation.

The foregoing observations go to show that the blastokinetic
processes are essentially the same throughout the suborder
Saltatoria. Each family presents certain deviations from the
type, which is probably most closely adhered to in the Gryllidee.
Anatrepsis is aberrant in the Locustidz, while the Acrididz
are aberrant in the tardy separation of the procephaleum from
the lower pole. Notwithstanding these deviations the Salta-
toria form a clearly circumscribed group embryologically as
well as anatomically, and were it not for Gry//otalpa would be
separated by a wide gap from all other Orthoptera. Grydlo-
talpa is a generalized form, as Brauer has pointed out from
a study of its anatomical peculiarities (86), and his conclusions
are to some extent substantiated by the large size of the germ-
band as compared with the yolk mass.

In the Cursoria, as represented by Blatta germanica, move-
ments of the embryo are far less apparent. The germ-band
never leaves the ventral surface, on the middle of which it first
appears. I have shown, nevertheless ('89, text-figures, p. 348),
that it moves down the yolk after the rupture of the envelopes
till its tail reaches the lower pole. The tail then remains sta-
tionary, while the head gradually rises to the cephalic pole as
the body walls develop and invest more and more of the yolk.
Slight as are these movements, they nevertheless recall the
blastokinesis of the Saltatoria. I would regard the movement
of the whole B/atta embryo towards the caudal pole as ana-
treptic ; katatrepsis is probably represented only by the up-
ward growth of the embryo. The very late occurrence of the
former movement may be due to its rudimental character, since
it is too weak to carry the germ-band around the caudal pole.

Few observations have been published on the relations of
the embryo to the yolk in the Gressoria. In Manfzs, as I have
shown, the germ-band when first formed lies somewhat nearer
the posterior than the anterior pole. The embryo never leaves

74 WHEELER. [Vot. VIII.

the ventral surface of the egg, but whether or not it exhibits
any traces of blastokinesis my limited material will not enable
me to decide, and Graber (77), Bruce (86), and Viallanes
(902, '90°), have contributed no observations bearing on this
point. It is clear, nevertheless, that in its development, J/antzs
resembles B/atta more closely than either of these forms re-
semble the Saltatoria. This merely confirms the view which
has long been held respecting the affinities of the Blattidz
and Mantide. From the structural similarity of the Phas-
midz and Mantide we may venture to infer a similarity of
embryonic development.

It thus appears that the Orthoptera are clearly separable
into two groups—the Saltatoria on the one hand and the
Gressoria and Cursoria on the other. The Saltatoria are
decidedly blastokinetic whereas the non-saltatory forms re-
tain only faint reminiscences of blastokinesis (B/atta). I
am inclined to believe that primitive embryological features
have been preserved more faithfully in the Saltatoria than
in other Orthoptera. That the habits of oviposition are more
primitive in this group is shown by Brongniart’s discovery of a
fossil Blattid provided with an ovipositor (89). Moreover, several
features in the development of the Saltatoria show great con-
servatism, ¢.g. the retention of the indusium in the Locust-
idz, the order in which the metameres arise, and the myriopod-
like habitus of the X7phzdium embryo in Stage D.

Not only does a study of the Saltatoria throw light on the
development of other Orthoptera, but it brings the order into
closer union with the Odonata and Rhynchota. The blasto-
‘kinesis of the Gryllidz agrees closely with that of the Hydro-
corisa among the Hemiptera—e.g. Corixa, as described by
Metschnikoff (66). Ranatra and Zaitha will bear even a closer
comparison with the Gryllidz. In the much elongated egg of
the former, which has the cephalic pole marked by the pair of
diverging pneumatic threads, the germ-band arises as usual on
the ventral surface with its head directed upwards. As the
envelopes develop it passes around the lower pole and finally
assumes an inverted position on the dorsal surface. During
katatrepsis it returns over the same path. The inclusion ob-

No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 75

servable in Corixa and probably also in Ranatra, of a small
quantity of yolk between the caudal amnion and the overlying
serosa when the embryo first passes to the dorsal surface, is
often observed in the Saltatoria. It is no great step to
pass from the conditions seen in the Hydrocorisa to the
«entoblastic” condition of other Hemiptera (Pediculus, Aphis,
Cicada) and the Odonata (Calopteryx), where the germ-band
instead of passing to the dorsal yolk during anatrepsis, comes
to lie in the middle of the yolk, or even near the ventral
surface (Pyrrhocoris). The Thysanoptera, as may be in-
ferred from Jordan’s brief statement ('88), the Corrodentia
(Mallophaga) according to Melnikow (69), and the Psocidee,
according to Packard ('84), are also referable to the ‘“ento-
blastic” type. Concerning the embryonic development of the
Plecoptera and Dermaptera nothing is known.

So far only the Homomorpha have been considered. The
eggs of the Heteromorpha, as I have attempted to show, con-
tain less yolk. Blastokinesis is nearly or quite lost in this
more recent group, a fact that perhaps indirectly tells in favor
of my view that the movements of the embryo have been ac-
quired for the purpose of ventilating the yolk and supplying
the growing embryo from time to time with fresh pabulum.
The transition to the Heteromorpha is probably represented by
the Ephemeridea. According to Burmeister’s account of the
development of Palingenia horaria (I quote from Zaddach, '54):
«am dritten Tage, nachdem das Ei gelegt war, hatte sich der
Keimstreif gebildet, der zungenformig war, und sich iiber zwei
drittel der Eilange erstreckte, also in Form und Ausdehnung
ganz dieselben Verhaltnisse zeigte, wie im Phryganidenei.”
This may, perhaps, be taken to indicate that the Ephemeridea
exhibit no blastokinesis; but the subject requires urgent
investigation.

Among the Heteromorpha it is especially the Coleoptera
which still show distinct though abortive movements of the
germ-band. Hydrophilus may be taken as an example. As
may be seen from Heider’s figures, the germ-band forms on the
lower ventral surface of the egg. As it grows in length, and
the amnion is formed, the tail curls around the caudal pole on-

76 WHEELER. [Vou. VIII.

to the dorsal surface, but soon separates from the serosa so
that a small amount of yolk is enclosed between the two en-
velopes. Later the yolk is expelied from this region and the
envelopes become applied to each other. A true movement
then sets in and carries the anterior portion of the germ-band
forward up the ventral surface till the procephaleum overlaps
the cephalic pole (Cf. Heider’s figures, 4c, 6a, 7 a and 9,
Pl. II. (89). A certain similarity of these movements to those
exhibited in B/atta leads me to believe that they represent
a weakened blastokinesis.

Whether or not similar movements occur in the other so-
called “ectoblastic”” forms (Diptera, Hymenoptera, Siphonap-
tera, Neuroptera, Trichoptera) cannot be decided at present.
If such movements occur at all they are probably exceedingly
weak.

As stated above, the Lepidoptera have developed embryonic
movements peculiar to themselves. In all the members of the
order hitherto studied, the germ-band arises on the ventral sur-
face of the egg, and its envelopes are formed while it is still
in this position. As development proceeds the convex ventral
surface of the germ-band, with its adherent amnion, moves back
from the ventral serosa and soon comes to lie in the middle of
the yolk. Hereupon the ventral surface of the embryo be-
comes concave, and its dorsal surface is applied to the dorsal
serosa. I have already remarked that this movement of the
embryo may have been independently developed for the same
purely physiological purposes as blastokinesis in the Homo-
morpha. The fact that the movement is represented in the
Trichoptera only by the change in flexure of the longitudinal
embryonic axis, would seem to indicate that it has been
acquired since the Lepidoptera diverged from the Trichopteroid
ancestor.

Graber ('90) has recently made the interesting discovery that
the Phytophagous Hymenoptera closely resemble the Lepidop-
tera in the movements of the embryo and in the amputation of
the envelopes. This, taken together with the striking resem-
blance between the eruciform larve of the two groups, appears
to point to a closer relationship than has usually been claimed.

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 77

While studying the movements of the embryo and the for-
mation of the envelopes in the different orders and families of
insects, with a view to testing the current classification, which
is the outcome of a great amount of comparative anatomical
and paleontological work, I have been especially impressed
with two facts: First, the embryological data in no wise conflict
with the generally accepted classification of Brauer. The de-
velopmental variations within limited groups are never greater
than the post-embryonic differences in the members of the
same groups. Usually there is great uniformity in embryo-
logical development between systematically allied insects of
the same order; the wide gaps usually occur between the
orders just where gaps have long been pointed out by com-
parative anatomy and paleontology. Second, developmental
differences between members of different allied families of
Orthoptera are greater than the differences between remotely
related families in more recent orders. For example, the dif-
ferences between a Locustid and an Acridian or a Locustid
and a Gryllid embryo, or between any of the Saltatoria and
the Blattidz, or Mantidz, are greater than the differences be-
tween an embryo Hydrophilid and a Chrysomelid, a Tabanid
and a Chironomid, or a Bombycid and a Shingid. Frequently,
it is true, the differences between the extremes in the higher
orders are considerable, as between the Tenthredinidz and the
Proctotrupide among Hymenoptera, or the Chironomidze and
Muscide among Diptera. If any conclusions bearing on classi-
fication can be drawn from the few embryological data which
I have collected, they refer to the ordinal value of the vari-
ous Orthopteran families. It would appear that these groups
have really more than family value. They are older than the
families of more recent groups, and therefore exhibit greater
divergence. The Rhynchota will probabiy be found to present
conditions similar to the Orthoptera. There are certainly more
considerable differences between the embryos of such forms as
Pyrrhocoris and Ranatra than there are between the embryos
of widely separated families among the Coleoptera.

78 WHEELER. [Vou VIII.

4. The Elimination of the Embryonic Envelopes.

Anatrepsis and katatrepsis in the lower insect orders, or the
completion of the envelopes and their rupture in the higher
orders, are separated by a distinct interval, during which the
germ-band undergoes a considerable development. But during
this interval, the diapause, no change is noticeable in the
envelopes themselves beyond a thinning of the amnion with
the increased growth of the embryo. The elimination of the
envelopes is preceded by katatrepsis just as their formation
was preceded or accompanied by anatrepsis. This elimination
is immediately followed by the completion of the dorsal body-
wall and may take place in a variety of ways. Korschelt and
Heider (92) distinguish the following types in this process:

1. The amnion and serosa become continuous and, after the
eversion of the embryo, are drawn back over the yolk to form
a single layer of cells. As the dorsad growth of the body-walls
proceeds, both envelopes are drawn together and pushed into
the yolk to form a sack or longitudinal tube which is ultimately
enclosed by the walls of the mesenteron and absorbed. To
this type belong the Odonata, Rhynchota, some Orthoptera
(Blatta, Cecanthus, Gryllotalpa) and some Coleoptera (//y-
adrophilus).

2. The serosa is shed from the yolk and the amnion alone
contracts on the dorsal surface preparatory to being drawn into
the yolk and absorbed. (Certain Coleoptera, e.g. Doryphora.)

3. The serosa alone is agglomerated and drawn into the
dorsal yolk, the amnion being cast off. (Certain Diptera
[Chtronomus| and Trichoptera.)

4. Both envelopes are shed. (Lepidoptera and certain
Hymenoptera.)

In Avphidium we may perhaps recognize a fifth type, in
which as in the fourth, both amnion and serosa are shed. But
while the serosa is in great part shed as a simple membrane,
the indusium which is a modified portion of the serosa, together
with the amnion is drawn together in a mass and cut off from
the embryo. It is more than probable that other types of
envelope elimination will be discovered when more forms have

”

No.1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 79

been studied. /usca may perhaps be regarded as representing
a distinct type, since in this highly modified form the rudi-
mental amnion and the serosa are neither shed nor agglomer-
ated and engulfed in the yolk, but are supposed to form the
definitive body-wall. (Kowalewsky, '86; Graber, ’89.)

It is clear that the revolution of the insect embryo includes
three distinct processes: first, the eversion and katatrepsis of
the germ-band; second, the formation of the dorsal walls; and
third, the elimination of the envelopes. The mechanical cause
of eversion and katatrepsis is probably a contraction on the part
of the envelopes after their fusion and rupture over the ventral
surface of the embryo. After the embryo is everted from the
amniotic cavity, or exposed after the rupture of the amnion and
serosa, these envelopes temporarily form the dorsal covering of
the yolk. Do they ever form the definitive dorsal body-wall ?
For both envelopes this is claimed to be the case only in Musca.
In all other insects the serosa, at least, takes no part in forming
the permanent body-wall, as it is either shed or engulfed in the
yolk. The question is, therefore, restricted to the fate of the
amnion. In many insects (Lepidoptera, Hymenoptera Phyto-
phaga, some Diptera and Coleoptera), it has been shown that
the amnion takes no part in the formation of the definitive body-
wall, although a decision on this point is rendered difficult by
the fact that no hard and fast line can be drawn between the
ectoderm of the germ-band and the cells of the amnion. In
other insects the decision is even more difficult. Still, I may
say that I have seen nothing in the insects I have studied,
to convince me that the amnion is converted into a portion of
the permanent body-wall. Even in d/usca it seems probable
that the amnion and serosa only temporarily function as the
body-wall, and that their cells are ultimately replaced by true
ectodermal elements from the germ-band. In JAlatta and
Aiphidium I have seen appearances which lead me to believe
that at least a part of the amnion may be eliminated by such a
process of cell-substitution. I incline, therefore, to the views of
Korschelt and Heider (92), who hold that the envelopes are
probably completely eliminated, and that the entire body-wall
is derived from the ectoderm of the germ-band.

80 WHEELER. [VoL. VIII.

If this be the correct view, it follows that the dorsal body-
wall is formed in essentially the same manner in all insects —
by a growth and meeting of the germ-band edges. This pro-
cess is, therefore, remarkably simple and uniform compared
with the processes whereby the envelopes are eliminated. The
ereat variability in the latter case has been dwelt on by Graber
(88) in a paper devoted to dorsal-wall formation in the Insecta.
After reviewing all the literature on the subject and contrib-
uting many new facts, he proceeds to base a classification of
the insects hitherto studied, on the ‘ Keimhiillenzustande.”
He finds some fault with the current classification on the
ground that insects which systematists regard as closely re-
lated often present great differences in their respective methods
of dorsal-wall formation, whereas remotely related insects often
agree very closely in this respect. Thus Zzwa and f1ydro-
philus differ more than Hydrophilus and Gicanthus in the
processes whereby the dorsal-wall is formed. In considering
Graber’s views, I may pass over the awkward and kakophonous
nomenclature which he has introduced, to what I regard as his
main error, vzz. the superficial analysis of his subject. Graber’s
term “Keimhiillenzustinde,” I take it, includes the formation
of the envelopes as well as their condition preceding and
during their elimination. Now I have attempted to show that
there is nothing in the formation of the envelopes nor in the
concomitant anatrepsis of the germ-band in the different insect
orders to conflict with the current classification. Nor is there
anything in the closure of the dorsal-wall in different groups
—restricting this term to the confluence of the pleural edges
of the germ-band —to support Graber’s conclusion. His state-
ment must therefore be restricted to the elimination processes.
That these are highly variable must be admitted, but they are
probably of very little taxonomic value, as Graber would prob-
ably have observed, had he attempted to account for the wide
differences in allied forms and the agreement of remotely re-
lated species. It is my opinion that this high degree of varia-
bility in the elimination process is to be traced to the same
causes as the variability of the indusium, viz., the rudimental
character of the envelopes. Up to the close of the diapause

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 8I

the envelopes subserve a distinct function, but as soon as the
germ-band has invested the yolk with its own ectoderm, they
have become functionless, or rudimental. Long before this
time, in fact ever since their completion, the envelopes show
no traces of cell-division. Moreover, their involution into the
yolk or complete shedding shows conclusively that their mor-
phological value is at this time reduced to zz/. Whether both
envelopes are shed instead of being drawn into the yolk, or
whether one is shed and the other drawn into the yolk, may
depend to some extent on the ease with which the pleural
folds can close without their temporary assistance. But which
of these processes shall occur in a given insect is probably a
matter of no vital importance to the embryo, and has prob-
ably played no réle in the struggle for existence. The involu-
tion of the envelopes, it is true, may add assimilable matter to
the embryo, but enough energy to counterbalance this addition
is probably consumed in metabolizing the dead cells. Hence
the adoption of this process may be of no greater advantage to
the embryo than the complete sloughing of the useless en-
velopes.

The insect envelopes, therefore, present only another case of
an organ which has become specialized for a particular function
at the expense of its formative power. This same phenomenon
recurs in insect ontogeny. During cleavage certain cells are
segregated for the express function of yolk-metabolization (vi-
tellophags), while the remaining cells go to form the blastoderm.
Later the cells of the blastoderm separate into those of the
germ-band proper and those of the specialized envelopes. Still
later, if the insect be metabolic, another splitting occurs, a por-
tion of the hypodermis being set aside in the form of the imag-
inal disks to supplant the specialized primitive larval hypoder-
mis. The formative material of the insect, like that of other
organisms, thus undergoes a successive splitting into a special-
ized and a comparatively non-specialized portion. The former,
being incapable of metamorphosis, is cast off or broken down,
while the latter persists until a new segregation takes place.
The analogy of this process to that occurring in rhizomatous
plants, Polyzoa, etc., need not be pointed out in detail.

82 WHEELER. [VoL. VIII.

V. NEUROGENESIS IN THE INSECTA.
1.- The Nerve-cord.

The first traces of the central nervous system of X7zphidium
make their appearance at a very early stage, before the blasto-
pore is closed and while the envelopes are still incomplete. In
this stage (Fig. 2) surface preparations made according to the
methods given in the latter part of this paper, show a number
of pale spots scattered over the procephalic lobes. They fre-
quently occur also in the maxillary region, and, were it possible
to remove the amnio-serosal fold without injuring the surface of
the germ-band, would probably be found to extend still further
caudad. The meaning of these spots is apparent when sections
of embryos in Stages B—D are examined. In a transverse sec-
tion (Fig. 25) through the middle of the abdomen of an embryo
in Stage D, the ectoderm, which bulges out somewhat on either
side of the median line, is seen to consist of two kinds of ele-
ments. First, there are a few large, clear, polygonal cells with
spherical nuclei (é.), lying in the deeper portion of the layer;
and second, a much greater number of small and more deeply
stainable cells (dd.), differing in no essential respect from the
cells forming the remainder of the ectodermal layer, such as
the appendages. The latter cells have smaller, oval or cuneate
nuclei, which appear to contain more chromatin than the large
inner cells. While the small cells form a continuous layer, the
large elements make their appearance singly or in small clusters,
as seen in the figure. It is these pale clusters underlying the
darker cells which produce the pale spots seen from the surface.

The large pale cells may be called neuroblasts — since it is
they that give rise to the purely nervous elements of the cord.?

1 The term “neuroblast” was originally used by Whitman ("78 and ’87) to
designate the two offspring of the large posterior macromere of the Clepsine egg,
which give rise by a process of budding to two rows of cells — the “neural rows.”
From these rows the nerve cord arises. His ('89) subsequently employed the
same term “neuroblast” to designate such of the offspring of the “ Keimzellen”
as give rise directly by differentiation and not by further divisions to the ganglion-
cells, or, to use Waldeyer’s term, to the neurons of the vertebrate central nervous
system. More properly the term would have been applied to the “ Keimzellen ”
themselves, and by mistake it has been thus used by at least one recent writer

(C. L. Herrick, ’92, p. 430, Fig. 10). Haeckel (Anthropogenie, 4th ed. p. 268,
91) uses ‘neuroblast’ in the sense of ectoderm in general.

No.1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 83

The remaining cells which cover the neuroblasts and extend
down between them in the median line, give rise to purely
integumental structures and may therefore be called dermato-
blasts. The two thickenings of the ectoderm are to become
the lateral cords (Seitenstrange). They extend from the
anterior edge of the eleventh abdominal segment, just in front
of the anus, to the mouth, where they diverge and pass without
interruption into the brain. The groove which separates the
lateral cords and which is very faint in Fig. 25, is the neural
furrow (Primitivrinne). It appears soon after the closure of
the blastopore and takes the place of this depression. It is
deepest anteriorly.

All the neural structures develop in an anteroposterior direc-
tion, beginning with the brain; hence different stages in the
development of the lateral cords may be studied in the same
embryo. Fig. 26 shows a section passing through the first
abdominal segment of the embryo from which the section
in Fig. 25 was taken. Here we see a distinct advance in
structure. The neural furrow (z.g.) is more clearly marked
and the neuroblasts (zé.), four in either lateral cord, have
arranged themselves side by side in a regular layer in the
deepest portion of the ectoderm. Over them the dermatoblasts
(db.) also form a single regular layer, while the cells lying in
the median line on either side of the neural furrow have grown
more elongate. Sections further forward show essentially the
same conditions —the neuroblasts which were at first differ-
entiated as small clusters or as isolated cells, have arranged
themselves throughout the anterior portion of the embryo as
an even layer entad to the dermatoblasts.

The further changes in the development of the nerve-cord,
are brought about — first, by a proliferation of the neuroblasts ;
second, by a proliferation of the dermatoblasts and a deepening
of the neural furrow; third, by the development of the median
cord; fourth, by the formation of the connectives and com-
missures, and fifth, by the development of the neurilemmata.
These changes which occur simultaneously may be described
singly for the sake of convenience.

As cross-sections show, the neuroblasts are arranged in from

84 WHEELER. [Vot. VIII.

3-5 longitudinal rows in either lateral cord. In surface view
these rows may often be followed through one or two seg-
ments as continuous strings of cells. I assume that there
were originally four of these rows, but that owing to the
pressure exerted by the developing appendages on the lateral
edges of the cords and to a more rapid growth of the neuro-
blasts than of the germ-band, the primitive regular arrange-
ment has been considerably obscured. The neuroblasts are
polygonal in outline from mutual pressure. When they divide,
as they very soon do, their spindle axes are directed at right-
angles to the surface of the body. As soon as one cell has
been given off, the nucleus rests for a short time and then
again divides in the same direction. This process continuing,
a column of cells is budded off from each neuroblast and stands
at right angles to the surface of the germ-band. The divisions
do not take place simultaneously in all the cells although cor-
responding neuroblasts in either cord will frequently be found
in the same phase of caryokinesis, especially in the earlier
stages of their proliferation. A section (Fig. 27) through the
first maxillary segment of an embryo in Stage F shows that
each of the eight neuroblasts has produced a row of daughter-
cells. The large succulent mother-cells are evenly rounded on
their outer surfaces which are overlaid by the dermatoblasts.
Their inner faces are flat or concave and in every case closely
applied to the latest daughter-cell. The nuclei of the mother-
cells are spheroidal and take no deeper stain than the pale suc-
culent cytoplasm which surrounds them. The neuroblasts are
in all essential respects typical proliferating cells like the ter-
minal cells in plant-shoots and the teloblasts of annelids. The
daughter-cells (g!) are at first characterized by their small
size, cuneate outline and deep stain. Their nuclei are con-
siderably flattened, probably from mutual pressure. These
characters are retained by the daughter-cells till they have been
pushed some distance from the neuroblast by later offspring,
when they become larger and considerably paler and assume
the appearance of definitive ganglion cells (g?).

Turning now to a somewhat older embryo (Stage G, Fig. 28)
we see that the columns of daughter-cells have greatly increased

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 85

in length, while the neuroblasts remain to all appearances un-
altered. The increase in the number of daughter-cells is so
great that they are forced to arrange themselves in several
rows. In the figure this is best shown in the progeny of the
innermost neuroblasts, and the tapering columns there formed
may be regarded as typical.

In my preliminary note (91°) I held that the daughter-cells
themselves divide to form the multiple rows in each pillar. I in-
cline to think that I was mistaken on this point. The daughter-
cells probably never divide but are directly converted into
ganglion cells. All reproductive powers seem to be confined to
the neuroblasts. Some of the nuclei of the daughter-cells exhibit
peculiar chromatic structures which I may have mistaken for
caryokinetic figures ; this being an easy error to make in the
case of small cells killed by means of heat, since the achromatic
portions of the spindles are obliterated by this method.

The last stages in the proliferation of the neuroblasts are
shown in Fig. 31, which is taken from an advanced embryo
(Stage J). The columnar arrangement is no longer visible
since the individual cells are now converted into the de-
finitive ganglionic elements. On the outer periphery of the
ganglia, however, neuroblasts are still to be found, and ex-
tending from them short series of small flattened cells (¢”),
their latest progeny, still distinguishable from the ganglion
cells by their deeper stain. It will be noted that these cells,
which like their precursors will become ganglion cells, are no
longer budded off at right angles to the surface of the nerve-
cord but parallel to it, a condition undoubtedly due to a lack
of space. Finally the neuroblasts stop proliferating and shrink
to the size of their progeny. Their chromatin then shows
signs of senility. Beyond this point I have been unable to
trace them satisfactorily. They are probably broken down and
absorbed by the growing ganglion cells. Some of them may
persist as ganglion cells of a particular character and function,
though I deem this improbable.

The dermatoblasts play an important part in the develop-
ment of the ventral nerve-cord, as will be seen by returning to
the younger stages. We left these cells as a layer covering

86 WHEELER. [Vou, VIII.

the neuroblasts and continuous laterally with the general ecto-
derm. In the median line they extend to the deepest portion
of that germ-layer in the form of a few compressed cells (Fig.
26, db.). These compressed cells form the walls and bottom of
the neural furrow. The proliferation of the neuroblasts has
caused the lateral cords to bulge out enormously (Fig. 27), so
that the dermatoblastic layer becomes stretched and attenuated.
Such divisions as occur in the cells of this layer seem to be
confined to the outer surface and do not extend into the furrow.
The spindle axes lie parallel to the surface, as shown at wd.
The bulging of the lateral cords naturally brings about a deep-
ening of the neural furrow, since the cells at its bottom have a
fixed attachment. At this point in Fig. 27 there is seen a
triangular cell-mass, capped by a single large element (0.), a
true neuroblast which resembles in nearly all respects the
neuroblasts of the lateral cords. Its more pyramidal outline is
obviously the result of its position between the converging
walls of the furrow. To the same mechanical cause is due the
shape of the cell-mass, which consists of the heaped up
daughter-cells of the neuroblast. Inasmuch as the proliferating
cell occurs in the median line, and together with its offspring
and the dermatoblastic cells of the median furrow, is equivalent
to the “Mittelstrang’’ of authors, I shall call it the median-
cord neuroblast. Its exact origin I have not been able to
determine. To judge from the number of cells which it has
given off it must have begun to proliferate at about the same
time as the lateral-cord neuroblasts. There can be no doubt,
it seems to me, that it originated as a polygonal ectoderm cell
like the lateral cells seen in Figs. 25 and 26, but whether it
was originally median in position or arose unilaterally I am
unable to decide. The pale surface spots of embryos in Stage
B show that neuroblasts are arising at a time when the blasto-
pore occupies the position of the neural furrow and hence, if
the median cells are median in position from the first, they
must arise somewhat later than their sister neuroblasts.

There is one important difference in the arrangement of the:
mother-cells of the lateral and median cord. Whereas the for-
mer, as has been stated, form continuous though irregular rows

No. 1.] CONTRIBUTION TO INSECT EMBRVOLOGY. 87

from mouth to anus, the latter constitute an interrupted series
between the same two points. They are single, isolated cells,
which occur only intersegmentally. That such is their distribu-
tion may be distinctly seen in frontal sections like the one rep-
resented in Fig. 30. This section passes through the first
to fifth abdominal segments at the level of the median cord
neuroblasts (#d.), which are seen to lie distinctly between
the segments, where the walls of the neural furrow dilate
at intervals for their accommodation. At first the daughter-
cells are given off in the same direction as those of the lateral
cords, but soon the triangular space to which they are confined
will no longer contain the older cells of the series and these
are pushed along the floor of the neural furrow. This produces
an angular flexure in the cell-column, but later the whole mass,
including the neuroblast, assumes a horizontal position. This
change in the position of the median cell-mass is seen to have
taken place in the median sagittal section from an embryo in
Stage G (Fig. 29). The neuroblast (#0) is in each segment
directed caudad, while the mass of small and deeply stainable
daughter-cells (#g) is wedged in under the commissures. Fhe
section passes through the sub-cesophageal ganglion, which
consists of the fused ganglia of the mandibular and both maxil-
lary segments (md. g; mx. g!; mx. 7); and through the pro- and
mesothoracic ganglia (f.g1; p.g?). Transverse furrows (2. 1;
7. g2), which I shall consider later, separate the unfused ganglia
from one another, and as the median cord cells lie in front of
these furrows, they must be regarded as belonging not to the
intersegmental region of the ectoderm, but to the posterior
portions of the separate ganglia. Each ganglion possesses a
median cord neuroblast, so that, beginning with the mandibular,
which is the first ganglion in the nerve-cord proper, and ending
with the tenth abdominal, there are in all sixteen median
mother-cells. Each of these, after producing its quota of
ganglionic elements, deteriorates in the same way as the
mother-cells of the lateral cords.

The development of the Punktsubstanz may be readily fol-
lowed in Xiphidium. It arises in each ganglion as two separate
masses. Each of the daughter-cells of the lateral neuroblasts

88 WHEELER. [Vor.;Vill.

sends out acytoplasmic process which soon ramifies. The mass
of fibres thus formed increases in size very probably by the addi-
tion of further ramifications till the Punktsubstanz is definitely
established as a scarcely stainable body, lying on either side of
the median line in the deepest portion of the lateral cord (Fig.
27, p.s.) In its earliest stages the formation of the substance
is easily followed, but very soon the felted fibres become too
dense for analysis by ordinary methods of investigation. It is
only after a distinct mass is formed in either half of a ganglion
that the longitudinal commissures, or connectives as they are
best called, make their appearance, and unite the hitherto iso-
lated centres in two longitudinal series. Very soon the trans-
verse commissures, or commissures proper, of which X7phidium,
like all other insects, has two in each segment, make their
appearance. The daughter-cells of the median cord neuroblasts
take no part in the formation of the anterior commissure.
Whether they contribute fibres to the posterior commissure or
not, I must for the present leave undecided. I have seen no
evidence in the median cord of a distinct and isolated Punkt-
substanz centre, such as is described and figured by Graber for
some Coleoptera (('90) Pl. V, Fig. 66). I deem it more probable
that in AX7phidium the commissures arise wholly from the
Punktsubstanz masses of the lateral cords. Both commissures
are distinctly seen in cross section in Fig. 29.

The connectives and commissures incompletely divide the
cellular portion of each ganglion into five parts, —two lying
laterad to the connectives and a median series of three smaller
portions separated from one another by the two commissures.
The former may be called lateral gangliomeres, and the three
median portions. respectively the anterior, central, and poste-
rior gangliomere.! Of the median divisions the posterior is
distinctly the largest from the first. This is due to its being
formed in great part by the progeny of the median neuroblast,
whereas the anterior and central gangliomeres consist of a
comparatively small number of cells, contributed by the lateral
neuroblasts.

1 These are equivalent to Graber’s laterale Zellenlager, vorderes, centrales, and
hinteres Medianlager.

No.1.) CONTRIGUTION TO INSECT EMBRYOLOGY. 89

It is not till after the commissures and connectives are
formed that the inter-ganglionic regions become clearly marked
out. Throughout the early stages, in fact till the embryo
reaches the ventral surface of the egg (Stage J), the ganglia
are as long as their respective segments and are separated
from one another only by the intersegmental constrictions.
These have grown very deep in Stage G, especially in the
thoracic and abdominal regions. In the median line, as shown
in sagittal section (Fig. 29, z.g!, z.g?.), they form deep tubular
ingrowths which may be called furcal pits. Since these pits
are median in position they are to be regarded as differentiated
interganglionic portions of the neural furrow. They therefore
belong to the median cord. They are not found between the
mandibular and first maxillary, nor between this and the second
maxillary ganglion, and are also wanting between the eighth
and ninth, and ninth and tenth abdominal ganglia. Evidently
their absence in these cases is due to early fusion to form the
infracesophageal and last abdominal ganglion. In the thoracic
segments the furcal pits are converted into chitinous apode-
matous structures which give attachment to some of the leg-
muscles. It is interesting to note that in the abdomen also
furcal pits are distinctly developed as late as Stage K. Here,
too, they serve for the attachment of a few weak muscle-like
structures, which run from their tips to points in the adjacent
abdominal wall, perhaps corresponding to the insertions of the
rudimental appendages. Later both muscle-like cords and ab-
dominal furcze disappear, —the latter bya very simple process.
It will be remembered that at this time the embryo is growing
in length and continually covering more and more of the yolk.
The tail end is practically fixed at the lower pole of the egg,
while the head slowly moves upwards. The body-wall is thus
stretched in both a longitudinal and lateral direction. Hence
the intersegmental constrictions, so deep in Stage J, must
gradually become shallower, and the furcal pits, which are
nothing but portions of these constrictions, are drawn out from
between the connectives to form part of the sternal integu-
ment. The stretching not only draws out the folds in the em-
bryonic body-wall, but also reduces it to a much thinner layer

90 WHEELER. [Vou. VIII.

of cells. The length of the individual segments is thereby
greatly increased and the nerve cord, which is firmly attached
in the infracesophageal region and more loosely in the terminal
abdominal segments, is compelled to lengthen. The separate
ganglia, besides assuming a somewhat fusiform outline, are
scarcely affected by this traction, whereas the connectives are
drawn out into thin threads denuded of all ganglionic cells and
covered only by the neurilemma.

The presence in the abdomen of temporary furcal pits cor-
responding to the persistent furcee of the thorax admits of an
easy explanation, if we take these structures to be correlated
with the development of ambulatory appendages. The tem-
porary abdominal appendages have usually been regarded as
the rudiments of once functional walking-legs, and they are still
so well preserved in the Orthoptera that it need not surprise
us to find traces of correlated structures which served for the
attachment of some of their muscles.

The progeny of the median neuroblast together with the
interganglionic portion of the neural furrow have been ac-
counted for; the former becoming the posterior gangliomere,
the latter a portion of the sternal integument ; but I have not
yet accounted for the remaining portion of the median cord —
viz. the intraganglionic walls of the neural furrow. This
portion of the groove is crossed by the two commissures and
separates those portions of the lateral cord which will ulti-
mately constitute the anterior and central gangliomeres. Its
cells are of an epithelial nature. Those of the opposite
walls of the furrow become applied to one another by the
swelling of the lateral cords. The lumen is thereby oblit-
erated though its walls are still continuous on the outer
surface of the ganglion with the integumental ectoderm.
The two lips of the furrow finally fuse and the ganglion
together with the portion of the furrow included between
its two halves is liberated from the ectoderm. It is these
epithelial walls thus set free from the integument which
appear to give rise to the outer and inner neurilemmata.
Both these neural envelopes are ectodermal; there are no
traces of mesodermal structures taking any part in their forma-

No.1.] CONTRIBUTION TO INSECT EMBRVOLOGY. gI

tion and it seems to me that they can have only two possible
sources —they either arise from some of the progeny of the
neuroblasts or from the intraganglionic portion of the median
cord. I deem it highly improbable that they should arise from
the former source, since the daughter-cells of the neuroblasts
have every appearance of being early specialized as ganglion
cells. Furthermore, the cells of the neurilemmata when they
definitely appear, closely resemble the cells of the neural fur-
row both in size and in the great avidity with which they
take the stain. The outer neurilemma covers first the inner
surface of the ganglion—then the outer or neuroblastic sur-
face;—the thin cellular membrane apparently progressing
ing laterad in either case and meeting near the origin of the
nerve-trunks. The inner neurilemma, which envelops the
Punktsubstanz is completed before the outer envelope. Histo-
logically both envelopes resemble each other in every respect.

The fusions of ganglia in the nerve-cord take place gradually
and may be easily followed in A7phidium. Several stages of
these fusions are represented in Fig. VII, A-D. In Fig. A,
the nerve-cord is shown much as it appears in Stage F. The
ganglia form an unbroken series from mouth to anus. The
connectives are very short and not as yet distinguishable from
the surface. Fig. B, is taken from an embryo just turning the
lower pole (Stage H). Here the mandibular and two maxillary
ganglia, and also the three terminal abdominal ganglia still
remain as in the preceding stage, while the other ganglia are
being drawn apart by the stretching of the embryo, so as to
show their short connectives. In Fig. C, the subaesophageal
and last abdominal ganglia are established as two fused masses.
The number of ganglia comprising each of these masses may
still be easily determined by counting the commissures. It
will be noticed that in this stage the first abdominal is closely
approximated to the metathoracic ganglion and that the second
and third abdominal also lie close together. Between the
other ganglia the connectives have lengthened. In the later
stage represented in Fig. D, the connectives are still longer;
the first abdominal ganglion has fused with the metathoracic,
and the second and third abdominal form a single mass.

92 WHEELER. [Morey iia:

These fusions become more intimate as the time for hatching
approaches, so that the ventral cord finally consists of only
ten ganglia instead of sixteen, the original number.

Fic. VII.

A-D. Diagrams of four consecutive stages in the development of the brain
and nerve-chain of the X7fhidium embryo. I, cephalic; II, thoracic; III,
abdominal region; s¢., stomodeum; az., anus; ¢, optic plate; fc*(o.g), first
protocerebral lobe, or optic ganglion; Ac, fc3, sccond and third protocerebral
lobes ; dc, deutocerebrum ; éc., tritocerebrum ; 1-16, the sixteen postoral ganglia ;
po.c.. postoral commissure ; /f., furcal pit; ac., anterior; fc., posterior ganglionic
commissure ; ag., anterior ; #g., posterior ; ¢g., central; /¢., lateral gangliomeres.

The description of the ventral nerve-cord of X7zphzdium here
given applies equally well to the other Orthoptera which I have
studied (Blatta germanica, Melanoplus femur-rubrum). The

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 93

points in which the Blattid and Acridian nerve-cord differ from
that of the Locustid are so insignificant that I need not burden
the reader with their enumeration. I will stop to mention only
two peculiarities in latta. Here I fail to detect the pale
spots in the “slipper” stage of the germ-band, and sections
show that the neuroblasts do not differentiate as early as
they do in A7phidium. They are, however, readily detected
in late stages, when they stand out with even greater distinct-
ness than in the Locustid. The median cord neuroblasts,
though present and occupying positions corresponding to their
homologues in A7phzdium, are more difficult to trace, probably
on account of the smaller size of the embryo.

Neuroblasts, or cells of a similar character have been de-
scribed and figured by a number of investigators of Arthropod
development. Perhaps the earliest mention of these cells is
to be found in Reichenbach’s beautiful As¢acws monograph
(86), where the nerve-cord is described as consisting in an
early stage of two kinds of cells—a few large pale elements
arranged in a single layer and confined to the periphery, and
a much greater number of small and more deeply stainable
cells forming the bulk of the ganglia. The developing ganglia
of the cray-fish resemble the ganglia of the Orthoptera in many
particulars. The number of large cells in the lateral cords in
Reichenbach’s figures (notably his figures 114-133) is 3-6, the
average being 4 or 5, the same as in AXzphzdinm, Blatta, etc.
Furthermore the ganglia of Astacus show a foliated arrange-
ment of the smaller cells, which is not unlike the condition
seen in the older ganglia of the Orthoptera. Some of the
figures (188 and 189 for example) show a single neuroblast-
like cell surmounting the median cord cell-mass. There are,
however, two points in Reichenbach’s work, which throw some
doubt on the homology of his large cells with the neuroblasts
of the Orthoptera. First, Reichenbach neither figures nor
describes these cells as dividing to form ganglion cells. This
negative observation, however, loses much of its force when
we consider that caryokinetic figures are singularly absent
from all of Reichenbach’s figures, excepting his surface views
of young embryos, and when we recall the fact that amitotic

94 Wg SPS Pa BI Oye [Vo.. VITI.

division is a very general phenomenon in the Crustacea (ac-
cording to Carnoy, '85). A more serious objection to the
homology under consideration is Reichenbach’s statement that
the large cells ‘gehen schliesslich in die von Leydig, Dietl,
Krieger und anderen beschriebenen, grossen Ganglienzellen
iiber.’ This, too, is an objection only if the neuroblasts really
degenerate, a point on which I am still doubtful.

Nusbaum found huge succulent cells in the young nerve
cord of the embryo J/yszs (87). He compares them with the
large cells of Reichenbach and believes that they have a similar
fate. Similar cells were also observed in the brain of Onzscus
murarius “pendant les stades relativement jeunes.’ On one
point only does he add to Reichenbach’s observations: he
depicts (Fig. 78) a caryokinetic figure, which from its size
and position must be referred to one of the large cells. Its
spindle axis is directed at right angles to the surface of the
body. This observation, small and incidental as it is, would
tend to show that the large cells proliferate as in the Orthop-
tera. I am inclined to think that a renewed study of the
Crustacean nerve cord will show that the ganglion cells are
budded forth from the large cells and that these are equivalent
to the insect neuroblasts.

Korotneff (85) was the first to find gangliogenic cells in
the Insecta. At p. 589 in his description of the Gryllotalpa
embryo he says: ‘“Einige der Ektodermzellen, welche die
Nervenauftreibung bedecken, fangen an zu wachsen, ihre
Kerne vergréssern sich bedeutend und zeigen dabei eine karyo-
kinetische Figur. Grésstentheils sind diese Zellen (Ganglien)
so angeordnet, dass einer einfachen platten Ektodermzelle
eine wachsende Neuroektodermzelle folgt. Hat sie eine be-
stimmte Grosse erreicht, so sinkt jede wachsende Zelle in die
Tiefe des Ektoderms und wird von den benachbarten, unver-
anderten Zellen bedeckt. Jede Ganglienzelle theilt sich dabei,
eine ganze Folge von neuentstandenen Zellen bildend, nur an
der Fig. 60 ist leicht zu unterscheiden, welche Gruppe von
Zellen der oben gelegenen Ganglienzelle entspricht. Durch
eine solche Vermehrung von Zellen wird der Nervenstrang
mehr und mehr in die Hohe getrieben.”’ The Fig. 60 referred

Nort.) CONTRIBUTION TO. INSECT EMBRVOLOGY. 95

to in this description is taken from a stage corresponding to
my Fig. 28. Both in this figure and in Fig. 61 he represents
four neuroblasts in one of the lateral cords. Korotneff seems
not to have seen the early stages of proliferation.

In the developing nerve-cord of Doryphora I observed (89, p.
366) that “the outer layer of cells continuous with the hypo-
dermis stands off somewhat from the ganglionic thickenings,
leaving a space which is in early stages occupied by several
large, clear, oval cells which divide rapidly by caryokinesis, and
might be called ganglioblasts, as the products of their divisions
reinforce the mass of ganglion cells.” In my figures the polar-
axes of the neuroblast spindles he parallel to the surface of the
ganglia. Re-examination of my preparations has convinced
me that this observation is essentially correct. I find also that
the newly-formed daughter-cells of the neuroblasts occasionally
divide caryokinetically and thus give rise to further generations
of daughter-cells. The daughter-cells are not budded forth in
regular rows, but very irregularly. I am not sure that I can
distinguish the median cord neuroblasts in Doryphora, though
I believe that I have detected homologous structures. In my
figure 72 I represented circular intersegmental patches in the
median line between the lateral cords. Closer examination
shows these to be clusters of cells of the same appearance and
dimensions as the lateral-cord neuroblasts. They are very
clearly brought out by Graber in his figures of Hydrophilus
(89, Figs. 40, 41, and 43, Pl. III) and are described as taking
part in the formation of the posterior gangliomere of each
ganglion. JI doubt whether the large cells constituting the
posterior gangliomere of Periplaneta in Miall and Denny’s
Fig. 43 (86) to which Graber refers, are to be regarded as the
equivalents of the median-cord clusters in Doryphora and
Hydrophilus. Periplaneta very probably has in each segment
only one median-cord neuroblast, which atrophies before the
close of embryonic life, and the large cells in Miall and
Denny’s figure probably arise from the daughter-cells and are
therefore merely large ganglionic elements.

Graber figures and describes (89, p. 47, Pl. X, Fig. 130) a
cross-section through an abdominal ganglion of a Melolontha

96 WHEELER. [Von Vif.

embryo in which he finds a neuroblast in either lateral cord
and three symmetrically arranged cells of the same character
in the median cord. Similiar median cells were seen in Luczlza.
He refers to Korotneff’s observations on Gryllotalpa and states
that he has found the lateral “ ganglionare Grosszellen”’ in Lzna.
He is inclined to regard them as a widely occurring differen-
tiation of the ectoderm.

In a subsequent paper (90) Graber describes and figures a
foliated condition of the ganglia in the nerve-cord of Steno-
bothrus. In Fig. 52 the neuroblasts are distinctly seen, and in
one lateral cord five, in the other four pillars of cells may be
distinguished. So far as the neuroblasts are concerned, he
cannot be said to have added anything to Korotneff’s account.

Nusbaum (91), in a recent Polish paper on the development
of AMZeloé, figures neuroblasts in the lateral cords. They are
frequently represented in mitosis—the spindle-axes being in
some cases perpendicular to the surface of the ganglia (Figs.
94, 95) while in others (Fig. 107) they are parallel to the
surface, as in Doryphora. Such portions of the text as were
translated for me contained nothing new on these structures.

Viallanes, in two recent papers (90%, ’90) on the structure
of the nervous system in the embryo Mantis, comes to con-
clusions agreeing with my own, which were arrived at inde- |
pendently. His observations on the neuroblasts may be
briefly summarized in his own words (90°, p. 293): “A Tori-
gine le bourrelet, primitif n’est qu’un simple €paississement
de l’exoderme, c’est-a-dire une région de ce feuillet dont
les cellules sont devenues columnaires et ont augmenté de
volume. Bientdt ces cellules se multiplient et se divisent en
deux couches, l'une superficielle (dermatogene), autre profonde
gangliogéne). A une période plus ou moins tardive, suivant la
région considérée, la couche des cellules dermatogenes se
sépare de la couche des cellules gangliogénes et devient I’hypo-
derme. Les cellules gangliogénes en se multipliant donnent
naissance aux cellules ganglionnaires.”

Viallanes’ figures do not show a regular arrangement of the
cells budded forth from the neuroblasts, and he has not de-
scribed the neuroblasts of the median cord, probably because

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 97

his attention was concentrated on the structure of the brain.
He has observed the degeneration of the gangliogenic cells, or
neuroblasts. Ina late stage (90°, p. 301), he says “Ils montrent
des signes évidents de décrépitude; beaucoup des cellules gang-
liogénes ont deja disparu, les autres sont en voie d’atrophie.”

Our knowledge of the median cord cannot be said to have
made much advance since this structure was first described by
Hatschek (77). While all writers agree that it originally ex-
tends as an uninterrupted structure from the mouth to the
anus, there is wide difference of opinion respecting the ultimate
fate of its inter- and intraganglionic portions. Hatschek (77),
Tichomiroff (82), and Korotneff (85) maintain that the inter-
ganglionic portions remain attached to the integument when the
nerve-cord is liberated and that they ultimately disappear.
Ayers ('84) on the other hand holds that the whole median
cord is liberated from the ectoderm, but does not affirm that
the interganglionic portions form a constituent part of the
ganglia.

Graber (90) has very recently come to a conclusion which
differs from the views hitherto advanced. With Ayers he
holds that the interganglionic portions of the median cord are
delaminated from the ectoderm along with the intraganglionic
portions, but he goes further and claims (p. 103) that
“das Zellenmaterial des interganglionalen Mittelstranges,
zum Theil wenigstens mit den Ganglien vereinigt wird, oder
mit anderen Worten, dass eine Vergrosserung des ganglionalen
Mitteltheiles auf Kosten des interganglionalen erfolgt.”

As will be inferred from the above descriptive paragraphs,
I hold to Hatschek’s view that the interganglionic portions
of the median cord take no part in the formation of the
ganglia but are drawn out from between the connections and
constitute a portion of the sternal integument. Graber’s re-
searches on this portion of the nerve cord are limited to the
Coleoptera and as the insects of this order certainly differ to
some extent from the Orthoptera in the formation of the
nervous system, I have no grounds for doubting the correctness
of his observations. I believe, however, that Ayers’ account
of the median cord in @canthus is open to criticism. After

98 WHEELER. [Vox VIII.

clearly implying that the median cord is set free from the
ectoderm along its whole extent he remarks (p. 252): ‘“ Be-
tween the successive pairs of ganglia the median ingrowth
atrophies, and at the time of the closure of the dorsal wall of
the body there is seen between the connecting cords of two
adjacent pairs of ganglia, a small triangular or cylindrical mass
of cells, concerning the fate of which I am not absolutely
certain. I believe, however, that they go to form a part of
the internal skeleton. The chitinous rods in the thoracic
region to which the muscles of the legs and wings are attached
probably arise from the remnants of the median invagination,
but in the abdominal region they may disappear entirely with-
out giving rise to such structures.” If I understand this
passage correctly, Ayers implies that the chitinous rods are
originally interganglionic portions of the median cord. But if
this is the case, how can the median cord separate completely
from the ectoderm unless we are to suppose that there is a
reunion of the interganglionic portions with the integument
to form the endoskeletal structures? The chitinous rods are
directly continuous with the chitin of the integument so that
until observations are forthcoming to show that portions of the
integument can loosen and pass into the body-cavity and subse-
quently reunite with the integument, I must regard Ayers’
account as inadequate.

I am still in some doubt as to the exact origin of the commis-
sures. Grassi ('84), Ayers (84), Heider (9), and Graber ('90),
all maintain that the commissural fibres arise from the median
cord cells. A priori, there are no reasons why the daughter-
cells of the median neuroblast should not send out processes to
form Punktsubstanz and thus form a commissure. From the
position of these cells, however, I regard it as highly improbable
that anything but the posterior commissure could be formed in
this way. The isolated Punktsubstanz masses in the Coleopte-
ran median cord in Graber’s and Heider’s figures may arise
from cells equivalent to the daughter-cells of the median neuro-
blasts of the Orthoptera. It is very improbable that the der-
matoblastic cells which form the walls of the median cord in
the region of the anterior commissure, and which I regard as

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 99

non-hervous, should take part in forming the fibres of that
structure.

Regarding the origin of the neurilemmata in insects, there is
still considerable doubt. The inadequacy and inconsistency of
Nusbaum’s observations on lata ('83) have been sufficiently
pointed out by Eisig (87) and Korotneff (85). Nusbaum derived
the median cord (which, by the way, he did not recognize as the
median cord) from the entoderm, and compared it with the
vertebrate chorda. So far his observations and conclusions
were erroneous, but he derived the inner and outer neurilemma
from the cells of this ‘“chorda’—an observation which agrees
essentially with my own.

Korotneff’s view that the neurilemmata arise from migrant
mesoderm cells has not been confirmed by recent writers,
who are inclined to derive these envelopes from the ectoderm
(Heider, ’89 ; Graber, '90). Though I venture to say that my
own observations are somewhat more definite than those hith-
erto published, I cannot regard them as in any way final.

2. Thé Brain.

In the following account of the X7zpAzdium brain, I shall use
the nomenclature employed by Viallanes in his recent papers
(90%, 90°), since his studies on the brain-development of
Mantis religiosa agree very closely with my own. Before
passing to a description of my sections I would refer the
reader to the diagrammatic figure (VII) which represents the
main points in the structure of the embryonic brain. Here it
is seen that the ventral nerve-cord bifurcates just in front of
the mandibular segment and passes on either side of the
mouth, where it forms two successive pairs of ganglia. The
posterior of these (¢c.) is the ¢rztocerebrum, or third brain seg-
ment. Its two halves are united by the zxfrawsophageal com-
missure, shown in the figure as a broad white band connecting
the Punktsubstanz-masses of the ganglia. The anterior pair
of swellings (dc.) constitute the deutocerebrum, or second brain
segment. From this portion the antennz are innervated.
Further forward the deutocerebrum passes into a large paired

cele) WHEELER. [Vot. VIII.

supracesophageal inass, the protocerebrum, or first brain seg-
ment, which constitutes the greater portion of the brain.
Each of its halves may be separated into three lobes; the
first, or outermost lobe (fc! [0.g.]) forms the optic ganglion of
the larva and imago, while the second and third lobes (fc?,
pc 3) ultimately form the bulk of the brain proper. The third
lobe is united with the contralateral lobe by the broad szpra-
wsophageal commissure. Such is the structure of the
Orthopteran brain reduced to its simplest terms. It may now
be considered a little more in detail.

Like the nerve-cord, of which it is simply a modified portion,
the brain arises from neuroblastic cells. These first make
their appearance in clusters (the spots seen on the procephalic
lobes in Fig. 2). Later they form a single layer of proliferat-
ing centres continuous with and in every way comparable to the
neuroblasts of the ventral nerve-cord. Like the latter they are
covered externally by a layer of dermatoblastic cells.

That the deuto- and tritocerebral ganglia are strictly
homodynamous with the ganglia of the nerve-cord is clearly
shown in AXzphidium. In the first place these brain segments
are directly continuous with the segments of the cord; second,
they have at first the same size and shape as the latter, and
third, they present on the average four neuroblasts in cross-
section on either side. The suppression of the median cord in
the deutocerebrum (if it be not drawn forward into the proto-
cerebrum), is perhaps sufficiently explained by the presence of
the stomodzal invagination. A partial suppression of the
median cord in the tritocerebrum may be due to the same
cause. The infracesophageal commissure is perhaps the
morphological equivalent of both the commissures of a ventral
ganglion.

The early clustered condition of the neuroblasts is seen in
Figs. 32-34 at 2b. At the edges of these cross-sections a
rounded mass of pale cells (fc! [0.g.]) is distinctly marked off
from a more deeply stainable layer which encloses it on nearly
all sides. This mass, the future optic ganglion (first proto-
cerebral lobe), is delaminated from the ectoderm at a very
early stage. The cells of the mass agree with the neuroblasts

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. IOI

in their slight affinity for stains; they differ in the more
elongate shape of their nuclei and cytoplasm. The dark layer
enclosing the optic ganglion on all except its innermost face is
the optic plate and will give rise to the compound eye. Passing
towards the median line in these sections (especially in Fig. 33)
two other thickenings may be distinguished (fc?, fc3) — the
second and third protocerebral lobes.

The cross-section, Fig. 36, runs through the labrum of an
older embryo (Stage F) and shows a considerable advance in
the structure of the brain. The three protocerebral lobes are
distinctly marked out. In the second and third, the neuro-
blasts have arranged themselves in a row and have budded
forth strings of ganglion cells. In the first lobe (fc! [o.g.]) no
teloblastic arrangement is ever present. The cells are small
and narrow and early assume a radial arrangement around the
Punktsubstanz core at the base of the mass. The cells of the
optic plate, which stand away from the surface of the ganglion,
already show a tendency to differentiate in that they have
become smaller and narrower than the dermatoblasts covering
the two other lobes of the protocerebrum. At the juncture of
the second with the third lobe, several large dermatoblastic
cells are intercalated (ég/.). They are continuous with the
dermatoblasts covering the second protocerebral lobe. This
intercalated mass is called by Viallanes the dourrelet ectoderm-
igue intraganglionnaire. I shall call it the intraganglionic
thickening.

A still more advanced stage in the development of the brain
is seen in Fig. 37 (Stage G). This section passes above the
base of the labrum. Owing to the active proliferation of the
neuroblasts, the mass of the protocerebrum is greatly aug-
mented. The Punktsubstanz has made its appearance as a
confluent mass.!_ The optic plate is much thickened and its
small cells are about to arrange themselves to form the
ommatidia. The intraganglionic thickening (zg/.) presents
an interesting appearance. The edge of the optic plate is
united with the inner edge of the optic ganglia, but between it

1] have seen nothing to corroborate Cholodkowsky’s view ('91) that there are
at first three distinct and separate pairs of Punktsubstanz masses in the brain.

102 WHEELER. [Vot. VIII.

and the second protocerebral lobe there is a fissure (w) which
extends in some distance. . Examination of a number of succes-
sive sections and stages has convinced me that this fissure is
not the result of artificial rupture during sectioning but that it
is brought about by an infolding of the intraganglionic thicken-
ing. The shape and position of the involuted mass may be
clearly seen from the surface in Stages H and I (see Figs. 8
and 9, zg/.).

In the frontal section (Fig. 39) are shown the relations of
the protocerebrum to the outer brain-segments and to the
ventral cord. Only a small portion of the optic plate (¢) is
cut. Beneath it lies the optic ganglion (fc! [o.g.]), the small
cells of which contrast with the large cells of the brain proper.
The second protocerebral lobe (fc?) still contains many neuro-
blasts which are budding forth their last progeny. The older
daughter-cells have already assumed an irregular arrangement.
The brain is separated from the attenuated dermatoblastic, or
integumental layer (dd.) beneath which the outer neurilemma
(enl.) is forming. The inner neurilemma (?/.) envelops the
Punktsubstanz portions of the brain. The broad supracesoph-
ageal commissure (s.c7z) connects the third protocerebral lobes
of the two sides. As shown in the figure, the deutocerebrum
is distinctly praoral. At az is shown the point where the
antennary nerve leaves the fibrous portion of this brain seg-
ment. Caudad to the deutocerebrum lies the tritocerebrum,
a pair of somewhat smaller ganglia united by the infracesoph-
ageal commissure. It is this segment which according to
Viallanes innervates the labrum and the frontal ganglion. Be-
sides the supra- and infracesophageal commissures and the
connectives which arise in the third protocerebral lobes and
traverse the deuto- and tritocerebrum to pass into the man-
dibular ganglion and thence through the nerve-cord, I may call
attention to two other masses of Punktsubstanz which lie in
front of the supracesophageal commissure. These are shown
at g. in the figure. They appear to be connected by a small
band running parallel to the robust supracesophageal commis-
sure. I did not succeed in finding these connected Punktsub-
stanz masses in all embryos of this stage, and as they were not

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 103

seen in later stages, their morphological value as indicating the
presence of another segment in front of the protocerebrum 1s
somewhat doubtful.

The three divisions of the protocerebrum may still be rec-
ognized in the transverse section (Fig. 40) of an embryo which
has passed beyond Stage J. In the median line at mc lies a
rounded and compact mass of cells which I regard as the pre-
oral representative of the median-cord cell-masses of the ventral
cord. A large cell, which I take to be a neuroblast, lies at the
outer periphery of this median cell-mass at a younger stage.
Structures at a corresponding position in the brains of other
insects (Hydrophilus, Musca) and likewise comparable to the
median-cord have been described by Heider (89) and Graber
(89, p. 49). It is not improbable that the brain neurilemmata
may have their origin near this median cerebral mass, just as
the neurilemmata of the ventral cord probably arise from the
non-ganglionic portions of the neural furrow.

Two other interesting structures are shown in Fig. 40: the
intraganglionic thickening (zg/.), now completely separated from
the integument and lying as a deeply stainable mass wedged in
between the optic ganglion and the second protocerebral lobe,
and a peculiar pale thickening at the edge of the optic plate (72).

In later stages it is very difficult to locate the intraganglionic
mass, so that I am unable to decide whether it atrophies or
persists in a modified form as a portion of the brain. The re-
searches of Viallanes on Mantis would seem to lend great
probability to the former alternative.

The thickening at the lateral edge of the optic plate is con-
stant in Stages G to J and somewhat later. It soon entirely
disappears, without taking any part in the formation of the eye,
so far as I have been able to observe. Does it represent an
abortive ocellus ?

The fully established optic nerve is shown in Fig. 41 (a. 7.).
In a much earlier stage it may be found as a delicate band of
cells connecting the posterior edge of the optic ganglion with
the optic plate (Fig. 38, 0..). I agree with Viallanes, that it
seems to arise from the ganglion and to grow outwards into the
ommatidial layer; for there is from the first a sharply defined

104 WHEELER. [Vor. VIII.

intercepting membrane between the nerve and the plate, whereas
the nerve passes without interruption into the ganglion. But I
am led to lay little stress on these appearances by the researches
of Watase (90) and Parker (90) on the adult ommatidia of a
number of Arthropods. They have shown in a very convinc-
ing manner that each retinula-cell is the termination of an
optic nerve fibre. The retinulz are undoubtedly modified optic
plate cells; and judging from recent observations on _per-
cipient cells in other forms (vertebrate eye, ear, olfactory
nerves, v. Lenhossék’s observations on Lumébricus (92), etc.),
we must suppose that the nerve fibres grow out from the
retinula-cells, pass through the optic nerve and enter the
ganglion. Such prolongations from all the retinule would be
amply sufficient to form the optic nerve, although it is probable
that some of its fibres are centrifugal prolongations from optic
ganglion cells. It has been suggested that the optic nerve
may be established at a very early stage, when the optic plate
and optic ganglion are still closely applied to each other (vde
Figs. 32-34), and that the nerve may not become visible till
the two Anlagen separate with further development. But I do
not think this is the case in A7zphzdium. Sections like Fig.
36 show a distinct separation of the optic plate and ganglion
in the region of the future optic nerve; and Viallanes has made
an exactly similar observation on Mantis.

The sympathetic nervous system arises in part at least from
the dorsal median wall of the cesophagus. At three separate
points (Fig. 61) the ectoderm becomes thickened and its outer
cells enlarge and assume the character of ganglion cells. The
most anterior of these thickenings (f g.) is the frontal gang-
lion. It arises just behind the base of the labrum. The two
other thickenings which are placed further back (vg, xg?) are
the second and third visceral ganglia. I have not followed the
development of the nerves which unite these ganglia and
ramify from them.

Concerning the origin of the peripheral nervous system I
have no positive data. Ina few cases I have seen appearances
which led me to believe that they arise as outgrowths from
their respective ganglia.

No. t.] (GONTRIBUTION “(TO INSECT EMBRYOLOGY. 105

The development of the brain of Blatta germanica and
Melanoplus femur-rubrum agrees in all essential respects with
the development of the X7phzdium brain. Certain Hemiptera,
e.g. Ranatra fusca, conform very closely to the type of brain
structure seen in these Orthoptera. I may mention in this
connection that the brain of Axurida maritima shows the
typical division into proto-, deuto- and tritocerebral segments
with great distinctness. The last segment especially is re-
markably distinct.

Until very recently the detailed study of the embryonic
Hexapod brain has been limited to the Coleoptera and the
results obtained have been naturally enough extended to
include not only other insects but other Arthropods as well.
The Coleoptera, however, are far from being primitive forms
and the rdle which they play in contemporary embryological
literature is largely attributable to the unusual technical ad-
vantages presented by their eggs. As far as development is
concerned, the simpler brain of the Orthoptera and Ametabola
in general offers many points of resemblance to the Crustacea
and Myriopoda,! whereas the brain of the Metabola, like so
many other points in their organization bears witness to a
considerable amount of modification. It is therefore more
consistent with our general views of phylogeny to reduce the
Coleopteran brain to the Orthopteran type than to proceed
vice versa.

We owe the most important contributions to the subject of
Orthopteran brain development to Viallanes. After a decade
of study devoted to the histological structure of the adult
Arthropod brain he has selected Mantis as a subject for
embryological investigation. His previous careful study of
the adult brain of other Orthoptera (dipoda coerulescens
and Caloptenus ttalicus, '87”) has enabled him to avoid the con-
fusion with which the inexperienced investigator is overwhelmed
when attempting to follow the rapidly increasing complication
of neural structures. With his usual skill and patience he has
traced the development not only of the main structural features
but of many details, so that we have a well-established point

1 See the papers of St. Remy ('90) and Viallanes ('879, ’87°).

106 WHEELER. (Vor. VIII.

of departure for further comparative studies. So far as my
own observations are concerned I am able to corroborate
Viallanes’ results on nearly all important points. I must state,
however, that I have not followed the development into such
detail. ‘

In the light of these researches a reconsideration of the
Coleopteran brain must be undertaken. Patten’s description
of the Acz/ius brain (88) and my description of the brain of
Doryphora (89) need revision and alteration. We described
the organ (see my Fig. 72, Pl. XIX) as consisting of three
segments, each of which was subdivided into a brain portion,
continuous with the ganglia of the ventral cord, an optic
ganglion portion and an optic plate portion. Between the
third brain and the mandibular segment, a segment was found
which I designated as intercalary. This segment is also
clearly seen in some of Patten’s figures (Figs. 2 and 27 s4 Pl.
VII). Thus according to our account there were four preman-
dibular segments or seven segments in the entire head. Our
results were obtained almost exclusively from surface views —
by itself a defective method. But greater error was incurred,
it seems to me, in ascribing segmental values to the various
prominences of the optic ganglion and optic plate.

In order to bring our observations into harmony with the
results obtained from a study of the Orthopteran brain, our
figures must be interpreted in a very different way from that
in which we chose to interpret them. Our first brain-segment
is probably no segment at all, but merely a slight elevation
often seen near the median line at the extreme anterior end of
the germ-band. Our second, third and intercalary segments
are equivalent to Viallanes’ third protocerebral lobe, deuto-
cerebrum and tritocerebrum. The three divisions of the optic
ganglion are not parts of three segments, but the whole struc-
ture belongs to the protocerebrum, of which it forms the
first lobe. In the same way we cannot regard the optic
plate as trisegmental since it has no connection with the
deuto- and tritocerebrum but only with the optic ganglion. It
follows that the ocelli of Coleoptera are not originally formed
on different segments as Patten would have us believe, but

No. 1.] CONTRIBUTION TO INSECT EMBRVOLOGY. 107

belong to one segment—the protocerebrum. That such is
the case I am convinced from a study of the eyes in embryos
of Dytiscus verticalis, a form closely related to Actlius.

As will be seen in the profile view Fig. 8, the optic ganglion
and optic plate of X7zphzdinm are at first folded back so as to
lie along side the deuto- and tritocerebrum. The antennal
furrow runs forward, separating the optic ganglion from the
brain but stops when it reaches the protocerebrum. The
value of this furrow as completely separating the second and
third brain-segments from the optic ganglion was overlooked
by Patten and myself: hence our false interpretation of the
structures lying laterad to it.

I believe that I am justified in putting this new interpreta-
tion on the Coleopteran brain, because it harmonizes with
Heider’s careful study of Hydrophilus ('89). He has failed to find
indications of segmental constrictions in the optic plate and
optic ganglion and his figure 4 A. B. at p. 37 agrees closely
with Viallanes’ description of the A/an¢zs brain. It should be
observed that the embryos of Aydrophilus are much larger
than those of Acz/zws and Doryphora and therefore much more
favorable for surface study. On the other hand it may be
urged that Heider evidently did not employ so good a method
of surface preparation as Patten.

The distinct invagination associated with the formation of
the optic ganglion in Coleoptera and described by Patten,
Heider and myself, is probably homologous, as Viallanes sug-
gests, with the intraganglionic thickening of the Orthop-
teran embryo. This structure in J/aztis, and probably also
in AXzphidium, takes no part in the formation of the optic
ganglion, which arises—at least in great part — by delamina-
tion as in the Crustacea (see Parker ('90)). Only the outer or
lateral portion of the optic plate becomes the compound eye,
so that in a later stage the intraganglionic thickening is separ-
ated from the edge of the eye by a considerable space. The
thickening then lies just laterad to the antennal furrow as
shown in Fig. 8. Whether or not the invagination in the
Coleoptera really plays any part in forming ganglionic tissue
as has been claimed, must be decided by renewed investi-
gations.

108 WHEELER. [VoL. VIII.

Concerning the researches of Cholodkowsky on the brain of
Blatta germanica, 1 must say a few words, since I have de-
scribed the brain and nerve-cord of this form as agreeing in all
essential respects with those of X7zphzdzum. Cholodkowsky lays
great stress on the existence of three distinct pairs of Punkt-
substanz masses in the supracesophageal ganglion as indicat-
ing the presence of three segments. When we come to exam-
ine his figures we find that he takes a very unusual view of
brain-segmentation, for the three pairs of Punktsubstanz masses
areseen) to ‘belong (Fig. 46 Pl IV3 Mig. o7 Bev to “the
protocerebrum and correspond to the centres of its three lobes.
He did not distinguish the deuto- and tritocerebral segments !
Such of his remarks on the development of the brain and
ventral nerve-cord as are at all comprehensible show similar
glaring defects in observation. Thus he has failed to detect
the small dermatoblastic cells which from the first cover the
brain and nerve-cord. He asserts that these organs are at first
naked and are only subsequently covered by an overgrowth of
the integument from the sides of the body. The antennal and
neural furrows do not play the part in development that he
ascribes to them. The last abdominal ganglion of the mature
embryo does not consist of four but of three fused ganglia;
the fusion of the second and third abdominal ganglia was com-
pletely overlooked.

3. General Remarks on the Nervous System.

The nervous system of Arthropods is by common consent
derived from the nervous system of annelid-like forms, and it
is to this group that we naturally turn in seeking an explanation
for certain structures in the Hexapod brain and nerve-cord.

In a brief preliminary paper Patten (88) made the statement,
that “the ventral cord and brain of Arthropods is at first
composed entirely of minute sense-organs, which in scorpions
have the same structure as the segmental ones at the base of
the legs.”” This would seem to indicate that the Arthropod
nervous system can be traced back to the condition seen in
Polychzeta — Lopadorhynchus, according to Kleinenberg ('86) —
where both brain and nerve-cord arise in connection with and

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 109

ultimately supplant certain larval sense-organs. So far, how-
ever, as the Hexapoda are concerned, Patten’s statement is, to
say the least, inapposite, since, as I have pointed out, both
brain and nerve-cord arise from peculiar ectodermal cells —
the neuroblasts — which under no circumstances can be re-
garded as primitively sensory. They are simply generalized
cells, like the teloblasts of worms and the meristem of plants.

The development of the nerve-cord in the Hirudinea and
Oligochzeta agrees more closely with the conditions seen in in-
sects. As Whitman has shown for Clepsine (87), and E. B. Wil-
son for Lumbricus (89), the nerve-cord is proliferated forward
from a pair of neuroteloblasts situated at the posterior end of
the germ-band. Hence, in these worms, the whole of the nerve-
cord is condensed, as it were, into two huge mother-cells,
whereas in the Insecta it is condensed into a single layer of
huge cells. There are reasons, however, for believing that
this layer is, in part at least, derived from a few large
cells situated just in front of the anus, and therefore corre-
sponding to the Annelid neuroteloblasts.'. That there are only
two rows of these cells in Annelids, while there are eight in
insects, can form no very serious objection to their homology,
as I pointed out in my preliminary note (90°).

Certain conditions in the Crustacea also lend probability to
the view that the Hexapod neuroblasts may be budded forth
from a prze-anal row of teloblasts.2_ Patten ('90) has pointed out
in Cymothoa a row of proliferating cells which form ectoderm,
and Nusbaum (91) has described a very similar condition in
Ligia. I have observed the same phenomenon in Porcellio,
and believe it to be of general occurrence throughout the
Isopoda. The cells are budded forth so as to form reguiar
transverse and longitudinal rows. Reichenbach describes and

1 What I have called the neuroblasts in insects therefore correspond to the
“neural cell-rows” in Annelids (the cells zg. c. in E. B. Wilson’s Fig. 59, Pl. XIX,
and the cells zc. in Whitman’s Figs. 9 and 11, Pl. V).

2 The neuroblasts of the last row (¢é., Fig. 56, Pl. VI) in the nerve-cord of
Xiphidium are always distinctly larger and clearer than the neuroblasts of the
remainder of the cord. They may be true neuroteloblasts and give rise to the
neuroblasts, but as I have never found unmistakable caryokinetic figures in
them, I am still in doubt as to their homology with the neuroteloblasts of worms.

L1© WHEELER. [Vou. VIII.

figures a very similar budding-zone of ectoderm cells in the
Decapod Astacus (86). That a portion of the ectoderm cells
thus budded forth in these forms goes to form the nervous
system, admits, it seems to me, of very little doubt. The
clustered condition of the neuroblasts in the young germ-band
of Xzphidium may be due to the rapidity with which the
germ-band grows in length and breadth; the original regular
arrangement of the cells being thereby disturbed and not
re-established till a somewhat later stage.

Although I have maintained a phylogenetic connection of
the insect neuroblasts with the neural cell-rows of Annelids, I
admit that they may be regarded from an entirely different
standpoint, vzz. as having arisen independently in insects by
a process of precocious segregation ; but it should be noted
in this connection that it is just the oldest Pterygota, the
Orthoptera, which show this segregation most clearly, while in
the more recent forms (Coleoptera, e¢c.) the process is more
obscure.

A comparison of the histogenesis of the insectean with the
histogenesis of the vertebrate nervous system brings out some
interesting analogies. The neuroblasts may be compared with
His’ Keimzellen which divide by caryokinesis to form his neuro-
‘blasts. These are directly converted into ganglion-cells by
sending out axis-cylinder processes. They correspond, there-
fore, to the daughter-cells of the insect neuroblasts, which
are likewise converted into ganglion-cells. The “ Keimzellen”
of vertebrates differentiate close to the central canal, which is,
of course, morphologically the outer surface of the cord, just as
the Arthropod neuroblasts lie at the surface of the cord. In
both groups the daughter-cells appear to be budded off in the
same direction morphologically; though in vertebrates the gan-
glion-cells migrate, while in insects they are pushed inwards by
their newly proliferated sister-cells. The early separation of
the neural ectoderm in vertebrates into Keimzellen and susten-
tacular tissue (spongioblasts of His) is paralleled in insects by
the precocious splitting of the same germ-layer into neuro-
blasts and dermatoblasts, the latter giving rise to supporting
tissue in the form of neurilemmata.

No.1.) CONTRIBUTION TO INSECT EMBRYOLOGY. BBs

The majority of authors hold that the Arthropod brain is
either wholly or in part homologous with the Annelid brain.
Patten (90) alone takes the view that the Annelid prostomium
is absent in Arthropods, and that the brain of the latter is
formed by the moving forward of three segments which are
postoral in the Annelids. Apart from the fact that we have as
yet no means of deciding whether what we call the first seg-
ment of the Arthropod head (protocerebrum) is really a single
segment or a complex of several, it is extremely improbable
that so highly important a structure as the Annelid brain should
have completely disappeared in the Arthropods. So great is
the resemblance between the Arthropods and Annelids in all
the more important morphological features and even in the de-
tailed structure of the ventral nerve-cord, that the complete
elimination of the brain certainly makes strong demands on
one’s credulity.

Will (8s) goes to the opposite extreme and regards the
preeoral portion of the Arthropod brain as the exact homologue
of the Annelid brain. He goes so far as to call the procephalic
lobes of Aphzs the “Scheitelplatte.” He finds that they lie at
the pole of the egg opposite the blastopore, or rather what
he takes to be the blastopore, and that they arise independently
of the nerve-cord. Now the “Scheitelplatte” of Aphzs must
include at least the proto- and deutocerebral segments — prob-
ably also the tritocerebrum. The deutocerebrum in all the
Orthoptera which I have examined is provided with a pair of
true mesodermic somites and witha pair of appendages, the
antennz. Each mesodermal somite sends a hollow diverticu-
lum into an antenna, which is thus shown to be homodynamous
with the other appendages of the embryo. The tritocerebral
segment also contains a pair of abortive somites and in Anurida
maritima, as I have lately ascertained, bears a pair of minute
but distinct appendages (see Fig. VI, zc. ap.). Viallanes (874)
and St. Remy (90) have found that the second pair of antennz
in Crustacea belong to the tritocerebral segment. These facts
go to show that the deuto- and tritocerebral segments are homo-
dynamous with the postoral segments and, as the “ Scheitel-
platte” of Aghzs must extend at least as far back as the

I12 WHEELEK. [Vo.. VIII.

tritocerebral segment, it cannot be homologized with the
Annelid Scheitelplatte, a structure which is not segmented.

A view midway between Will’s and Patten’s probably accords
best with the facts at our disposal. The Arthropod proto-
cerebrum probably represents the Annelid supracesophageal
ganglion, while the deuto- and tritocerebral segments,
originally postoral, have moved forward to join the primitive
brain. This is essentially Lankester’s view (81), according to
which in Arthropods “the praecesophageal ganglion is a syn-
cerebrum consisting of. the archicerebrum and of the ganglion
masses appropriate to the first and second pair of appendages
which were originally postoral, but have assumed a prezoral
position whilst carrying their ganglion-masses up to the archi-
cerebrum to fuse with it.”

In comparing the Arthropod with the Annelid brain much
stress has been laid on the fact so clearly brought out by
Kleinenberg (86) — that the Annelid supracesophageal ganglion
originates independently of the ventral nerve-cord. Several
investigators — Balfour (80), Schimkewitch (87), Will (ss) and
others —have fancied that they could detect a similar onto-
genetic discontinuity of the brain and nerve-cord in Arthropods.
But more recent observations all tend to prove that there is
a direct continuity of the central nervous system from the time
when the ganglia first make their appearance. So far as the
insects are concerned I may note that Will’s conclusions were
based on the defective surface observation of a form (Af/zs) ill
adapted to the study of the central nervous system.?

Even granting that the Annelid brain arises independently of
the nerve-cord —and this,is not yet settled —at least so far as
the Oligochzeta are concerned (see E. B. Wilson, '89) — Lankes-
ter’s view of the Arthropod brain is in no way invalidated. The
line of separation corresponding to the Annelid prototroch must
fall in front of the deutocerebral segment, since it has been
shown that this segment in some insects contains a pair of well-

1 Little value can be attached to Cholodkowsky’s assertion that in A/atta the
supracesophageal ganglion originates independently of the nerve-cord, since he has
failed to see the deuto- and tritocerebral segments which are quite as well
developed in A/atta as in other Orthoptera.

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. I13

developed mesodermic somites. When we stop to consider the
intimate union of the proto- and deutocerebral ganglia from
the time of their first appearance, we need entertain little hope
of finding traces of a separation which existed, if indeed it
existed at all, in a very remotely ancestral period.

VI. THE DEVELOPMENT OF THE REPRODUCTIVE ORGANS
IN THE INSECTA.

1. Zhe Gonads.

The following account of the development of the sexual
organs is based almost exclusively on Xzphidium. Some
attention was devoted to the study of Alatta, but this form
proved to be so much less satisfactory and to depart so little
from the X7phzdium type, that it was abandoned.

Before passing to a description of the sexual organs and
their ducts, it will be necessary briefly to consider the meso-
'dermal somites, since the history of the organs under consid-
eration is intimately bound up with the history of the middle
germ-layer. The mesoderm of AX7zphzdium, like that of other
insects, is coextensive with the blastopore and hence reaches
from the region of the definitive mouth to the region of the
definitive anus. At first a continuous cell-layer, it soon splits
up into segmental masses as metamerism sets in. These are
further divided in the median ventral line so that each segment
has a pair of mesoderm blocks. Each of these acquires a
cavity and the somites are established.

The appendages are from the time of their first appearance
intimately connected with the somites, since each of the latter
sends a hollow diverticulum into the appendage of the corre-
sponding half of the body. All the somites are fully formed
when the embryo has reached Stage F. There are then eight-
een pairs in all. The most anterior pair occupies the deuto-
cerebral segment and sends hollow diverticula into the
antenne. The walls of these somites are much thinner than
those of succeeding pairs and, curiously enough, persist much
longer. The pair in the tritocerebral segment is very small

114 WHEELER. [Vot. VIII.

and indistinct and disappears very early. Each of the succeed-
ing segments, with the exception of the eleventh abdominal
has a distinct pair of somites. In the last abdominal, meso-
derm is present, but I have been unable to find a trace of
a true ccelomic cavity.1

The youngest embryo in which I was able to detect repro-
ductive cells had almost reached Stage F. In still earlier
stages careful scrutiny failed to reveal any differentiation of
the mesoderm cells. These cells, it is true, vary considerably
in size and appearance but I have found it impossible to fix
on any one set of elements which might be brought into con-
nection with the germ-cells of older embryos. It is not,
therefore, till the somites are established as distinct sacs
that unmistakable primitive germ-cells make their appearance.
In frontal sections of embryos in Stage F (Fig. 52 gd!, gd@3),
the primitive germ-cells are seen to lie in the inner wall of the
somite. They are considerably larger than any of the sur-
rounding mesoderm cells, and much paler. The chromatin of
their nuclei is arranged in a more delicate skein. Like the
neuroblasts they stain very deeply in picric acid. Normally,
they occur only in the first to the sixth abdominal segments,
each cluster being confined to the inner wall of asomite. The
reproductive organs of AX7zphidiwm are therefore truly meta-
meric in their origin. There is nothing to show that they
arise from vitellophags which have migrated into the somitic
wall ; nor can they arise from the entoderm, since they are
differentiated before the entoderm-bands have reached the
basal abdominal segments in their growth backward from the
oral and forward from the anal formative centre. I conclude,
therefore, that the primitive germ-cells are enlarged and modi-
fied mesoderm-cells. In explanation of Fig. 52 it may be
noted that the plane of section is somewhat oblique so that it

1T mention this because Graber (’90) has recently described a ccelomic cavity
in the anal segment of Hydrophilus (Fig. 29, p. 62). Cholodkowsky also describes
and figures (’91, Fig. 49 Pl. IV) such a cavity in the eleventh abdominal seg-
ment of A/atfa. Every little slit in the mesoderm is not a ccelomic cavity, and
the figures referred to show only small spaces between the mesoderm cells of
the telson. This may have been produced artificially, for aught the figures show
to the contrary.

No.1.] CONTRIBUTION TO INSECT EMBRYOLOGY. I15

strikes only the lowermost germ-cell of the cluster in the third
abdominal segment (g@3) and in the fourth segment passes
completely under the cluster. Even at this time certain meso-
derm-cells, the future epithelial elements (ef.) begin to flatten
out and apply themselves to the surfaces of the germ-cells.

The exact relations of the primitive germ-cells to the walls
of the somite are readily seen in transverse section (Fig. 53).
The inner face of the triangular somite is applied to the surface
of the yolk and besides giving rise to the germ-cells will ulti-
mately form the splanchnic, or visceral layer. The remainder
of the coelomic wall is somatic, or parietal, and is converted
into fat-body and musculature. The heart arises where the
outer edge of the splanchnic passes into the somatic layer.
In the section figured the entoderm is still wanting on the left
side, whereas on the right side a single cell (ez) of the right
posterior band has already reached the segment. Similar ine-
qualities in the rate of growth of the entoderm-bands are by
no means infrequent.

In this stage some of the primitive germ-cells show a
tendency to leave the wall of the somite and to drop into the
ceelomic cavity. This is distinctly seen in Fig. 53. These
cells sometimes enlarge considerably, become vacuolated and
take on the appearance of young ova. A cell of this kind,
nearly filling the ccelomic cavity, is shown in Fig. 55. I do
not believe that the cells are loosened from the ccelomic wall
during the process of sectioning.

Although the clusters of germ-cells normally occur only in
the first to the sixth abdominal segments, in one somewhat older
embryo (Stage G) a well developed pair of clusters was
found in the tenth segment. One of these is shown in
sagittal section in Fig. 56 (gd@1°). It resembles the normal
clusters in every particular. The same section shows the
diverticulum of the tenth somite (mz. d.). In the next section
laterad to the one figured, the hollow tip of the diverticulum
is seen to terminate in the right appendage of the segment.
A similar relation of the coelomic diverticula to the append-
ages obtains in all the abdominal segments in front of the
tenth.

116 WHEELER. [Vo.. VIII.

The primitive germ-cells, which at first occupy only a limited
portion of the splanchnic wall, increase in number during a
stage midway between F and G. Beautiful caryokinetic
figures may then be found in some specimens — showing that
the primitive germ-cells themselves proliferate. New germ-
cells may be added to the clusters by a differentiation of
elements in the splanchnic wall but I have seen nothing to
convince me that this occurs. The epithelial cells become
much flattened and stain more deeply so that they stand out
distinctly among the pale rounded germ-cells which they
invest. The inner wall of the somite soon becomes too
small to contain all the rapidly accumulating cells and is
forced to send out a solid diverticulum. This is directed
anteriorly, and in a little later stage fuses with the wall of the
antecedent somite. This fusion is probably preceded by the
shortening of the embryo which takes place during a stage
immediately succeeding Stage F. The result of the fusion is
the formation of a continuous strand of germ-cells with their
accompanying epithelial cells. For some time the typical hexa-
metameric arrangement is still visible in the strand, but later
the whole mass shortens very decidedly to form the definitive
ovary and testis and all traces of metamerism are lost. In the
present paper I shall not follow the development of these
organs further, but will pass on to a description of the sexual
ducts. This will enable me to supplement the recent work of
Heymons (91) who has given us an extended and valuable
account of the development of the sexual organs in 4/atzta, but
has contributed only a few observations on the development of
the ducts.

2. The Male Ducts.

The sexual ducts like the germ-cells are modified portions
of the mesodermal somites. While considering the excep-
tional embryo in Fig. 56, attention was directed to the
diverticulum (m. d.) of the tenth abdominal somite. This
diverticulum, which is quite normal, is destined to form the
terminal portion of the deferent duct (spermaduct) and the
seminal vesicle of the adult insect. At the base of the divert-

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. ry

iculum a constriction is formed which converts the proximal
portion into a thin cord but leaves the distal end expanded as
a hollow sac, which I shall call the terminal ampulla. The
remainder of the deferent duct — vzz. the portion extending from
the sexual gland in the sixth to the anterior end of the tenth
segment is formed by a cord-like thickening in the splanchnic
wall of the three intervening somites. Anteriorly the cells
of the duct pass into the epithelium enveloping the germ-cells.
There is no lumen in the duct proper except towards its end
where it widens into the terminal ampulla. Thus only the ap-
pendage diverticula of the tenth segment go to form the ends
of the ducts ; in all the other abdominal segments the diverti-
cula break down and disappear, together with their respective
appendages, before the embryo reaches Stage H. In the
thoracic, oral and antennary segments, however, the diverti-
cula are converted into the muscles of the persistent ap-
pendages.

The further history of the male ducts is readily followed in
partially stained embryos mounted zz ¢oto. Sex is determined,
so far as I have been able to make out, during or soon after
katatrepsis, at which time the appendages of the second to
the seventh abdominal segments disappear. Fig. 42 represents
the caudal end of an embryo just after katatrepsis (Stage J).
Appendages still persist on the eighth to eleventh segments
while the pleuropodia, not seen in the figure, have begun to
degenerate. The testes (¢s.) and the spermaducts (m. d.) are
represented in blue. The former have shortened considerably
and moved caudad so that they now lie in the sixth and seventh
segments. The long terminal threads run forwards from the
anterior ends of the testes, while the spermaducts run back-
wards and end in the terminal ampullz (¢a. m.) which still fit
into the cavities of the tenth pair of abdominal appendages
(ap. 1°). A section through these appendages is seen in Fig.
57, showing very clearly the connection of the ampullz (¢a. m.)
with the ducts (m. d.). In front of this section the ducts have
no distinct lumen.

In Fig. 43, taken from a slightly older embryo, the append-
ages of the eighth segment have completely disappeared,

118 WHEELER. [Vou. VIII.

while those of the tenth have grown smaller and approached
the median ventral line. They have, in fact, grown too small
to contain the ampulla which are drawn away from them and
lie between the ninth and tenth segments a little in front of
the abortive appendages. At the same time the inner faces of
the ampullz have become flattened and applied to each other
in the median line. Each of these sacs has a pointed tip
directed caudad. The more arcuate course described by the
ducts in this stage is undoubtedly due to the mutual approxi-
mation of their terminal ampulla. The appendages of the
eleventh segment, the cerci (cc. [a11]), which in the pre-
ceding stage were rounded like the other abdominal append-
ages, have become oval.

A more advanced embryo is represented in Fig. 44 (some-
what younger than Stage K). The appendages of the tenth
segment (ap!°) have almost completely disappeared. Those
of the ninth segment, the future styli (a@f9) have lengthened
and now point outwards and forwards. The cerci have grown
more pointed. The terminal ampullz lie completely in the ninth
segment, having shifted their position headwards. The move-
ment takes place in such a way that what were the posterior
faces of the ampullz in the younger stage (Fig. 43) are applied
to each other, while the pointed tips are directed forwards.
The ducts are thereby rendered still more arcuate towards
their terminations. An intermediate stage in this singular
movement is shown in Fig. 45, where only small portions of
the posterior faces of the ampullz are as yet applied to each
other. Comparison of Figs. 43, 44 and 45 shows that the
pointed tips remain united and move forward while the sur-
faces of mutual contact are being shifted. Finally in Fig. 46,
which represents the abdominal end of an embryo ready to
hatch, we see that the terminal ampullze have increased con-
siderably in size at the expense of the thickness of their walls.
They have also lengthened, and brought still more of their
surfaces in contact in the median line. The pointed tips of
the ampulla extend into the eighth segment. It may also be
noted that the points where the spermaducts meet the ampullze
have moved forward. The appendages of the tenth segment

Novi eCONV TRIBUTION, TO INSECT EMBRVOLOGY. 119

have long since disappeared and the pleuropodia have lost
their organic connection with the embryo, so that only two
pairs of abdominal appendages persist, the stylets (s¢. [ap 9])
and the cerci (cc. [af1!]), both provided with sete. The
pointed fusiform cerci are now folded back so as to bring
their insertions on the anal segment into view.

Beyond this stage the development of the male ducts was
not followed in X7phidium ensiferum, but several larval stages
of an allied species, .. fasczatum, were studied for the purpose
of connecting the embryonic with the adult condition.

It will be noticed that in X7phidium ensiferum there exists
at the time of hatching no external opening to the sperma-
ducts; the ampullae are completely closed sacs applied to
the ventral hypodermis of the ninth abdominal segment, and
the ducts connecting them with the testes have no lumen.
In the X. fasczatum larva 10 mm. long the ejaculatory duct
has made its appearance as an unpaired invagination of the
hypodermis in the median line between the ninth and tenth
segments. Fig. 47 shows the sexual organs of such a larva
seen from within, the ventral scutes of the tenth and anal seg-
ments having been entirely removed. The prominent terminal
ampullz, which become the seminal vesicles of the adult, are
considerably enlarged and their walls have increased in thick-
ness. The short spermaducts, now provided with a small
lumen, run from the under surface of the sacs to the prom-
inent testes. Only the outer opening of the invagination
which is to form the ejaculatory duct is seen at m.o. It runs
forward as a fiattened chitin-lined depression beneath the
’ seminal vesicles. Sagittal sections show that there is as yet
no communication between the lumina of the mesodermal and
ectodermal portions ; it is not till a later stage that such a
communication is established.

3. The Female Ducts.

The oviducts, like the vasa deferentia, are derived from a
pair of ccelomic appendage-diverticula, but in the female the
diverticula belong to the seventh abdominal segment. The
diverticula of the female embryo also become constricted

120 WHEELER. (Vou. VIIL

proximally and end in terminal ampulle, which are from
the first somewhat smaller and more elongate than the homo-
dynamous structures of the male. The appendages to which
the ampulla belong are also less prominent than the tenth
pair of appendages in the male. Examination of several
series of cross-sections from embryos in Stage J —this being
the stage in which the sexes differentiate —reveals the curious
fact that in the female, besides the pair of ampullz in the sev-
enth, a pair is also retained in the tenth segment. Figs. 58 and
59 represent two sections taken from such an embryo— the
former passing through the tenth, the latter through the seventh
abdominal segment. In Fig. 58, the two terminal ampullze
(ta. m.), and small portions of the ducts leading to them, are
still preserved, but the cells and nuclei, especially in the ducts,
are being broken down. The ampullz soon share the same
fate. In the seventh segment (Fig. 59, fd.) the cavity of the
diverticulum still opens into the coelomic cavity of the same
segment (coe). Its distal ampullar end is applied to the ecto-
derm where it bulges out to form the small seventh abdominal
appendage (af7). The condition of the diverticulum after the
constriction of its proximal portion is shown in Fig. 60, taken
from a somewhat more advanced embryo. In this figure, the
connection of the oviduct with the posterior end of the young
ovary (ov.) is distinctly seen. The cells of the duct pass over
into the epithelial cells of the ovary, just as the cells of the
spermaducts become continuous with the testicular epithelium.

We may now turn to surface views of the female reproduc-
tive organs. The specimen represented in Fig. 48 is in-the
same stage as the male embryo represented in Fig. 42. Five
consecutive pairs of abdominal appendages are still present
(ap7, apt!) Of these, the ninth and eleventh pairs are very
prominent, while the tenth pair has grown very small. The
ovary (ov.) extends back to the seventh segment where it joins
the oviduct. This ends in the terminal ampulla, which lies near
the posterior edge of the segment in the seventh abdominal
appendage. The terminal ampullz of the tenth segment have
not yet disappeared. They are represented in blue because I
regard them as the homologues of the persistent male ampulle.

No. 1.] COMTRIBUTION TO INSECT EMBRYOLOGY. F2u

In a more advanced embryo (Fig. 49, Stage K) the male
ampullz have disappeared completely, and the tenth pair of
appendages, while growing smaller, have moved up to the inner
posterior insertions of the ninth pair. The ampullz have in-
creased in size and have come to lie at right angles to the lon-
gitudinal axis of the embryo. This causes the oviducts to
describe an arc. It is thus seen that the movement of the
female ampullze is essentially the same as that of the male, but
considerably weaker. Traces of appendages on the seventh
segment are still apparent.

From this stage we may pass to a brief consideration of the
female embryo ready to hatch (Fig. 50). The ovaries (o0v.)
have now assumed their definitive characters. Although the
pointed and flattened ampulla have approached the median
ventral line, they are still separated by a wide space. Even in
this advanced stage slight thickenings of the integument over
the posterior edges of the ampulla may be taken to represent
the remains of the seventh pair of abdominal appendages. The
appendages of the tenth segment appear to have joined the
inner bases of the ninth pair. I must say, however, that my
observations on this pair of appendages are unsatisfactory,
notwithstanding I have taken considerable pains to follow their
history. The appendages of the eighth and ninth segments
undoubtedly form the two anterior pairs of gonapophyses.
The third pair has been described by Dewitz ('75) and others as
arising from the inner bases of the second pair and is therefore
supposed to belong to the ninth segment. I believe, however,
that the tenth pair of embryonic appendages persists and moves
forward to join the ninth pair, whence they grow out during
early larval life as the third pair of gonapophyses. In the
embryo the line separating the ninth and tenth segments is
certainly very vague, especially on the ventral surface, so that
the possibility of a fusion of the two pairs of appendages is by
no means precluded. That this fusion should occur is certainly
no more remarkable than the migration of the male ampullz
from the tenth to the ninth segment. Both of these forward
movements may be in some way connected with the forward
migration and fusion of the ganglia belonging to the eighth,
ninth, and tenth segments (cf. Figs. 42-46, agt-3).

122 WHEELER. [Vou VIET.

The female larva, like the male, has no external orifice to
the sexual organs at the time of hatching. It is even more
backward than the male, inasmuch as the terminal ampullz of
the oviducts have not yet met in the median ventral line. The
first traces of a vagina were found in X7zphidium fascvatum
larve about 10 mm. long (Fig. 51). Here the terminal am-
pullz meet, but the surfaces of mutual contact are limited to
the pointed tips. The vagina (vg) is a short and broad in-
vagination of the hypodermis between the seventh and eighth
segments. Its tip extends to the juncture of the terminal
ampullz. In a somewhat later stage the ampullz open into
each other and into the vagina. The three pairs of gonapo-
physes (0f!—0f3) are already assuming their definitive charac-
ters.

For an excellent résumé of the little work that has been done
on the embryonic development of the sexual organs in insects
I would refer the reader to Heymons’ recent paper (91). Here
I shall consider only three contributions, —two of Heymons’
(90 and '91) which treat mainly of the sexual glands, and Nus-
baum’s paper on the development of the ducts (84).

Although the results of my study of AX7zphzdium, so far as
they go, agree in many respects with Heymons’ account of
Blatta, several not unimportant differences must be pointed
out. The first difference relates to the stage in which the
germ-cells make their appearance. Heymons ('91) claims that
he can detect them before the somites are established, at a time,
in fact, when the abdominal region of the embryo is still un-
segmented. This would correspond to a stage in X7phidiwm
midway between B and C. In Slatta certain mesoderm-cells
at this time enlarge and assume a clear and succulent appear-
ance. There is apparently no definite relation between the
position of these modified cells and the future segments, and
even in a later stage when they become integral portions of
the somite-wall, they are quite irregular in their distribution.
Heymons regards them as largely dissepimental in position.
In Xiphidium, which is, on the whole, a far more favorable
form for the study of the sexual organs than A/atia, I was

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 123

unable to detect the presence of germ-cells till the somites
were established (Stage a little younger than F). At this time
they formed strictly metameric cell-clusters each of which was
confined to the median portion of the splanchnic wall of its
respective somite. These cells rarely, if ever, strayed into the
dissepimental region during this stage. It is, of course, con-
ceivable, that X7phidiwm and Blatta may differ very consider-
ably in respect to the point under consideration, but I suspect,
nevertheless, that Heymons has mistaken the young vitellophags
for sexual-cells, notwithstanding his assertion to the contrary.
At any rate, to be complete, his figures should show the vitel-
lophags, which are undoubtedly present in the stages he
studied and which occupy the very location of his “sexual-
eclls”” in his Pigs: 2/and! 3:

Another point on which we differ is the distribution of the
germ-cells. According to Heymons they occur in Aletta in
the second to seventh abdominal segments, whereas I find
them in A7phzdium in the first to sixth. Heymons says
emphatically: “Im ersten Abdominalsegment treten niemals
Genitalzellen auf.” But this is certainly an error, for
in several Llatta embryos I find unmistakable germ-cells
forming a pair of isolated clusters in the first abdominal
segment. Here they also persist till a comparatively late
stage when they are drawn into the second segment during
the contraction of the sexual Anlage. The peculiarly modified
pleuropodia in 4/atta form so efficient a means for determin-
ing the exact position of the first abdominal segment and its
somite in series of sections both longitudinal and transverse,
that I feel confident of not being mistaken in this matter. I
admit, however, that fewer germ-cells occur in the first than in
the succeeding abdominal segments. According to Heymons
comparatively few germ-cells occur in the seventh pair of
abdominal somites. In these I have never seen traces of
germ-cells in Xzphzdium but I cannot, of course, assert that
they never occur, especially as I have shown that germ-cells
may be found even as far back as the tenth segment. It is
interesting to note that Heymons, too, found germ-cells in some
of the posterior abdominal segments in Blatza.

124 WHEELER. [Vor. VIII.

In his first paper (90) Heymons made the following state-
ment in regard to the genital ducts: ‘Von _ besonderer
Wichtigkeit scheint mir nun die Thatsache zu sein, dass
beim Mannchen der urspriinglich angelegte Ausfihrungs-
gang nicht in seiner ganzen Lange zum Vas deferens wird,
sondern dass sich sein distaler Abschnitt spater wieder zu-
riickbildet, ohne je functioniert zu haben. Der wirklich als
Ausfiihrungsgang dienende Endtheil des Vas deferens, welcher
sich mit dem ectodermalen Ductus ejaculatorius verbindet, ent-
steht erst nachtraglich an dem urspriinglich angelegten Aus-
fiihrungsgang. Beim Weibchen dagegen bildet sich der ganze
primitive Ausfiihrungsgang zum Oviduct aus.”

This is the very opposite of what I have found: in X7phzdium
it is the male duct which at first occurs in both sexes in the
tenth abdominal segment — whereas in the female the oviducts
are an independent formation, the original male duct being
soon broken down. In the female both pairs of ducts are
established simultaneously since they are both ccelomic
diverticula.}

In his more recent paper Heymons ('91) describes the genital
ducts as terminating at the posterior edge of the seventh
abdominal segment. As he mentions this fact before he
comes to a description of the embryo with determinate sex,
I assume that he regards these ducts as common to both sexes.
What he saw was without doubt the pair of oviducts, not the
deferent ducts. From personal observation I can state that
the male ducts of Aélatta end at first in terminal ampullz
enclosed by the appendages of the tenth abdominal segment
just as in X7phidium, whereas the female ducts terminate
in much flattened ampullze in the seventh segment. Whether
or not a rudimental male duct is present in the tenth segment
of the female B/atta embryo I have been unable to decide.
Perhaps Heymons found something of this kind and while
confounding the sex of the embryos he studied, was led to
make the above quoted remark.

1 For the sake of greater exactness, I may state that the anterior pair is, perhaps,
formed a little sooner than the posterior pair, since the somites develop from
before backwards.

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 125

Of the few observations which have been made on the
development of the genital ducts in insects, Nusbaum’s ('84)
are the most important. Their agreement with Palmén’s
anatomical researches on Ephemerids (84) has been regarded as
sufficient warrant of their accuracy. Nusbaum studied the
developing ducts in Mallophaga, Pediculidz, Blattide, and
Culicidz and came to the conclusion that the testes and defer-
ent ducts in the male and the ovaries and oviducts in the
female are mesodermal, while the seminal vesicles, ejaculatory
duct and accessory glands in the male, and the uterus, vagina
and accessory glands in the female are ectodermal. He
therefore draws the line between mesodermal and ectodermal
portions at the juncture of the seminal vesicles and deferent
ducts in the male and at the juncture of the oviducts and
uterus in the female. This is at variance with my results,
since I have found that the seminal vesicles and uterus are
mesodermal. These structures are described by Nusbaum as
if he had traced their derivation from the ectoderm step by
step. Yet he seems not to have studied embryos but only
larvee, and it is during embryonic life that the uterus and
seminal vesicles are formed.! Furthermore his investigations
were carried on without the aid of sections. The differentia-
tion of the uterus and seminal vesicles from the ectoderm is
far from satisfactorily shown in his figures. I cannot therefore
regard Nusbaum’s work as contributing any evidence in favor
of Palmén’s view. Palmén concluded from a careful study of
the Ephemeridz that the genital ducts of insects originally

1 This is Nusbaum’s description (’84, p. 40): “ Auf der Bauchseite des vierten,
von hinten an, Abdominalsegmentes entstehen zwei paarige Hautepithelverdick-
ungen die sich einander nahern um sich dann zu einem hufeisenformigen unpaaren
Korper zu vereinigen. Bevor aber noch die Vereinigung zu Stande kommt, lésen
sich diese Keime von der Haut ab und verwachsen, wie gesagt, mit den Enden
der noch soliden Vasa deferentia. . . . In dem vorderen Theile des soliden huf-
eisenfOrmigen Keimes entstehen zwei vordere geschlossene Hoéhlen; der mittlere
Theil bleibt noch weiter solid, der hintere verlangert sich in zwei seitliche solide
Auswiichse. Die zwei vorderen Hohlen verlangern sich nach vorn und differ-
enziren sich in die zwei Vesiculz seminales (innere Schliuche) des definitiven
birnformigen Kérpers. Mit denselben communiciren die sich aushéhlenden Vasa
deferentia.” An essentially similar description is given by Nusbaum for the
female (p. 41).

126 WHEELER. [VoL. VIII.

had paired openings on the surface of the body. This view,
which I fully endorse, has a good basis of anatomical data ;
but Nusbaum has not shown that there is a double opening
to the sexual organs or even a distinctly paired Anlage to the
ectodermal portion of the sexual apparatus. According to
his own figures the vagina and ejaculatory duct are unpaired
from the first ; the structures on which. he laid stress as being
paired ectodermal portions were nothing more nor less than the
unmodified terminations of the mesodermal ducts — the ter-
minal ampulle.

4. General Considerations.

The foregoing account of the development of the sexual
organs differs sufficiently from the accounts of other authors
to justify a brief consideration of some general questions.

First in regard to the germ-cells. These arise as six meta-
meric pairs of clusters in the splanchnic walls of the meso-
dermal somites. Since the single layer of cells forming the
walls of each somite corresponds to the peritoneal epithelium
of Annelids, Heymons’ conclusion that the germ-cells of insects
arise in essentially the same manner as the germ-cells of
Annelids, is certainly well-founded. In both groups the germ-
cells are local proliferations of the epithelium lining the body
cavity. In Annelids the germ-cells lose their connection with
the peritoneum and drop into the body cavity where they
undergo maturation. I have called attention to a similar
process in the Xzphidium embryo. Whether these germ-cells
disintegrate or again attach themselves to the wall and become
invested with epithelial cells, I must leave undecided. I am
inclined to adopt the latter alternative, since I have found no
traces of germ-cells in the coelomic cavities in stages but little
older than the one figured. (Fig. 53.) _Heymons has observed
in Blatta a similar migration of the germ-cells into the
coelomic cavity.

I have alluded to the fact that Xzphzdium exhibits more
pronounced metamerism in the early arrangement of the
germ-cells than B/atta. Strictly speaking the germ-cells in
the form studied by Heymons are not at all metameric since

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. Re 7

they arise, if his account is correct, before metameres are
established. It is only on the supposition that the germ-cells
of Llatta are precociously segregated that their method of
origin can be satisfactorily compared with the conditions seen
in Annelids, for in this group the germ-cells are not differ-
entiated—so far as I am aware—until after the somites have
reached a considerable degree of development. Providing,
therefore, that I have not overlooked the germ-cells in preccelo-
mic stages, AX7phidium must be regarded as presenting more
primitive conditions than Blatta.

In Xzphidium and &latta six, and therefore more than half
the total number of abdominal segments, produce germ-cells.!
In one case I found well-developed clusters in the tenth
segment, so that if we omit the eleventh or telson-segment,
which is rudimental and hence cannot be expected to produce
germ-cells, and if, moreover, Heymons is correct in stating that
reproductive elements occur in the seventh, only two abdominal
segments fail to produce germ-cells! This consideration lends
support to Heymons’ suggestion that ‘iirspriinglich die Sexual-
zellen auch in den hinteren Segmenten des Abdomens noch in
derselben typischen Weise auftraten.’”’ The resemblance of
the insect-embryo to Annelids in which a great number
of consecutive segments produce ova and spermatozoa, is very
obvious. The high development of the appendages and their
musculature in the thoracic and oral segments of insects per-
haps sufficiently accounts for the complete elimination of the
germ-cell clusters in these regions. It may also be noted that
they are normally absent in the abdomen in the very segments
which longest retain traces of geondam ambulatory append-
ages —viz. the eighth to the eleventh.

The indications of metamerism which are so transitory in
the sexual Anlage of the Orthoptera would appear to be retained
throughout life in some of the Thysanura. In /afyx, according
to Grassi (89), the arrangement of the egg-tubes is ““nettement
métamérique,” and his Fig. 44, Pl. IV, represents in either
ovary seven egg-tubes, occurring in consecutive abdominal seg-

1Jn a single instance (Fig. 55, Pl. VI) what I took to be a sexual cell was
found in one of the ccelomic cavities of the metathoracic segment !

128 WHEELER. [Vot. VIII.

ments, beginning with the first. The solid testes show no
traces of metamerism. In the -young Lefzsma, according to
the same authority, the egg-tubes are also segmental, there be-
ing five in either ovary, a pair in each of the five basal abdominal
segments. In the adult this character is no longer noticeable.
In certain species of Lefzsmina, there are six sacs in either
testicle, united in pairs on either side. These also lie in the
basal abdominal segments. ‘“ L’organisation segmentaire des
ovaires chez /apyx se répete chez Machzlis. Cette répétition
dans des formes trés éloignés l’une de l’autre comme le sont
précisément Japyx et Machilis, tend a donner au fait une
sérieuse valeur morphologique. Les tubes ovariques de Wachilis
sont au nombre de sept de chaque cété.”’ In the latter form
Oudemans ('87) also figures seven egg-tubes strung along the
oviduct, but without a clearly marked metameric arrangement.
In the male there are three pairs of testicular sacs.

In all these Thysanura the female, as might be expected, ad-
heres more tenaciously than the male to the metameric scheme.
It will also be observed that the number of egg-tubes (five to
seven pairs) is about the same as the number of germ-cell clus-
ters in embryo Orthoptera. The position of the organs is also
identical, viz. in the first to seventh abdominal segments. We
might conclude, therefore, that the sexual organs of the higher
Thysanura represented an embryonic or arrested condition.

A difficulty is encountered, however, when we stop to ask
the question: Is the individual egg-tube in such a form as MWa-
chilis homologous with an individual egg-tube in 4la¢ta or any
other Pterygote? So far as structure is concerned, this would
appear to be the case. We should also say that each egg-tube
of Japyx or Machilis was a metameric unit. But the lowest
number of egg-tubes in the A/a¢za ovary is sixteen, and as this
is more than double the number of metameres which contribute
germ-cells in the embryo, the egg-tube in this form cannot be
regarded as a metameric unit. We must conclude, therefore,
either that the individual egg-tubes are not homodynamous in
the Pterygota and Apterygota, or that the ovaries in /apyz,
Machilis, etc., are not primitively metameric. The possibility
of there being an acquired metamerism, or pseudometamerism

No. 1.] CONTRIBUTION TO INSECT EMBRVOLOGY. I29

in these cases is suggested by Grassi: “Cette disposition qui
est nettement métamérique préserve l’ovaire du danger d’étre
détérioré de quelque maniére que ce soit. Le danger existe
particuli¢rement quand les oeufs sont pres d’arriver a maturité
et provient de ce que, chez /apyx, la differentiation segmentaire
de la musculature et de la cuticule est avancée au point que les
métaméres ont acquis une grande independance de mouvement.
Cette independance est beaucoup moindre chez Campodea et
c'est pour cela que cet insecte n’offre pas la disposition indiqué
plus haut.” If this be an adequate explanation, the resemblance
of the sexual organs in the Thysanura to those in the Orthoptera
is due to secondary causes. At all events, this question must
remain open till Thysanuran embryos can be studied.

The metameric mesodermal origin of the germ-cells in
embryo Orthoptera is too much like the origin of the germ-
cells in Annelids to be considered as secondary and I fully
agree with Heymons (91), and Korschelt and Heider (92) in
regarding the sexual organs of such forms as the Rhynchota
(Aphidee, Cicadide) and Diptera (Chironomus, Cecidomyia) as
derived by a process of precocious segregation from metameric
gonads like those of the Orthoptera. These exceptional forms
frequently exhibit peculiar and aberrant features (partheno-
genesis, peedogenesis) like the Crustacea which have a similar
precocious segregation of germ-cells (¢.g. Motna, according to
Grobben, ’79).

The genital ducts of the insect embryo are not so readily
reduced to the Annelid type. Many authorities, it is true,
have regarded them as modified nephridia but apart from their
paired mesodermal origin and tubular structure there was very
little to support such a view.! But now the prevailing view
receives fresh support from the fact that the ducts in both
sexes arise as hollow diverticula of the ccelom. Though
temporarily obliterated the lumen of the duct is very probably
a persisting remnant of the ccelomic cavity. This is certainly

1 My statement in a former paper (’89) that the genital ducts might arise from
tracheal involutions is erroneous. What I saw and figured (Fig. 80, Pl. XIX)
was a section through the terminal ampulle of the deferent ducts, and not as I
supposed, through their orifices.

130 WHEELER. [Vor. VIII.

the case with the cavities of the terminal ampullz which are
never obliterated.

In seeking for some clue to the true nature of the coelomic
diverticula one naturally turns to Peripatus. Unfortunately
the two accounts of the development of the nephridia and genital
ducts in this curious Arthropod—the one by v. Kennel (85
and '88), the other by Sedgwick (85 and '8s) —- contradict each
other in many particulars. Both authors, however, agree in
deriving the mesodermal portion of the nephridium from hollow
diverticula of the somites (similar to those seen in the Orthop-
tera, in that they extend into the appendages!), and both agree
in regarding the sexual ducts as modified nephridia. But Sedg-
wick derives the nephridium from the portion of the diverti-
culum located in the appendage, while Kennel derives it from
the inner lower angle at the base of the somite. According to
Kennel only the funnel-portion arises in this way, the long
duct being formed by a tubular invagination of the ectoderm.
On the other hand, Sedgwick derives the funnel and the greater
portion of the duct from the mesoderm and believes that only a
very small portion of the duct arises by invagination from the
ectoderm. These differences apply, of course, to the sexual
ducts as well. According to v. Kennel’s account not only their
unpaired terminal portion (opening in his form on the ante-
penultimate segment) but also the deferent ducts and uteri
are ectodermal; only a short piece, corresponding to the
nephridial funnel, and uniting the uteri to the ovaries, and the
deferent ducts to the testes, has a mesodermal origin. Accord-
ing to Sedgwick the dorsomedian portion of the coelom persists
in the segments caudad to the fifteenth and is constricted off
from the remainder of the somite. The dissepiments are
broken down between the adjacent abstricted portions of the
somites, so that a hollow tube is formed on either side. These
tubes receive the germ-cells from the entoderm and form the
sexual glands... In the segment bearing the anal papillex

1 Sedgwick claims that the germ-cells originate in the entoderm and later on
migrate into the coelomic wall. In this particular I prefer to adopt v. Kennel’s
account, according to which the germ-cells have a mesodermal origin, since it

accords better with the facts of Annelid development and with my own
observations.

No.1.] CONTRIBUTION TO INSECT EMBRYOLOGY. RAE

(which in all probability are reduced ambulatory appendages)
a complete separation of the ccelom into a lateral (diverticular)
and a dorsomedian (genital) portion does not take place, so
that the two cavities remain confluent. The portion of the
coelomic wall surrounding the proximal cavity joins the sexual-
gland while the diverticular (nephridial) portion acquires an
external opening, ‘“ which, however, is much nearer the middle
line than in the case of the anterior somites, and, indeed, may
be described as being common with that of the opposite side.
However this may be, the two openings soon become definitely
united to form a single opening, while the tubes themselves
persist as the generative ducts. Whether any large portion of
the latter are ectodermal in origin, that is to say, derived from
a growth of the lips of the opening at its first appearance, it is
impossible to say.”

If we accept Sedgwick’s account it is easy to reduce the
genital ducts of insects to the type seen in Pervifatus and con-
sequently to Annelid nephridia. In the first place, everything
goes to show that the appendage diverticula of the ccelom are
homologous both in Pervzpatus and Orthoptera. In both cases
the oviducts and deferent ducts arise from these diverticula by
partial constriction. Just as the ducts of Peripatus run into
the cavities of the anal papilla, so the sexual ducts of Blatta
and A7phidiwm run into the rudimental abdominal appendages.
In both cases there are terminal ampullz, for as such I venture
to regard the slight distal widening of the caelomic diverticula
in Sedgwick’s Figs. 42 and 44. A comparison of these
figures with my Figs. 56 and 59 will show the close resem-
blance between insects and Pervifatus better than paragraphs
of description.

As the exact limits of the ectodermal portions of the ducts
of Peripatus have not been clearly ascertained, further com-
parison with the Insects cannot at present be undertaken. In
the Insecta only the vagina and ejaculatory ducts with their
respective accessory glands arise from the ectoderm. These
structures are median and unpaired in all insects except the
Ephemeridea, one of the oldest and most primitive groups. In
this group, as Palmén has shown (84), the ducts of both sexes

LQ 2A WHEELER. [Vor ovilt:

have independent openings. The oviducts open at the posterior
edge of the seventh, the deferent ducts, which are continued
into a pair of penes, at the posterior edge of the ninth segment.
There is no ductus ejaculatorius proper since, according to
Palmén, the chitinous cuticula covering the surface of the body
does not extend in beyond the lips of the orifices.1 In the
females of Heptagenia the oviducts open at the bottom of an
infolding of the hypodermis between the seventh and eighth
segments. This infolding, the ovivalvula, accommodates the
mature eggs till the time for oviposition, and may be regarded
as a structure on the way to becoming a vagina. Morphologi-
cally it is simply an intersegmental depression differing from
those which separate the sternites of other segments only in
being somewhat exaggerated. Palmén observed that the male
genital ostia are not opened till the last nymphal ecdysis.

A comparison of the nymphal Ephemerid with the Ortho-
pteran embryo is very instructive. In A7phzdiwm and Slatta the
female ampullz lie at the hind end of the seventh, the male at
the hind end of the ninth abdominal segment. Just as the
deferent ducts of Ephemerids extend into the penes and open to
the exterior, so the terminal ampullze originally extend into a
pair of appendages, albeit on the tenth segment and not opening
to the exterior. If the penes of Ephemerids are really modified
ambulatory appendages they would be homologous with the
styli of Orthoptera. The curious persistence of these append-
ages in existing Orthoptera may be due to their having once
functioned as penes, long after the other abdominal ambulatory
appendages had disappeared. It would be necessary to suppose,
if this view were adopted, that the terminations of the male
ducts had moved backwards. But this whole matter is very

1 Palmén claims ('84, p. 82) to have found no chitinous lining in the terminal
portion of the ejaculatory ducts and oviducts of Ephemerids—an observation
from which he naturally infers that these ducts are mesodermal throughout their
entire length. I have found, however, that there is in the nymph of a species of
Blasturus very common in the ponds of Worcester, Mass., a distinct chitinous
lining to the ejaculatory ducts for some distance inward from the orifice of either
penis. My attention was attracted to this lining during the ecdysis of the insect,
when I saw the membrane withdrawn from the ducts along with the cuticle
covering the external surfaces of the penes and terminal abdominal segments.

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 133

obscure, for why should the ampulle in X7phzdium move from
the tenth into the ninth segment? The answer to this enigma
depends on further comparative embryological research. The
long persisting closure of the ostia of the male ducts in Ephe-
merids is probably an embryonic trait. That the vagina and
ejaculatory duct of higher insects may have arisen from a
simple intersegmental depression like the ovivalvula receives
some support from the fact that the ectodermal portions of the
sexual apparatus make their appearance so late ontogenetically.
To obtain in X7phidium a condition essentially like that in
Ephemerids it would only be necessary to have each terminal
ampulla in both sexes open to the exterior.

The original termination of the sexual ducts in modified
ambulatory appendages—which is so clearly seen in both
sexes in embryonic Orthoptera—is very probably a primitive
feature. In the Malacostraca among Crustacea and in Dip-
lopod Myriopoda the sexual ducts terminate on more or less
modified ambulatory limbs; in both sexes in the former group,
only in the males in the latter. In the insect embryo the
male genital appendages are larger than those of the female;
hence, perhaps the larger size of the ampulle filling their
cavities. The ampullz are probably very important structures
from a phylogenetic standpoint. They may perhaps represent
the nephridial Endblasen of Peripatus and Aunelids, providing
these latter structures are mesodermal. In Annelids the End-
blasen occasionally function as temporary receptacles for the
sexual products, a function which seems to have been retained
in the male insect, where they become the vesiculz seminales.

Within the group Eutracheata! the position of the sexual
openings is subject to great variation. Thus in Diplopods and
Pauropods the ducts open behind the second pair of legs,
usually between the second and third segments. In Chilo-
poda, on the other hand, they open on the penultimate seg-
ment. In the Symphyla the unpaired genital orifice is situated
on the fourth segment, which probably corresponds to the first
abdominal segment in insects. Even within the division
Apterygota great variation is observable. In the Collembola in

1 Under this heading I would include the Myriopoda and Hexapoda,

DAA WHEELER. [Vou VIII.

both sexes the orifice lies at the posterior edge of the fifth
abdominal segment. In the Thysanura the female opening
usually lies on the eighth, and that of the male on the ninth
abdominal segment. In female Orthoptera and Ephemeridea
the sexual organs open behind the seventh, in the male behind
the ninth abdominal segment, while in the Plecoptera and
many other insects the female orifice is said to lie at the pos-
terior edge of the eighth segment. Although these facts of
adult anatomy point to great instability in the segmental ter-
mination of the sexual ducts, the evidence from embryology is
more conclusive. The female X7zphzdium embryo has at first
two pairs of ducts, and in the male the single pair shift their
position from the tenth to the ninth segment. The former
fact proves conclusively that the male and female ducts are
not homologous but homodynamous structures, and the latter
that ducts may shift their insertions from one segment to
another during ontogeny. The inference is, that sexual ducts
may arise in any nephridium-bearing segment from the pair
of nephridia which best subserve the sexual function and at
the same time interfere least with the development and func-
tion of other organs, and that a phylogenetic shifting of the
ducts has probably taken place repeatedly. The position of
the genital ostia on a particular segment cannot therefore be
regarded as a character of high morphological value, at least
for the larger groups.

Two conflicting views have long been entertained respecting
the morphological significance of the gonapophyses. Under
this term, introduced by Huxley (77), we may include the ap-
pendages of the eighth to the tenth abdominal segments in the
female and such of their homologues as persist in the male.
In the female, these appendages go to form the ovipositor.
According to Lacaze-Duthiers (49-53) they are not true append-
ages, z.e. homodynamous with the legs, mouth-parts, etc., but
simply modified ventral sclerites. Haase (’89), too, believes that
the gonapophyses are not true appendages but ‘“ Integument-
bildungen von etwas héherer Werthigkeit als die Griffel,” or
styloid processes which are found inserted at the bases of the
legs in some Myriopods and Thysanura. A similar view ap-

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 135

pears to be held by Grassi (89). All these authors base their
conclusions solely on comparative anatomical data.

Other observers, including Weismann (66), Huxley (77);
Uljanin, Kowalevsky (73), Kraepelin (72), Dewitz (75) and
Cholodkovsky ('912) regard the gonapophyses as homodynamous
with the true ambulatory appendages. Most of these authors
adduce support for their views from the origin of the ovipositor
during the larval and pupal stages. The ovipositor and sting
have been traced in Orthoptera and Hymenoptera to two pairs
of imaginal disks — one situated on the eighth, the other on the
ninth abdominal segment. On the latter segment the pair of
disks gives rise to a bifurcate or double pair of appendages.
(Dewitz, Kraepelin, etc.) But the mere fact that these append.
ages arise from imaginal disks is not sufficient evidence of their
homodynamy with ambulatory appendages, since the wings of
the Metabola also arise from imaginal disks, yet cannot belong
to the same category as the ambulatory appendages. The
imaginal disks of the gonapophyses must be traced into the
embryo and a connection clearly established between them and
the embryonic appendages, before the view advocated by
Huxley, Uljanin and others can be said to rest on a secure
foundation. X7zphzdium supplies this hitherto missing evidence.
In this form there can be no doubt concerning the direct con-
tinuity of the embryonic appendages with the gonapophyses.
One embryo which had just completed katatrepsis still showed
traces of all the abdominal appendages. The pairs on the
eighth, ninth and tenth segments were somewhat enlarged.
In immediately succeeding stages the appendages of the second
to sixth segments disappear ; the pair on the seventh disappear
somewhat later. Up to the time of hatching the gonapophyses
could be continuously traced, since in Xzphzdium there is no
flexure of the abdomen as in other forms to obscure the ventral
view of the terminal segments. From the time of hatching
Dewitz ('75) has traced the development of the ovipositor in
another Locustid (Locusta viridissima) so that now we have
the complete history of the organ.

While there can be no doubt about the appendages of the
eighth and ninth segments, which go to form the two outer

136 WHEELER. [Vor. VIII.

sheaths of the ovipositor or sting, the development of the
innermost pair of blades is by no means so satisfactory. But
whether this pair is only a portion of the ninth pair of ap-
pendages, as most authors claim, or represents the tenth pair
of appendages, as I maintain, the main question at issue is
in no wise affected ; for it still remains true that the ovipositor
consists of two or three pairs of modified ambulatory limbs.

In the male X7zphidium embryo it was claimed that the pair
of appendages on the ninth segment persists to form the defini-
tive styli; those of the eighth and tenth segments disappear-
ing very early. The continuity of the styli with the embryonic
appendages was quite as satisfactorily observed as the con-
tinuity of the ovipositor blades. Cholodkowsky has made an
exactly similar observation on A/atta (91%). The styli are,
therefore, the homologues of the second pair of gonapophyses.
Haase must therefore have gone astray in seeking to homo-
logize the styli with the styloid processes, or “ Griffel,” for
the styli are modified ambulatory appendages. Moreover, if
my interpretation is correct, he cannot have found, as he
claims, the evanescent rudiments of styli in young female
Blattids, since the second pair of “anal palps”’ are the homo-
logues of the styli (vzde Huxley, '77).

VII. THe SuBG@:SOPHAGEAL Bopy IN XIPHIDIUM AND BLATTA.

This structure, of which I have elsewhere (92) given a brief
preliminary account, makes its appearance in the AX7phidium
embryo, in a stage a little earlier than F. The somites in the
oral and thoracic segments are then established as closed sacs.
The stomodzum is still a relatively shallow depression, and
the entoderm-bands starting from its inner end have made but
little progress. Sagittal (Fig. 61) and frontal sections (Fig.
62), through the heads of embryos in Stage F, show several
interesting details. A pair of somites (coe) lie in the mandi-
bular segment, and previous to this stage there was also a pair
of small somites with indistinct cavities in the tritocerebral
segment (éc). The planes of section in the two figures are
such that the deutocerebral somites are not shown. A mass

No. 1.] CONTRIBUTION TO INSECT EMBRVOLOGY. 137
of cells, the subcesophageal body, colored pink in the figure,
extends between the cesophagus and the mandibular somites.
The origin of this mass is obscure. It may arise from the
ectoderm of the cesophagus, to the inner end of which it is
attached (Fig. 61), or it may come from the entoderm (nyo 1
believe, however, that it arises from neither of these sources,
but from the mesoderm, which in a preceding stage formed
the abortive somites of the tritocerebral segment. In frontal
section (Fig. 62) the mass of cells is A-shaped, with the
juncture of its two arms attached to the lower surface of
the cesophagus. The distal ends of the arms are applied to
the anterior walls of the mandibular somites. The separate
cells are often sharply wedge-shaped and appear to be sepa-
rated by clear spaces. They grow somewhat, lose their triangu-
lar outline and become more rounded. At the same time they
tend to fuse in curved strings, with their broad edges applied
to one another. This condition is seen in Fig. 63, which is
taken from a section through the organ of an embryo in Stage
G. The cytoplasm is now very granular, and has a distinctly
yellow tint even in unstained sections; like the neuroblasts
and germ-cells it absorbs picric acid with avidity. Vacuoles
have begun to make their appearance, and the walls between
adjacent cells are disappearing. . The volume of the nuclei
remains constant, but the cytoplasm enlarges up to the time
of hatching. Fig. 64 is a part of a section through the sub-
cesophageal body of a 7 mm. larva of Xiphidium fasciatum.
The condition of the organ is essentially the same as at the
time of hatching. The increase in volume of the cytoplasm is
clearly shown. Instead of being granular, as in the younger
stages, the protoplasm is now so filled with small vacuoles that
it is reduced to a coarse reticulum.

In the suboesophageal body of a larva 9 mm. long, signs
of degeneration have begun to appear. The small vacu-
oles fuse in the centres of the cells, leaving only the cell-
walls as ragged envelopes. The nuclei become somewhat poly-
gonal in outline. At this time the organ is found attached to
the anterior ends of the salivary glands and to the large trunks
which run forward into the head from the first thoracic trachez.

138 WHEELER. [Vou, VIII.

>

A section from a very young nymph (10 mm. long), is shown
in Fig. 65. The cytoplasm of the fused cells is reduced
to a ragged mass in which the irregular nuclei are suspended.
Their chromatin is aggregated in rounded masses—a sign of
advanced degeneration. ~In this stage the organ is much
shrunken in size so that one is led to conclude that part of it
has already been absorbed. In a little later stage the last
traces of the organ have disappeared.

A subcesophageal body essentially like the one here described
occurs also in Llatta. It, too, has the characteristic yellow
tint. In his study of the development of 4/atta Cholodkowsky
appears to have seen this peculiar structure, though he regarded
it as a portion of the fat-body. At page 52 ('918), he says:
“Die Entodermlamelle umwachst den Nahrungsdotter dorsal-
warts und von allen Seiten; der Vorder- und Hinterdarm
liegen nun ausserhalb des Nahrungsdotters und werden vom
homogenen Dotter umspiilt, in welchem (besonders neben dem
Esophagus) kleine blasse Zellen liegen, die sich in Fettkorper zu
verwandeln scheinen.’1 The organ is shown in Cholodkowsky’s
Fig. 68, Pl. VI. In other writers on insect embryology I find
no mention of this interesting structure.

In the Rhynchota, to judge from a few observations on the
embryos of Zaztha fluminea, the subcesophageal body occurs in
a slightly modified form. Here it consists of a number of loose
spherical cells lying on either side and a little below the
cesophagus. The nuclei are large and spherical and the com-
pact and finely granular cytoplasm has a distinct yellow cast.
Though these cells vary in size (11-15) they are always larger
than the cells of the surrounding tissues (6.34). Beyond this
stage I could not trace the organ in Zaz¢ha on account of lack
of material.

The subcesophageal body may always be readily distinguished
from the fat-body of the oral and more posterior segments by
the peculiar structure and arrangement of its cells and by its
yellow tint. I therefore regard it as an organ suz genesis. It
belongs to the category of embryonic or early larval organs,

1 The italics are mine.

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 139

and this alone would suffice to distinguish it from the fat-body
which persists throughout life.

It is perhaps premature to advance any hypothesis as to the
function and morphological significance of the subcesophageal
body, but I may call attention to its possible homology with an
organ in the Crustacea. The researches of Viallanes and St.
Remy go to show that the tritocerebral segment of insects is
homologous with the second antennary segment of Crustacea.
In the latter group of Arthropods this segment is provided
with the green-gland, a structure which develops from the
mesoderm and is generally regarded as a modified nephridium.
The subcesophageal body, providing it arises from the meso-
derm of the tritocerebral segment, may be all that remains
of this same pair of nephridia in the cephalic region of
insects.

VIII. TECHNIQUE.

Aiphidium eggs, like those of other Orthoptera are not
easily sectioned in the younger stages, because their yolk
bodies are rendered so brittle by the hardening fluids and are
cemented together with so little protoplasm that they disin-
tegrate during the process of cutting. After the appearance
of the appendages the embryo may be readily dissected away
from the yolk either in the fresh or hardened egg and mounted
or sectioned by itself. In the study of the envelopes, where
it is necessary to section the whole egg, the following method
gives fairly good results :—

The eggs are taken from the galls and killed by being
placed for about a minute in water heated to 80° C1 They
are then transferred for preservation to 70 per cent alcohol in
which they should remain for several weeks, if not months, in
order to allow the yolk to harden and to shrink away from the
chorion. The neglect of this simple precaution has led many
to exaggerate the difficulty of studying insect eggs or to
abandon them altogether. After remaining in the alcohol
for some time, the chorion may be removed by tearing it

1 Alcoholic picrosulphuric acid also proved to be an excellent killing reagent.

I40 WHEELER. (Vou. VIIL

open at the broad pole and gently pushing against the narrow
pole of the yolk with one needle, while holding on with
the other to the chorion at the same pole. In the earliest
and latest stages the chitinous blastodermic membrane comes
off with the chorion, in other stages it adheres firmly
to the yolk and prevents satisfactory staining. If aqueous
stains like Orth’s lithium carmine or Grenacher’s alum car-
mine are used, the eggs should be left in them but a short
time and carefully watched as the yolk-bodies have a peculiar
tendency to absorb water till they lose the polygonal shapes
they acquired by mutual pressure, finally swell and _ fall
asunder. This is especially liable to occur in the younger
stages when the blastodermic membrane is removed. I have
as yet found no other insect egg with yolk capable of imbibing
so much water. In Grenacher’s borax carmine there is no
swelling, a reason which has induced me to use this stain in
preference to the aqueous solutions ; though the two stains
mentioned give excellent results if used with due precautions.
After dehydrating and clearing with cedar oil, the eggs are
kept from two to three hours in melted paraffine (55°C.).
Older embryos in which most of the yolk has been metabolized
need not remain in paraffine more than an hour.

Embryos isolated from the yolk in the anatreptic stages, as
well as later embryos used in sectioning, were stained in Czo-
kor’s alum cochineal. The bluish color of this stain is prefer-
able to the borax carmine in serial sections, as it is less
wearisome to the eye.

In the study of the entire embryo three different methods
may be followed with advantage.

Metuop I.—The isolated embryo is stained with borax car-
mine, all excess of the stain is removed by prolonged immersion
in acid alcohol, and the preparation mounted in clove oil or
balsam. In such preparations many of the details of internal
structure, such as the arrangement of the coelomic sacs, may
be very clearly distinguished. This method was very exten-
sively used by Graber; in fact it seems to have been the only
method which he employed for surface study. In this respect
it is decidedly inferior to

No. 1.] CONTRIBUTION TO INSECT EMBRYOLOGY. IAI

Metuop II.— The hardened eggs or embryos, freed from
their envelopes, are transferred from seventy per cent alcohol
to Delafield’s or Ehrlich’s hzematoxylin, in which they are left
not longer than thirty or forty seconds. Then they are suddenly
returned to seventy per cent alcohol, and a drop of twenty per
cent HCl is allowed to fall through the alcohol onto the em-
bryos, which almost instantly change color. As soon as they
pass from a red to a salmon tint the fluid must be hastily re-
moved and replaced by fresh seventy per cent alcohol, to which
a trace of ammonia has been added. The nuclei gradually turn
blue and throw the embryo out in bold contrast to the pale
yellow yolk. In older isolated embryos, the stain faintly tinges
the surface protoplasm, accentuates the shadows, and leaves all
the sharp depressions unstained. When embryos thus treated
are mounted in glycerin or balsam and examined with widely
opened diaphragm and Abbé condensor under a moderately
low power (about sixty diameters), the surface relief is
exquisitely sharp and clear. The exact delimitation of the
appendages, both permanent and evanescent, the tracheal
orifices, cenocytic invaginations, segments of the brain and
nerve-cord, etc., may be traced with great precision, as the
figures on Plate I will testify.

The methods here given with several modifications of my own,
was taught me by Dr. Wm. Patten, who has used it with great
success in his studies of Arthropod development, more especially
in his work on the brain and eye of Acz/ius. A very similar
method seems to have been used by other investigators (vide
Foster and Balfour, Elements of Embryology, 1883). Un-
fortunately, surface preparations with hematoxylin are not
permanent, probably on account of the acid used to extract
the stain. The color gradually fades, often disappearing
completely in the course of a few weeks. I therefore prefer
Czokor’s alum cochineal, washing in water instead of acidu-
lated alcohol. These preparations are nearly or quite as clear
as the hematoxylin preparations and keep indefinitely.

Metuop III.—This is really only a compromise between
Methods Iand II. Embryos in the katatreptic stages are
allowed to remain in Czokor’s alum cochineal till the stain has

142 WHEELER. [Vot. VIII.

penetrated as far as but not into the yolk. They are then
washed in water, dehydrated and mounted in balsam. The
sexual ducts together with their ampulle may be distinctly
traced on the yellow background of the yolk and structures
which lie just beneath the integument, like the cenocyte clusters
and the nerve-cord, may be more readily studied than in
specimens prepared by Methods I and II. The figures on
Plate V and Fig. 10, Plate I were drawn from such partially
stained embryos.

The methods here described give good results, not only with
AXiphidium and Blatta, but also with all the other insects and
crustaceans which I have examined.

The outlines of the figures in the plates were drawn with
an Abbé camera.

CLARK UNIVERSITY,
WORCESTER, MAss., May Ioth, 1892.

No.1.] CONTRIBUTION TO INSECT EMBRYOLOGY. 143

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'86a RypeER, J. A. The Origin of the Amnion. Am. Waturalist. Vol. 20.
1886.

’86D RypeErR, J. A. The Development of Anurida maritima, Guérin. Am.
Naturalist. Vol. 20. 1886.

90 St. Remy, G. Contribution A l’étude du cerveau chez les Arthropodes
tracheates. Arch. zool. Expér. (2.) Toms, and Supl. (1887), 1890.

No.\t.]) (€ONTRIBCLTION TO INSECT EMBRYOLOGY. 147

’87 SCHIMKEWITSCH, W. Etude sur le Développement des Araignées.
Arch. de Biol. Tom. 6. 1887.

85-88 SEDGWwICK, A. The Development of the Cape Species of Peripatus.
Part I-VI. Quart. Journ. Micr. Sct. Vol. 25-28. 1885-1888.

91 STILEs, C. W. Bau und Entwicklungsgeschichte von Pentastomum
proboscideum und Pentastomum subcylindricum. Zedéschr. f. wiss.
Zook 452: Bd), 189%.

82 TicHOMIROFF, A. The Development of the Silkworm (Bombyx mori)
in the Egg. (Russian.) Works Lab. zool. Mus. Moskow. Vol. 1.
1882.

872 VIALLANES, H. Sur la morphologie du cerveau des Insectes et des
Crustacés. Compt. Rend. Tom. to4. 1887.

’87b VIALLANES, H. Etudes Histologiques et Organologiques sur les
centres nerveux et les organes des sens des animaux articulés.
V. Mémoire. Le cerveau du Criquet (Oedipoda coerulescens et
Caloptenus italicus), etc. Azz. d. Sci. Natur. Zool. Tom. 4. 1887.

90 VIALLANES, H. Sur quelques Points de l’Histoire du Développement
embryonnaire de la Mante religieuse (Mantis religiosa). Revue
Biol. du Nord dela France. Tom.2. No. 12. 18go.

"91 VIALLANES, H. Sur quelques Points de I’Histoire du Développement
embryonnaire de la Mante religieuse (Mantis religiosa). Anal. des
Sct. Natur. Tom.11. 1891.

89 VoELTzKOw, A. Entwicklung im Ei von Musca vomitoria. Ard. zool.
soot. Inst. Wiirzburg. 9. Bd. 1889.

90 WaTASE, S. On the Morphology of the Compound Eyes of Arthro-
pods. Stud. Biol. Lab. Johns Hopkins Univ. Vol. 4. 1890.

’66 WEISMANN, A. Die Metamorphose der Corethra plumicornis, ein
weiterer Beitrag zur Entwicklungsgeschichte der Insecten. Zeztschr.
f. wiss. Zool. 16. Bd. 1866.

’89a WHEELER, W. M. Ueber driisenartige Gebilde im ersten Abdominal-
segment der Hemipterenembryonen. Zool. Anz. 12. Jahrg.
1889.

'89b WHEELER, W. M. The Embryology of Blatta germanica and Dory-
phora decemlineata. /ourn. of Morph. Vol. 3. 1889.

908 WHEELER, W. M. On the Appendages of the First Abdominal Seg-
ment of Embryo Insects. Zvaus. Wis. Acad. Sct., Arts and Letters.
Vol.'8: “18090.

°90b WHEELER, W. M. Note on the Oviposition and Embryonic Devel-
opment of Xiphidium ensiferum, Scud. Jusect Life. Vol. 2.
Nos. 7. and 8. 18go.

790° WHEELER, W. M. Ueber ein eigenthiimliches Organ im Locustiden-
embryo. Zool. Anz. 13. Jahrg. 1890.

91a WHEELER, W. M. The Embryology of a Common Fly. Psyche.
June, 1891.

’91b WHEELER, W. M. The Germ-band of Insects. Psyche. July, 1891.

148 WHEELER.

'91¢ WHEELER, W. M. Neuroblasts in the Arthropod Embryo. /Jourz.
of Morph. Vol. 4. 1891.

'92 WHEELER, W. M. Concerning the “Blood-tissue” of the Insecta.
Psyche. ¥eb.—April, 1892.

'78 WHITMAN, C. O. The Embryology of Clepsine. Quar. Journ. Micr.
Sct. Vol. 18. 1878.

’87 WuitmAn, C. O. A Contribution to the History of the Germ-layers in
Clepsine. Journ. of Morph. Vol.1. 1887.

’'88 WiLL, L. Entwicklungsgeschichte der viviparen Aphiden. Sfengel’s
zool. Jahrb. Abth. f. Anat. u. Ontog. 3. Bd. 1888.

’81 Witson, E. B. The Origin and Significance of the Metamorphosis of
Actinotrocha. Quart. Journ. Micr. Sct. Vol. 21. 1881.

’'89 Witson, E. B. The Embryology of the Earth-worm. Journ. of
Morph. Vol. 3. 1889.

'90 Witson, E. B. The Origin of the Mesoblast-Bands in Annelids.
Journ. of Morph. Vol. 4. 1890.

'54 ZADDACH, G. Untersuchungen iiber die Entwicklung und den Bau der
Gliederthiere. I. Die Entwicklung des Phryganideneies. Berlin.

1854.

150 WHEELER.

EXPLANATION OF PLATE I.
(Xiphidium ensiferum, Scud.)

Fic. 1 (A). Surface view of embryo during gastrulation. 4.0., indusium;
pel, procephalic lobe; 4/., blastopore; a., anal bifurcation of the blastopore ;
ams., amnioserosal fold.

Fic. 2 (2). Surface view of embryo with amnioserosal fold closed over trunk-
region. /c/.z., neuroblast-centres on the procephalic lobes. o., anterior widening
of the blastopore. Remaining letters as in Fig. t.

Fic. 3 (C). Surface view of embryo with the amnioserosal fold encroaching
on the indusial thickening ; 40.am., amnioserosal fold of the indusium ; z., pedicel
temporarily uniting the indusium with the head; az, antenna; md.s., mandibular
segment; mx.s.," first maxillary segment; m.x.s.,7 second maxillary segment ;
p.5.,'—p.s.,3 first to third thoracic segments ; a.s.," first abdominal segment. Re-
maining letters as in Fig. 1.

Fic. 4 (D). Surface view of embryo just after the separation of the indusium
from the head. Letters as in Figs. 3 and 1.

Fic. 5 (Z). Indusium spreading over the yolk. View of embryo nearly
completely submerged in the yolk, on its way to the dorsal surface. ¢éc.s.,
tritocerebral segment. Other letters as in preceding figures.

Fic. 6 (7). Surface view of elongate embryo on dorsal surface of yolk. 70.,
labrum ; md., mandible; mx.,' first maxilla; mx.,7 second maxilla; ~.*—7.,3 the
three thoracic appendages (legs); coe., coelomic sac of first abdominal segment
showing through the body wall; A/. (af.1), pleuropodium (appendage of the first
abdominal segment) ; v., yolk ; ezv/., cellular envelopes torn away from the ventral
face of the embryo; cc. (af."), cerci (appendages of the eleventh abdominal
segment).

Fic. 7 (G). Surface view of shortened embryo on dorsal yolk. c.,2 second
protocerebral lobe ; fc.,3 third protocerebral lobe; dc., deutocerebrum ; ¢c., trito-
cerebrum ; ¢., eye; x., metastigmatic, or cenocytic invagination ; af.,4 fourth ab-
dominal appendage. Remaining letters as in Fig. 6 (7).

Fic. 8 (7). Surface view of embryo turning the lower pole of the egg. 5»,
inner indusium functioning as the serosa; am., amnion reflected back over the
yolk and continuous with the membrane s7.; a¢, antenna; //., right pleuro-
podium ; 7¢/., intraganglionic thickening.

Fic. 9 (/). Surface view of embryo just after returning to the ventral face of
the egg. ¢., eye; other letters as in Fig. 8 (/7/).

Fic. 10 (A). Surface view of advanced 9 embryo just before the secretion of
the larval cuticle. d.o., “dorsal organ”; fd., oviduct; ¢a., terminal ampulla of
oviduct ; of.! (0f.°), of.? (ap.), first and second pairs of gonapophyses (append-
ages of the 8th and oth abdominal segments). cc. (a."'), cercus.

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EXPLANATION OF PLATE II.

(Figs. 11 and 12, Stagmomantis carolina; Figs. 13 and 14, Gryllus luctuosus ;-
Figs. 15-20, Xzphidium ensiferum.)

Fic. 11. Surface view of gastrula of Stagmomantis. X 150. f.0., rudiment
of indusium?; 4/., blastopore; ams., amnioserosal fold extending just over the
edge of the oval germ-band.

Fic. 12. Outline of egg of Stagmomantis showing the position and relative
size of the germ-band during gastrulation.

Fic. 13. Surface view of gastrula of Gry//us. X 150. ams., amnioserosal fold
extending just over the edge of the germ-band ; é/., deeper posterior end of blasto-
pore.

Fic. 14. Outline of egg of Gry//ws showing the relative size and position of
the germ-band during gastrulation.

Fic. 15. Surface view of a A?phidium embryo during the closure of the
amnioserosal fold over the mouth. Eight segments in the trunk. p.o., indusium ;
y- pale area between the indusium and the head; fc./., procephalic lobe ; ams.,
edge of amnioserosal fold. o., stomodeum ; md.s., mandibular segment ; m-x.s.1,
first maxillary segment ; 7.g., neural groove; s., serosal nuclei; a.s.,’ first abdom-
inal segment.

Fic. 16. Surface view of head during the separation of the indusium. Stage
of Fig. 3 (C), Plate I more highly magnified. /.0., indusium ; 7., shrunken nuclei
of the indusium ; s., serosal nucleus; z., pedicel connecting the indusium with the
head; fc./., procephalic lobe; é., labrum; 0., stomodeum; az. antenna; éc.s.,
tritocerebral segment ; md.s., mandibular segment ; mw.s.,* first maxillary segment.

Fic. 17. Median transverse section through the indusium while still a simple
thickening of the blastoderm (serosa). X 230. s., serosa; #.0., portion of indu-
sium with normal nuclei; .,! shrunken nuclei; .,7 less shrunken nuclei; @.,
thickened periphery of the organ; v., yolk.

Fic. 18. Median transverse section of the indusium while the amnioserosal
fold is closing over the disk. X 230. Letters as in Fig. 17.

Fic. 19. Median transverse section through the indusium after the complete
closure of the amnioserosal folds. X 230. a@m.,' outer indusial layer; /.0., inner
indusial layer ;_ 77.," shrunken nuclei.

Fic. 20. Median transverse section through the indusium just after it has
begun to spread. X 230. Letters as in the preceding figures.

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EXPLANATION OF PLATE III.

(Xiphidium ensiferum.)

Fic. 21. Median longitudinal section through the head of an embryo over
which the envelopes have just closed. X 230. o0., stomodzum ; am., amnion;
s. serosa; 7.0., indusium; z., pedicel uniting the indusium to the head of tha
embryo; ec., ectoderm; ms., mesoderm; wfz., vitellophag; v., yolk.

Fic. 22. Median tranverse section of the indusium when it has reached half
way round the egg. X 230. /.o., inner indusial layer ; a.,' outer indusial layer ;
J. serosa.

Fic. 23. Three normal cells from the indusium. X 700.

Fic. 24. Three cells with shrivelled nuclei from the indusium. X 700.

Fic. 25. Transverse section through basal abdominal region of an embryo
passing to the dorsal surface of the yolk. X 230. 0., neuroblasts; @é., derma-
toblasts; am., amnion; ms., mesoderm ; ec., ectoderm.

Fic. 26. Transverse section through first maxillary segment of an embryo
passing to the dorsal surface of the yolk. X 230. mg., neural groove; e7., ento-
derm. Other letters as in Fig. 25.

Fic. 27. Transverse section through the second maxillary segment of an
embryo in the stage of Fig. 6,(/) PlateI. X 230. mvwx.,* second maxilla
(trilobed) ; #7d., median-cord neuroblast ; fs., Punktsubstanz; w/z., vitellophag.
Other letters as in Figs. 25 and 26.

Fic. 28. Transverse section through the mesothoracic ganglion of an embryo
in a stage somewhat younger than G (Fig. 7, Pl. I). X 230. g.,l younger offspring
of the neuroblasts ; g.,? older offspring of the neuroblasts (ganglion cells) ; ecd.,
ectoderm; mc., median cord (“ mittelstrang”’). Remaining letters as in preceding
figures.

Fic. 29. Sagittal section through nerve-cord a little to one side of the median
line. Embryo in Stage G (Fig. 7, Pl. I). 175. md.g., mandibular ganglion ;
mx.g.,* first maxillary ganglion ; m.x.g.,7 second maxillary ganglion ; zg.,” zg.,” first
and second interganglionic depressions; #.g.,7 first thoracic ganglion; /.¢.,?
second thoracic ganglion; #d., median cord neuroblasts ; mg., their progeny ;
zzd., inner neurilemma.

Fic. 30. Frontal section through the base of the abdomen of an embryo
somewhat older than Fig. 6, (F), Pl. I. 175. é., neuroblasts; m7d., median
cord neuroblasts in the intersegmental regions; #.,3 metathoracic leg ; A/., pleuro-
podium ; a/.,?-af.,5 second to fifth abdominal appendages grazed by the knife.

Fic. 31. Transverse section through the mesothoracic ganglion of an embryo
in a stage between Fig. 9 (/) and 10 (A) Pl. I. X 230. hy., hypodermis (product
of dermatoblasts) ; 7., neuroblasts; g.2, latest progeny of the neuroblasts; s,
ganglion-cells; z/., inner neurilemma; ev/., outer neurilemma; /s., Punkt-
substanz.

Journal of Morphology Vol. Vil

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156 WHEELER.

EXPLANATION OF PLATE IV.
(Xiphidium ensiferum.)

Fic. 32. Transverse section through the head of an embryo in the stage of
Fig. 5 (E) Pl. I. X 230. J.,labrum; Zc. (0.g.), Ac.,” pe.,3 first, second and third
protocerebal lobes ; 0.f., optic plate ; e7. ms., mesentoderm.

Fic. 33. Next following section to that represented in Fig. 32. X230. 7.0.
neuroblast ; az., amnion. Remaining letters same as in Fig. 32.

Fic. 34. Next following section to that represented in Fig. 33. X 230. st,
stomodzum. Remaining figures the same as in Figs. 32 and 33.

Fic. 35. Transverse section through the labrum, in a stage intermediate be-
tween E and F (Figs. 5 and 6, Pl. I.) X 230. fc.,3 third protocerebal lobe; Z.,
labrum ; e7. ms., mesentoderm.

Fic. 36. Transverse section through labrum and brain of an embryo in Stage
F (Fig. 6, Pl. I.) X 230. 7@é., labrum ; Zc.,* pc.,? pc.,3 first, second and third proto-
cerebral lobes; of., optic plate; 7d., neuroblast; @d., dermatoblasts ; coe., head-
celom; ms., mesoderm cells; zg/., intraganglionic thickening ; sz, stomodzum ;
am., amnion.

Fic. 37. Transverse section through prelabral region of an embryo in stage
somewhat later than F (Fig. 6, Pl. I). X 145. mc., median cord; fc." (0.g.), first
protocerebal lobe (optic ganglion) ; w., orifice of involution of the intraganglionic
thickening. Remaining letters as in Fig. 36.

Fic. 38. Transverse section through optic ganglion and optic plate of an em-
bryo in Stage G (Fig. 7, Pl. I). X 145. /., clear thickening in the optic plate ;
é., eye; on., optic nerve; fc." (0.9.), optic ganglion ; a., amnion.

Fic. 39. Frontal section through the brain of an embryo somewhat older than
I (Fig. 9, Pl. I). X 175. £., problematical brain segment; fc." (0.g.), optic gan-
glion; fc,.? fc.,3 second and third protocerebral lobes; dc., deutocerebrum ; ¢éc.,
tritocerebrum ; ¢., eye; md., mandible ; »d.g., mandibular ganglion ; ~g., recurrent
ganglion ; sz, stomodzum.

Fic. 40. Transverse section of brain through the supracesophageal commis-
sure of an embryo in Stage K (Fig. 10, Pl. I). X 175, #é., neuroblasts ; zg/.,
interganglionic thickening; ¢#., clear thickening in the optic plate; coe., head-
ceelom ; /s., Punktsubstanz; o7., optic nerve. Remaining letters as in Fig. 39.

Fic. 41. Transverse section from same series as that represented in Fig. 4o,
but passing through the frontal ganglion. 175. dc., deutocerebrum; /fg.,
frontal ganglion; sé., subcesophageal body. Remaining letters as in Figs. 39
and 4o.

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EXPLANATION OF PLATE V.
(Xiphidium ensiferum, Figs. 42-46, 43-50. X. fasciatum, Figs. 47 and 51.)

Fic. 42. Tip of abdomen in surface view from a ¢ embryo which has just
passed the lower pole. (Stage J, Fig. 9, Pl. 1.) ¢5-7,° fifth to eighth abdominal
stigmata ; ¢s., testis ; m.d., vas deferens ; fa.m., terminal ampulla ; a/.,° appendages
of eighth abdominal segment; st. (ap.9), stylets (appendages of ninth abd. seg.;
ap.,*° appendages of tenth abdominal segment, in the cavities of which the
ampulle lie in this stage; cc. (ap.,") cerci.; prd., proctodzum ; az., anus.

Fic. 43. Tip of abdomen in surface view from a ¢ embryo somewhat older
than the one shown in Fig. 42. Letters same as in Fig. 42.

Fic. 44. Tip of abdomen in surface view from a ¢ embryo somewhat older
than the one shown in Fig. 43. v., yolk; ag., last abdominal ganglion. Re-
maining letters as in Fig. 42.

Fic. 45. Vasa deferentia (#.d.), with their terminal ampulle (éa.m.), from an
embryo just before the development of the larval cuticle.

Fic. 46. Tip of abdomen of a $ embryo ready to hatch. Letters same as in
Figs. 42 — 44.

Fic. 47. Tip of abdomen opened and seen from within from a ¢ larva I cm.
long. m.o., sexual orifice, c.z., connectives ; remaining letters as in Figs. 42-44.

Fic. 48. Tip of abdomen of Q embryo in a stage corresponding to that
represented in Fig. 42. ov., ovary; fd., oviduct; ¢a.f, terminal ampulla; m.d.,
vas deferens and terminal ampulla of the male type, still persisting ; 4.°—7,°, sixth
to eighth abdominal stigmata; a/.,’ persisting appendage of the seventh abdominal
segment ; 0f.! (af.°), of.? (ap.9), 0f.3 (ap.%°), three pairs of abdominal appendages
which become the gonapophyses (ovipositor) ; @z.,anus ; cc. (af.""), cerci; v., yolk.

Fic. 49. Tip of abdomen of 9 embryo seen in full surface view in Fig. to. (x),
Pl. I. mxd., location of median cord neuroblast ; c#., posterior commissure ; ¢7.,
connective ; ag., last abdominal ganglion. Other letters as in Fig. 48.

Fic. 50. Tip of abdomen of 9 embryo ready to hatch. Letters same as in
Figs. 48 and 49.

Fic. 51. Tip of abdomen of @ larva 1 cm. long, opened and seen from within;
all the parts being dissected away except the reproductive organs. vwg., vagina ;
op. (ap.*), op.” (ap.9), op.3 (ap.'°), three pairs of gonapophyses. Other letters as
in Fig. 48.

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CONTRIBUTION TO INSECT EMBRYOLOGY. 159

EXPLANATION OF PLATE VI.
(Xiphidium ensiferum, Figs. 52-63; X. fasciatum, Figs. 64 and 65).

Fic. 52. Frontal section through first to fourth abdominal segments, show-
ing segmental arrangement of the gonads. Embryo in Stage F (Fig. 6, Pl. I).
X 230. m/., longitudinal ventral muscle; gd.'¢d.2¢d.,3 gonads of first, second and
third abdominal segments ; coe., coelomic cavity; ecd., ectoderm; ef., epithelium.

Fic. 53. Transverse section through the third abdominal segment of an
embryo in Stage F, (Fig. 6, Pl. I). X 230. c., nerve cord; am., amnion, sm-s.,
somatic wall of somite; sfms., splanchnic wall of somite; v., yolk; ez., ento-
derm. Remaining letters as in Fig. 52.

Fic. 54. Frontal section through the fourth abdominal segment of an embryo
in Stage F (Fig. 6, Pl. I). X 230. The diverticula point towards the head. v.,
yolk ; ¢d.,4 gonad of fourth abdominal segment ; cve., ccelomic cavity ; ef., epithe-
lium.

Fic. 55. Section through a somite from the third thoracic segment showing
a single enlarged germ-cell protruding into the ccelomic cavity. X 230.

Fic. 56. Sagittal section through the end of the abdomen of an embryo in
Stage G (Fig. 7, Pl. I). 7., neuroteloblast?; 7é., neuroblasts; coe.’—coe.,! ccel-
omic cavities of the seventh to tenth abdominal segments ; m.d@., diverticulum of the
tenth abdominal somite which becomes the vas deferens and its terminal ampulla ;
gd@.,*° gonad in tenth abdominal segment (abnormal and atavistic); 7.c., nerve cord
in unflexed portion of abdomen.

Fic. 57. Transverse section through ninth abdominal segment of embryo
represented in Fig. 42, Pl. V, cutting the tenth pair of appendages. X 175. msc.,
muscular tissue; frd., proctodzum ; z.c., nerve cord; %., heart; ecd., ectoderm ;
m.d., vas deferens; éa.m., terminal ampulla; a/.,1° appendage of the tenth ab-
dominal segment.

Fic. 58. Transverse section of the abdomen of an embryo in the stage repre-
sented in Fig. 48, Pl. V. The greater portion of the section passes through the
ninth, its anterior portion through the tenth abdominal segment. X 175. coe.,!°
coelom of tenth abdominal segment ; 47. blood corpuscle; a.m., terminal ampulla
of vas deferens ; m.d., vas deferens disintegrating ; 09.3 (af.'°), third pair of gona-
pophyses ; z.c., nerve cord; frd., proctodzeum ; ec., ectoderm; %., heart.

Fic. 59. Transverse section through the seventh abdominal segment, taken
from the same embryo as the section in Fig. 58. X 175. vw., yolk; ez. ento-
derm; 4/. blood-corpuscle dividing; %., heart; coe.” ccelom of the seventh
abdominal segment; fd., oviduct; ¢a., terminal ampulla; o¢., oenocytes; ec.,
ectoderm ; wc., nerve-cord; af.,” appendage of seventh abdominal segment.

Fic. 60. Transverse section through the seventh abdominal segment of an
embryo somewhat older than that in Fig. 48, Pl. V. X 17 5: 0v., OVATY; mCc.,
median cord; ad., fat-body; sfms., splanchnic mesoderm. Other letters as in
Fig. 59.

Fic. 61. Sagittal section through the head of an embryo in Stage F (Fig. 6,
Pl. I). X 175. /g., frontal ganglion; 7..,! first recurrent ganglion; 7g.,2 second
recurrent ganglion; v., yolk; fc.,3 third protocerebral lobe; ms., mesoderm; JZé.,

160 WHEELER.

labrum; s¢, stomodeum; ¢c., tritocerebrum; md.g., mandibular ganglion; ¢x.,
entoderm; s.d., subcesophageal body ;_ coe.,3 ceelom of mandibular segment.

Fic. 62. Frontal section through the head of an embryo in Stage F (Fig. 6,
Pl. I). X 175. pc.! (0.g), optic ganglion; md., mandible. Remaining letters as
in Fig. 61.

Fic. 63. Transverse section through the subcesophageal body from an em-
bryo in Stage G (Fig. 7, Pl. I.). X 500.

Fic. 64. Section through subcesophageal body of larva 7 mm. long. X 500.

Fic. 65. Section through the subcesophageal body of a nymph 1 cm. long.
X 500.

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