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~ * Development of the Nine-Banded Armadillo

- from the Primitive Streak Stage to
i i; C <r Birtll . with Especial Reference

to the Question of Specific
Polyembryony

••'.I

II. H. /NFAVMAN AND J. THOMAS PATTERSON

From «Ae Zoological Laboratory, University of Texas

FIFTEEN TEXT FIGURES AND NINE PLATES

Reprinted from the JOURNAL OF MORPHOLOGY
Volume 21, No. 3

L13RARV

!

THE DEVELOPMENT OF THE NINE-BANDED ARMA-
DILLO FROM THE PRIMITIVE STREAK STAGE TO
BIRTH; WITH ESPECIAL REFERENCE TO THE
QUESTION OF SPECIFIC POLYEMBRYONY1

H. H. NEWMAN AND J. THOMAS PATTERSON
From the Zoloogical Laboratory, University of Texas

FIFTEEN TEXT FIGURES AND NINE PLATES

CONTENTS

I. Introduction 360

A. Review of the literature 360

B. Material and methods 363

C. Purpose and scope of the present paper 364

II. The female genitalia 365

III. Number, arrangement and sex of embryos 367

A. Number of embryos 367

B. Airangement of embryos 368

C. Sex of embryos 370

IV. The early embryology 371

A. The earliest stages of Fernandez 371

B. The primitive streak stage 374

C. The five to seven somite stage 380

V. History of the placenta 384

VI. History of the amnion 393

VII. History of the allantois and the umbilicus 39&

VIII. Pairing of the embryos 397

IX. Conditions in vesicles containing five foetuses 401

X. The question of identity of embryos 405

XI. Specific polyembryony and the determination of sex 406

XII. Summary of evidence for specific polyembryony 409

Bibliography 411

1 Contribution from the Zoological Laboratory of the University of Texas,
No. 105.

JOURNAL OF MORPHOLOGY, VOI . 21, NO. 3.

473380

360 H. H. Newman and J. T. Patterson.

I. INTRODUCTION
A. Review of the Literature

It is not our present purpose to attempt any comprehensive
review of the literature dealing with the development of the
Edentata, nor even of that treating especially of the armadillos.
It seems advisable, rather, to limit our survey to those contri-
butions, a knowledge of which is essential to an understanding
of the problem of specific polyembryony.

That certain species of armadillos bring forth at a birth young
all of one sex has been known for over a century. According
to Azara,2 a writer of the eighteenth century, the natives of
Paraguay and of the Argentine Republic knew that this was true
for the Mulita (Tatu hybridum). Any observant hunter, who
had been fortunate enough to capture a litter or two of young
animals in a burrow with the mother, might readily have noted such
a unique state of affairs, for the sexes are easily distinguishable.

In the latter part of the nineteenth century Herman von Jher-
ing, ('85 and '86), met with similar statements on the part of
the natives of Brazil and was sufficiently interested to attempt
a scientific confirmation of what had been until then merely an
interesting piece of folklore. Two pregnant females came under
his observation, the uterus of each of which contained eight male
foetuses, all in exactly the same stage of development. Each
foetus was described as having its own separate amnion: but all
were surrounded by a common chorion.

These conditions were interpreted in a* subsequent paper by
the same author as indicating the origin of the several embryos
from a single fertilized egg, and it was further assumed from the
facts in hand that the splitting of the original single germ into
separate embryonic primordia occurred at some period after
fertilization. Von Jhering apparently saw nothing more funda-
mental in this situation than' the discovery of a new type of ani-
mal reproduction to which he gave the name "temnogenesis."
Its bearings on the problems of sex determination and of heredity

2 Referred to by von Jhering.

Development of the Nine-Banded Armadillo. 361

were not appreciated. To him however belongs the credit of
having discovered specific polyembryony in the Mulita.

No attempt was made to secure evidence, either internal or
external, of the validity of von Jhering's suggestion until Rogner,
('01), took up the subject in connection with his studies of human
monochorial twins. On the basis of a histological examination
of the ovaries of one pregnant female of the South American nine-
banded armadillo he attempted completely to discredit the idea
that the several embryos of a litter arise from a single fertilized
ovum. Since his observations strike at the very foundations of
the question of polyembryony in the armadillos it seems neces-
sary to review his work in some detail.

The genitalia of two pregnant females were sent to him by
von Jhering, and an examination showed that the ovaries of only
one specimen were sufficiently well preserved to admit of histo-
logical examination. Sections of the other pair of ovaries showed
that a large -percentage of follicles contained more than one egg.
There were in all 52 large follicles : 11 with 2 eggs, 7 with 3, 2 with
4, 1 with 5, and 1 with 7. The two largest follicles contained four
eggs, exactly the number necessary to produce the four embryos
habitually brought forth in a litter of this species. Since the
youngest follicles never contained more than one egg the condi-
tions seen in the older ones must have resulted from secondary
fusions of adjacent follicular walls, which subsequently disap-
peared in such a way as to form a common cavity. The author's
figures are evidently accurate representations of actual observa-
tions and are calculated to convince the reader. Especially strik-
ing is the figure of a reconstruction of a series of sections through
a large pluriovular follicle in which each of the eggs has its own
thick coating of discus proligerus cells.

Rosner believes that the observed condition of four embryos
surrounded by a common chorion is to be explained by the fol-
lowing sequence of events: four adjacent follicles fuse in such a
way that four eggs are thrown into a single cavity ; on the rupture
of this compound follicle the four eggs are discharged simultan-
eously, descend the fallopian tube held together in a mass by
means of their discus proligerus cells, become fertilized, undergo

362 H. H. Newman and J. T. Patterson.

cleavage and come to a common point of attachment in the uterus;
subsequently the contiguous walls of the four blastocysts atrophy
and a single vesicular chorion is produced.

Were Rosner's observations a record of the normal conditions
in the armadillo ovary the question of specific polyembryony
would assume an aspect entirely different from that suggested
by von Jhering, and we would need to seek no further for an expla-
nation of the observed conditions. The observation that all the
embryos in a litter are of the same sex was summarily dealt with
by Rosner who considered it as interesting but in no way connected
with the presence of a common chorion. Fortunately however
there is now every reason to believe that Rosner's material was
pathological or otherwise exceptional, for no subsequent investi-
gator has been able to find in the armadillo ovary conditions such
as he described.

Cuenot, ('03), while engaged in the study of the problem of the
determination of sex, examined the ovaries of one pregnant and
of one virgin female of the species investigated by Rosner. In
the ovaries of the pregnant specimen there occurred only one
follicle of the pluriovular type and this contained only two small,
rather abnormal ova.(j3ut of 119 follicles in the ovaries of the
virgin female however three contained two or three eggs, but none
was found with the number requisite to give rise to the number
of young habitually born in a litter.

Until quite recently no further progress was made toward the
solution of the problem. In 1909, however, there appeared almost
simultaneously and quite independently, two contributions to the
subject, one by Fernandez, (J09), on the Mulita (Tatu hybridum),
and the other a preliminary report by the present writers, ('09),
on the North American armadillo (T. no vemcinctum) . The iwo spe-
cies evidently agree very closely in many of the more fundamental
details of development but differ sufficiently to make it both inter-
esting and valuable, from the comparative standpoint, to have
the developmental history of both species worked out in the ful-
lest detail.

Fernandez presents somewhat detailed descriptions of seven
rather early embryonic stages and enters upon a brief discussion

Development of the Nine-Banded Armadillo. 363

of some of the more important questions involved. He was espec-
ially fortunate in securing in a good state of preservation two very
young embryonic vesicles in which the demarkation of the several
embryonic primordia had not yet manifested itself. For the equi-
valent of this stage we have looked in vain and hence,. for the pre-
sent at any rate, are compelled to rely on Fernandez's description
for an explanation of our own earliest stages. Since it is neces-
sary constantly to refer to Fernandez's work in the body of the
text no further comment of an introductory character is needed
here.

At this point it becomes necessary to refer to our own pre-
liminary report in order to correct the description of fig. 3 in
that paper. The specimen there figured was presented to us
with the statement that it was intact in every respect, except
that the uterus and the contained vesicle had been slit open along
the mid- ventral line. On the basis of this statement, together
with a study of the external features, we reconstructed the vesicle
in situ. Our subsequent investigations of fresh specimens has
led us to suspect that what we took to be a young vesicle was in
reality only the villous portion of a somewhat later stage.

B. Material and Methods

During the past two years we have had the opportunity of
examining 137 females of the native armadillo, together with a
considerable number of males. During the breeding season hun-
ters employed to collect material for us covered a wide range of
territory in south-central Texas. These men were frequently
obliged to haul the living animals through rough country for dis-
tances of fifty miles or more in order to reach an express office
whence they could be shipped to our laboratories. As a rule
a number of days elapsed between the capture of the annuals
and their arrival in Austin. This delay would serve in part to
explain our ill success in securing the earliest embryonic stages.
In order to obtain a complete series we believe it will be necessary
either to breed the animals in captivity or to accompany the hun-
ters on their expeditions so as to lose no time in examiningjfreshly

364 H. H. Newman and J. T. Patterson.

fertilizecjlfemales. Although we fully expect to secure the earliest
stages in the course of time it seems inadvisable for us to post-
pone the publication of the results thus far obtained, results suffi-
ciently clean cut in themselves to form the basis of a self-consis-
tent and fairly well rounded embryological account.

At present we have in our possession seventy embryonic vesicles
comprising a close series of stages ranging from the primitive streak
stage to birth.

Little need be said about the methods employed. To each ani-
mal that reached the laboratory was given a number and a page
in a ledger where all facts that might be of interest were recorded.
In case the carcase was to be thrown away complete records of
all data that might be useful in the future were kept. The ovaries
of the majority of the females were fixed in the standard cytologi-
cal fluids. Every part taken from a given specimen was numbered
accordingly. Much of the data thus gathered proved useful
during the course of the work and we have no doubt that all of it
will ultimately serve to throw light on future investigations.

C. Purpose and Scope of the Present Paper

In this our second contribution to the developmental history of
the armadillos the main purpose in view is to establish the fact of
specific polyembryony and thus to clear the way for future in-
vestigation. A more or less tentative explanation of its causes
and of the conditions and relations that result from it is hazarded
on the strength of the evidence now in hand, which is internal in
contradistinction to that derived from an examination of the ovar-
ies and testes, no detailed discussion of which is attempted at
present.

Although the question of polyembryony is the central problem
it is impossible to treat of it as an isolated phenomenon for the
reason that many curious developmental processes are intimately
associated with it. The history of the amnion and of the placenta,
for example, would be indecipherable apart from the fact of poly-
embryony, and the inter-relationships of the embryos admit of
a rational explanation on no other basis. The associated phenome-

Development of the Nine-Banded Armadillo. 365

non of germ layer inversion is also (indissolubly) bound up with poly-
embryony and in turn involves many peculiar and interesting
relations.

Any adequate treatment of the principal problem will therefore
necessitate the presentation of a somewhat complex array of
facts whose combined verdict will, we trust, establish our main
contention.

Except in the case of the two earliest stages described no at-
tempt is made to present a detailed account of the organogeny of
the species. No doubt such a study would reveal many facts
of interest to the specialist in mammalian embryology, but would
serve only to cloud the main issue with obscuring details.

II. THE FEMALE GENITALIA

The uterus is simple and not unlike that of the primates in
form. In the non-pregnant condition it varies somewhat in size
and shape according to the previous history of the individual.
In old females that have produced a number of litters the organ
though non-pregnant may be distended to several times its normal
size, often leading the observer into the vain hope of finding the
earliest stages. The uterus of the virgin adult presents a less
modified condition and will furnish a basis for the accompanying
detailed description.

The average dimensions of the non-pregnant uterus are as fol-
lows : 13 mm. from the tip of the fundus to the junction of the cer-
vix with the vagina, 15 mm. between the points of entrance of
the* two fallopian tubes, and 10 mm. deepdorso-ventrally. Viewed
from the dorsal aspect the uterus appears to be broadly kite-
shaped (fig. 7) with the posterior angle blending into the vagina.
The fallopian tubes are approximately straight where they enter
the uterus, but near the ovaries are strongly convoluted, each
ending in a hood-shaped fimbriated infundibulum, which, with
the aid of a posteriorly directed flap of the broad ligament,
covers a large part of the ovary and thus renders the escape of
the ovum into the body-cavity well-nigh impossible. The points

366 H. H. Newman and J. T. Patterson.

of entrance of the fallopian tubes are about equadistant from
the tip of the fundus and the vagina, thus rendering the cavity of
the uterine body much larger as compared with that of the cervix
than is the case in the human uterus, where the tubes enter prac-
tically at the distal end of the organ.

The ovaries are kidney-shaped having the convex side directed
anteriorly, with reference to the axis of the animal. In virgin
females the two ovaries are approximately equal in size, but in
individuals that are or have been recently pregnant there is always
a considerable difference in the size of the two ovaries. The larger
one may be two or three times as large as the smaller, and this
greater size is invariably due to the presence of a single enormous
corpus luteum, the actual bulk of which may be much greater than
that of the remaining ovarian tissue. There are found not infre-
quently smaller bodies (resembling in histo!6gical appearance the
large corpus luteum) which are crowded to one end of the ovary and
suggest by their shrunken and irregular form that they are either
relics of a previous pregnancy or simply the lutea of ova which
were never fertilized. It may be stated without hesitation how-
ever that there is never more than one large and prominent corpus
luteum in the ovaries of a pregnant female.

The mucosa of the uterus is undoubtedly deciduate in charac-
ter, as may be seen in the illustration of a section taken from a series
cut through a pregnant uterus and its contents (fig. 1). Even
at the comparatively early period represented it can readily be
seen that the mucosa is separated from the outer layers of the uterus
by a lymph space of considerable magnitude.

Since the young embryonic vesicle always gains attachment to the
mucosa near the tip of the fundus it is not a difficult matter to
orient it with reference to the uterine axis. It will be found con-
venient to refer to the fundus and cervix ends of the vesicle, the
former being the original attached and the latter the original free
end. The axis of each embryo is also related to that of the
uterus, in that its anterior extremity is directed towards the
cervix end of the vesicle, except in advanced conditions when the
length of the umbilical cord occasionally permits an embryo to
reverse its position within its amniotic sac.

Development of the Nine-Banded Armadillo. 367

The pregnant uterus assumes a variety of shapes in different
individuals. At approximately the same period of pregnancy it
may be either elongated or comparatively broad, either blunt
or pointed at one or both ends, and either simple or clearly
bilobed dorso-ventrally at the fundus end (figs. 42 and 43).
These various forms are not due to the position or arrangement
of the foetuses, which in this respect are practically constant, but
probably to individual variation influenced by the previous func-
tional history of the organ.

III. NUMBER, ARRANGEMENT AND SEX OF THE
EMBRYOS

A. Number of Embryos

In sixty-five out of seventy cases there were four normal embryos
in a vesicle. It may be assumed then that four is typical for
the species. Three atypical conditions occurred which may be
listed as follows:

1. Vesicles containing five normal embryos (three cases, nos.
28, 91, 108).

2. Vesicle containing three normal embryos each measuring
15 mm. and one decidedly abnormal embryo 7 mm. in length
(no. 57). No doubt this vesicle was destined to produce a three-
embryo litter.

3. A case of twins (no. 137 ). These were born in captivity.
A very careful examination of the uterus and intestines of the
mother convinced us that there were no other young born. This
may have been a case somewhat like the preceding except that
two embryos degenerated instead of one.

There appear not infrequently in otherwise normal embryonic
vesicles small amniotic sacs that usually contain the more or
less completely degenerated remains of what may once have
been extra embryos. In one case (no. 108), a vesicle with five
normal 'embryos, such a sac appeared, which, if truly the repre-
sentative of an extra embryo, would furnish an example of a six-

368 H. H. Newman and J. T. Patterson.

embryo vesicle. In another case (no. 17), which is peculiar in
several other respects, there occurred a small empty amniotic
sac fused firmly to the wall of the Trager and connected with
the amniotic sac of a normal embryo by means of an amniotic
canal similar to those of the other embryos. In still another
case (no. 9) a fairly large sac in the Trager region was connected
by means of a perfect amniofcic canal with that of a normal
embryo (fig. 44) . There is little doubt but that these sacs repre-
sent the remnants of supernumerary embryos and as such are
the equivalent of those described by von Jhering and Fernandez.
It is interesting to note in this connection that Tatu novem-
cinctum shows a stronger tendency toward stability in the number
of foetuses in a litter than does T. hybridum. There is evident,
however, in the latter species, a tendency to produce eight young
in a litter, just twice the number typical for our species. The
numbers of individuals in a litter ranges, however, from seven to
twelve.

B. Arrangement of Embryos

In order to clear the way for the description of the early embry-
onic conditions it should provisionally be pointed out that the
four embryos of this species are arranged in pairs, one pair to
each lateral half of the uterus. The upper embryo of the left
hand pair usually occupies the dorsal amniotic quadrant and is
therefore referred to as the-" dorsal embryo " (no. Ill) . The lower
embryo of the left hand side occupies the left lateral amniotic
quadrant and is referred to as the "left lateral" embryo (no.
IV) . The lower embryo of the right hand pair occupies the ven-
tral amniotic quadrant and is the "ventral embryo" (no. I),
while its mate, occupying the right lateral quadrant is spoken of
as the "right lateral" embryo (no. II). Nos. I and II constitute
the right hand pair and nos. Ill and IV the left.

The orientation of the vesicle in the uterus and the arrange-
ment of the four embryos with reference to the vesicle and to
one another is rather precise, so that a plane running from the
mid-dorsal to the mid-ventral line of the uterus would divide

Development of the Nine-Banded Armadillo. 369

FIG. 1. Outline camera drawing of a transverse section through a pregnant
uterus measuring about 15 mm. long by 14 mm. wide. Line D -V is drawn from the
points lying at the middle of the dorsal and ventral sides of the vesicle. It divides
the section of the vesicle into halves. Embryos I and II lie in the left hand half, and
II and IV in the right hand half. a. a., line of attachment of the amnion to the ves-
icle; e.v., a small extra chorionic vesicle, which is not fused with the larger one;
i.L, intestinal loop; l.s., lymph sinus between the wall of the vesicle and the uter-
ine mucosa, urn. X 9.

the two pairs of embryos and their placental areas from each
other. There may be a secondary shifting of the positions of the
various amniotic sacs, so that in the definitive condition one may
find the upper embryo of the right hand pair occupying the dorsal
position, which in the great majority of cases is occupied by the
upper left hand embryo. Such a shifting might easily occur at
any time before the walls of the various amnia fuse firmly with
the chorion, a process that does not occur until a late period of
gestation. Previous to this time each amnion is attached to the
chorion only along a meridional line, an attachment that would
permit the whole sac to swing almost as readily to one side as to

JOURNAL OF MORPHOLOGY, VOL. 21, NO. 3.

370 H. H. Newman and J. T. Patterson.

the other. Reference to fig. 1 will show that the amnion of embryo
II, especially after the amnia have increased considerably in
size, might readily overlap the line D-V, so its embryo would
occupy the dorsal amniotic quadrant. The same shifting might
equally well occur on the ventral side. Such shiftings might take
place however without affecting in any way the point of the em-
bryonic attachment, which is immediately adjacent to the origi-
nal amniotic attachment (fig. 1, a. a.). Such departures from the
typical arrangement of embryos in the vesicle are rather rare,
and are not to be considered as of prime importance, for they
in no way affect the pairing of embryos, a relationship depending
on the point of attachment of the latter which is equivalent to
their point of origin. The significance of this arrangement is
discussed in a subsequent chapter.

C. Sex of Embryos

In thirty-eight embryonic vesicles the foetuses are sufficiently
advanced to permit of the accurate determination of their sex.
There is no exception to the rule that all embryos in a vesicle are
of the same sex.

Although the armadillo hunters claim that males are consider-
ably more numerous than females we find no inequality of sexes
in the sets of embryos in our collection, exactly half of which are
male and half female. In the small collection of nine advanced
sets of mulita embryos Fernandez found that six were female and
three male. On this basis he proceeds to discuss the significance
of the apparent disproportion of sexes in the species. No doubt
a larger collection of embryonic sets would have shown no such
disproportion, for in our earlier survey of the subject of sex dis-
tribution we found a much larger proportion of males.

Development of the Nine-Banded Armadillo. 371

IV. THE EARLY EMBRYOLOGY

In the development of the nine-banded armadillo we find that
striking peculiarity, met with in the rodents, of germ-layer inver-
sion. In the case of the armadillo the inversion is intimately
bound up with the formation of the four embryos, and without
it the mechanics of specific polyembryony, as found here, would
be inexplicable. The possession of a common amnion by the em-
bryos at an early stage could only occur as a sequence to inversion,
and strongly suggests that the embryos are the product of a sin-
gle fertilized egg.

In the present' description of Tatu novemcinctum we shall
begin with the 'primitive streak stage, and leave out of account
the younger embryos (except for a brief reference to the work of
Fernandez) until we shall have secured a series covering that im-
portant period. In dealing with the following stages considerable
emphasis is placed upon the embryological details, and especially
upon the relations existing between the embryos. This is done
because these stages furnish the strongest internal evidence for
polyembryony that has been brought forward.

A. The Earliest Stages of Fernandez

It will be necessary to refer to the work of Fernandez, especially
to the part in which he describes his youngest two stages; because
they hold the key not only to the morphology of the older embryos
of Tatu hybridum, but also, we believe, to that of the stage of
T. novemcinctum which we are about to consider.

Fernandez secured two specimens of his earliest stage, and the
one he describes in detail was cut longitudinally into twenty-
three sections (10 microns thick). It was found attached to the
mucus membrane at the bottom of a fold at* the fundus end of
the uterus.

Fernandez correctly interprets the condition presented in this
early stage as one having been brought about through the process
of germ-layer inversion, and compares the vesicle to corre-

372 H. H. Newman and J. T. Patterson.

spending stages of the rat and the mouse, described respectively
by Selenka, '84, fig. 29, Taf . XIV., and Melissinos, '07, figs. 38
and 39 Taf. XXXIV. He thus finds the vesicle composed of
three sacs lying one within the other: the innermost one is the
ectoderm, the middle the entoderm, and the outer the tropho-
blast (hinfalJigen Ectoderm), which at the proximal or attached
end of the vesicle is differentiating into the Trager. The simil-
arity between the vesicle of Fernandez and those figured by Melis-
sinos (his figs. 38 and 39) is particularly striking, though, as he
points out, there are several differences. In the first place, the
mesoderm is not yet formed and the so-called Trager cavity scarce-
ly can be regarded as homologous with that of the mouse. In the
second place, the parietal layer of the yolk-sac entoderm is not
complete, but is wanting in the distal portion of the trophoblast.
If, however, we may be allowed to make a suggestion based on a
study of his photograph (fig. 6, Taf. XIX), what appear to be
scattering cells lying along the inner surface of the distal tropho-
blast might well be interpreted as representing the remains of the
parietal layer of the yolk-sac. This would make this early stage
of the Mulita very closely resemble the corresponding stages of
several other forms, as illustrated in the figures of such investi-
gators as Selenka ('84), Robinson ('92), Jenkinson ('00), and
Mellissinos ('07).

The most interesting portion of this young vesicle of the Mulita
is the inner sac, for it is the primordium out of which the ecto-
derm of the several embryos later differentia tes. Fernandez
points out the significant fact that it gives no indication of being
a multiple structure, such as one would expect to see if the vesicle
were the product of the fusion of several eggs.

The second stage of Fernandez is decidedly more advanced than
the preceding, and was found lying loose in the fund us end of
the uterus. In the preserved condition it measured 3 mm. long
by 2.3-2.5 mm. wide. The general condition of the germ layers
in this vesicle is made clear in the slightly modified copy of his
second text-figure (fig. 2). The figure, which is a diagram of a
median longitudinal section passing through two embryos, is
shaped like a horse shoe. The entire convex anterior and lateral

Development of the Nine-Banded Armadillo. 373

ex.c.

— en

— ec

-am. c.c.

-V--m.p.

— 777 S

FIG. 2. A diagrammatic longitudinal section of an early stage of the Mulita.
ex.c., extraembryonic body cavity; en., entoderm; am.c., amniotic cavity of the
embryo; am.c.c., beginning of the amniotic connecting canal; c.am.c., cavity of the
common amnion; ms., mesoderm ',m. p., medullary plate ; i r. c., Trager cavity; tr.e.,
Trager epithelium; (slightly modified after Fernandez).

margins represent the entoderm of the inverted yolk-sac, while
the concave posterior margin is covered with Trager epithelium.
Between these two regions occurs a narrow zone where the vesicle
was attached to the uterine wall (marked X) .

The Trager cavity (tr. c.) is situated in the concave space roofed
over by the Trager epithelium. While in some respects this cavity
is comparable to that of the rodents, yet for the most part any
such comparison would appear to be strained. The difficulty
standing in the way of pointing out any true homologies, however,

374 H. H. Newman and J. T. Patterson.

must be attributed to the incompleteness of the history of these
early stages — a fact which Fernandez freely admits.

Within the limits of the vesicle there are two distinct cavities :
one the general cavity of the vesicle (ex. c.), and the other the com-
mon amniotic cavity (c. am. c.). The former is lined throughout
with mesoderm, and the latter with ectoderm.

The embryos, which are in the medullary plate stage, lie in
pocket-like diverticula from the lateral margins of the floor of
the common amnion; and each embryo is connected with the lat-
ter by a short tube, which is the beginning of the amniotic con-
necting canal. The common amnion, together with its accom-
panying embryos, is the product of the inner ectodermal sac of
the earlier stage. It is not at all easy to explain fully the manner
in which the various structures presented in this vesicle develop
out of the primordia of the preceding vesicle, although the history
of several of them is self evident. To go from this to the succeed-
ing stage is, however, an easy step, and we shall therefore pass
directly to it as exemplified in our youngest vesicle of Tatu
novemeinctum.

B. The Primitive Streak Stage

We were fortunate in being able to secure from the uterus the
entire embryonic vesicle in practically a perfect state of preserva-
tion. The opportunity was thus afforded not only to make a de-
tailed study of the relations existing between the different embryos
but also to obtain a drawing of the vesicle as a semi transparent
object (fig. 12). In the preserved condition it measured 7 mm.
wide by 9 mm. long. It is slightly flattened dorso-ventrally but
in general outline is shaped like an inverted balloon, with two lat-
eral horn-like projections which fit into the openings of the fal-
lopian tubes. These horns persist for a considerable time and are
of great service in aiding one to maintain the correct orientation
of the vesicle during its early development.

The surface of the vesicle presents two distinct regions, the
lower of which fits into the fundus end of the uterus and is recog-
nized as the Trager. It is therefore covered by Trager epithe-

Development of the Nine-Banded Armadillo. 375

Hum. At the extreme lower end there is a small cap-like area
where the primitive attachment knots or cords of the Trager epi-
thelium are beginning to disappear. The other region occupies
the upper two-thirds of the vesicle and differs from the preceding
both in its greater transparency and in the complete absence of a
trophoblast. This region is the yolk-sac of the inverted type,
and consequently is covered with the entoderm. It is rather in-
distinctly divided into two portions : (1) the central zone occupied
by the embryos and their vascular areas, and (2) the cap-like
upper third in which the almost complete transparency is ob-
structed by the presence of the common amnion and its connecting
canals.

Two of the embryos lie on the upper side (corresponding to the
ventral side of the uterus) and two on the lower side of the vesicle.
Each embryo is connected with the Trager region by a rather
broad band, the belly-stalk, and is surrounded by an amnion.
Since there is an inversion of germ layers, the embryos when
viewed from the outside of the vesicle are seen from their ventral
aspects; hence, the posterior portion of each amnion is invisible
except as seen through the semi-transparent embryo. Anteriorly,
however, the lateral margins of the amnia are clearly distinguish-
able and are seen to pass forward as the tube-like, amniotic, con-
necting canals. These lie on the inner or mesodermal surface of
the yolk-sac, to which they are loosely attached, and in passing
forward they converge and finally enter the common amnion.
They do not communicate with this by four distinct openings,
but by two, for just before reaching it, the canals belonging to the
dorsal and left lateral embryos unite to form a single tube, as do
also those belonging to the ventral and right lateral embryos.
As will be pointed out in another section, this fusion of the canals
is an indicatiqn of the pairing of the embryos since the union in
each case is between individuals of a pair.

The common amnion at this stage is a comparatively small
vesicle lying at the extreme cervix end of the vesicle. The man-
ner in which this condition has been evolved from that seen in
the second stage of Fernandez is not difficult to figure out. On
the one hand, the cavity of the embryonic vesicle has undergone

376 H. H. Newman and J. T. Patterson.

an enormous extension, due in part to the natural growth of the
vesicle and in part to the modification in the shape of the Trager
wall, which has changed from concave to convex; on the other
hand, the common amnion not only has failed to keep pace with
this rapid expansion of the embryonic vesicle, but has actually
ceased to grow at all, and is destined soon to degenerate and dis-
appear. In the rapid growth of the embryonic vesicle the embryos
gradually have been drawn away from the common amnion,
and consequently their connections with it have been pulled out
into the long, slender, tube-like canals.

The embryo viewed from the dorsal side shows the exact rela-
tions existing between it and the amnion (fig. 13)-. In general
outline the embryo is slipper-shaped and throughout the greater
part of its length the amnion conforms to this contour. Both an-
teriorly and posteriorly the amnion narrows down rapidly — in
the former direction to produce the amniotic canal (am. c. c.)
and in the latter to form the posterior amniotic process (p. am. .p),
which ends blindly above the Trager. The level at which the am-
nion becomes narrower than the belly-stalk varies in different
embryos. In the embryo in question it cuts in some distance
posterior to the mouth of the allantois, but in other cases it may
cut in at a level somewhat anterior to this point.

The entire embryo, fr^m the anterior end of the medullary plate
to the posterior tip of the amnion, measures 3.5 mm., but the
embryo proper is only 2.5 mm. long. Running through the cen-
tral part of the medullary plate is the elongated primitive streak,
in which is a well developed primitive groove with a faintly de-
fined primitive pit at its anterior end. The primitive streak is
exactly 1 mm. long, and has at its anterior end a distinct head
process measuring 0.28 mm.

The outline of the allantois is seen through the embryo, and
begins a short distance back of the posterior end of the primitive
streak and extends through the mesoderm of the belly-stalk, fi-
nally ending some distance anterior to the tip of the amnion. Fer-
nandez does not describe the development of the allantois in the
Mulita, and this stage is, of course, too far advanced to give any
clue to the exact nature of its origin.

Development of the Nine-Banded Armadillo. 377

Lateral to the embryo is seen the beginning of the yolk-sac or
vitelline circulation. At this time the blood islands are well
developed and incipient blood vessels are represented by a net-
work of anastomosing cords of mesoderm. About midway be-
tween any two contiguous embryos there is a band-like area extend-
ing from the Trager to the upper limit of the area vasculosa.
The band represents the region where the boundaries of the vas-
cular areas of adjacent embryos come together, and thus corres-
ponds to the sinus terminalis of other forms, except that it is
double in composition. At the anterior margin of the vascular
area of each embryo the sinus terminalis tends to form the arc
of a circle, a tendency which, if not inhibited by the crowding of
four embryos, would result in the production of a circular sinus
exactly as in other forms. As a result of this retardation by crowd-
ing the anterior margin of the vascular zone of the four embryos
is in the form of a series of scallops.

For an appreciation of the condition of the germ layers it is
necessary to turn to a study of representative sections. In the
most typical of these, such as that taken through the primitive
pit, the neural portion of the ectoderm is thick and has the general
appearance of that of corresponding stages of other forms (fig.
19). The outer ends of the section curve decidedly upward,
especially the one on the right, but for the most part this is due
to the fact that the embryo conforms to the general curvature of
vesicle. At the ends of the section the medullary plate turns up-
ward to form the amniotic ectoderm, which is composed of a single
layer of cells.

In the central part of the section the entoderm is composed
of rather flattened cells, which, however, remain distinct from
the overlying mesoderm. Beyond the limits of the primitive
streak it becomes thicker and its cells are cuboidal in shape.
It must be kept in mind that the entoderm actually forms the
outer surface of this region of the vesicle; for the trophoblast has
practically disappeared and there are found only a few of its cells
scattered here and there along the outer surface of the entodermal
layer.

378 H. H. Newman and J. T. Patterson.

The mesoderm is arising from the primitive streak region in
the characteristic manner, and laterally it thins out and, at the
point where the ectoderm turns up to give rise to the amnion,
divides into two layers, one following closely the amniotic ecto-
derm and the other the yolk-sac entoderm.

Through the middle of the head process (fig. 18 h. p.) the ento-
derm at the center of the section is barely distinguishable from
the mesoderm, and in many places the union of these twro layers
is very intimate. This must be looked upon however as a condi-
tion which is in all probability secondary. In the region of the
head process proper the mesoderm cells are closely packed to-
gether, but are entirely separate from the neural plate.

Anterior to the head process the mesoderm rapidly thins out
practically to a single layer of cells and is easily distinguishable
from the entoderm (fig, 17).

Anterior to this section the mesoderm passes into a thickened
region of the entoderm, which obviously has nothing to do with
the mesoderm, but owes its existence to a proliferation of ento-
derm cells (fig. 16, p. p. h.). It was not detected in the whole
mount preparations of the embryos, but its extent is easily deter-
mined by a study of sections. The thickening runs through the
first five sections beginning with the anterior tip of the embryonic
shield, and its width is equal to its length, and it therefore forms
a circular plate about 45 microns in diameter. In every respect
this circular spot corresponds to the "protochordal plate" of
Hubrecht, (J08), who has laid especial emphasis upon it as a re-
gion where the entoderm is clearly a source of mesoderm forma-
tion. Whatever may be one's conviction regarding Professor
Hubrecht's interpretation one can at least be certain that the
thickening is purely of entodermal origin in this species. Our
series is here too incomplete to permit of tracing out the
history of the protochordal plate, and thus to see whether its
definitive condition is simply that of mesoderm formation, or
whether it contributes to the formation of the fore-gut or the oral
plate.

It should be stated here that the protochordal plate at the stage
under discussion thins out to a single layer towards its margin,

Development of the Nine-Banded Armadillo. 379

where it gradually passes into the surrounding entoderm. In
many places the mesoderm cells are beginning to migrate in between
the plate and the ectoderm, and especially is this true in the more
anterior sections (fig. 21). In this section, which shows six of the
mesodermal cells, the anterior limit of the protochordal plate is
represented. A very short distance in front of this the sections
pass through the amniotic canal (fig. 20), which is seen to be com-
posed of two layers, a rather thick inner ectodermal layer, and a
thin outer mesodermal layer. In some places the canal is loosely
connected with the underlying mesoderm of the yolk-sac, but for
the most part it merely lies in contact with the latter. *

In sections lying posterior to the primitive pit there is nothing
of especial note until we come to the region where the allantoic
tube takes its origin. The mouth of the allantois is in the form of
a deep groove traversing the ventral side of the anterior end of the
belly-stalk (fig. 22, al) . This is lined with an especially thick ento-
derm and gradually fades out anteriorly, but posteriorly suddenly
narrows down to form the tube. The mesoderm of the belly-stalk
appears to extend laterally to form the two wing-like processes,
which are to be interpreted as representing cross section of the
belly-stalk bands (b. b.). Externally these are covered with an
epithelium, but within are composed of a loose mesodermal tissue
in which run the umbilical blood vessels together with their accom-
panying sinuses. In section the posterior amniotic process is
triangular in shape, and is not much more than half the width
of the belly-stalk.

In sections taken through the posterior end of the embryo
(fig. 23) the allantois is reduced to a slender tube, having a small
lumen. The amnion is here triangular in cross section with the
lower angle coming in close proximity to the allantoic entoderm.
The mesoderm has much the same shape as in the preceding figure,
but may be divided rather indistinctly into two portions: (1) the
allantoic mesoderm which surrounds the entodermal tube, and
has the cells compactly arranged; (2) the more distal wings or
belly-stalk, bands through which the blood vessels run.

The semidiagrammatic longitudinal section of the primitive
streak stage is shown in fig. 24, and in connection with what has

380 H. H. Newman and J. T. Patterson.

been said above concerning the transverse sections, this may be
studied with profit. The entoderm in this section can be traced
from the protochordal plate back along the entire length of the
embryo. Throughout the greater part of its length it is composed
of flattened cells, but near the posterior end of the primitive streak
these cells become cuboidal, and in the region of the mouth of
the allantoic tube (al) take on a columnar appearance. Posterior
to the allantoic opening the yolk-sac passes back and ends abrupt-
ly at the margin of the Trager epithelium (tr. e.}.

WMle the median section does not show the lateral belly-
stalk bands which form the main connections between the embryo
and the Trager, it does, however, bring out with clearness the
union between these two as seen at the extreme tip of the embryo.
This connection (ms. co.) is simply a backward and downward
continuation of the allantoic mesoderm, which passes over into
the general mesodermal lining of the Trager region.

C. The Five to Seven Somite Stage

The general relations existing between the various parts of the
embryonic vesicle in this stage closely resemble those of the pri-
mitive streak stage, but the vesicle is almost twice as large,
measuring 15 mm. long by 14 mm. wide (fig. 14). Owing to this
increase the horns are not only relatively but actually shorter
than in the preceding stage. The Trager has undergone marked
differentiation and shows a tendency to overgrow the yolk-sac
region. The common amnion with its canals presents the same
general features as before.

The most interesting changes have occurred in connection with
the development of the embryos, and it is to these that we would
direct attention. In the first place emphasis should be placed upon
the fact that the embryos are not equally differentiated, for the
dorsal and left lateral have each, five pairs of primitive segments
while the ventral and right lateral embryos have seven. In other
words, the individuals of the same pair are in the same stage
of development.

Development of the Nine-Banded Armadillo. 381

In the five somite embryo (fig. 30) the neural folds have not yet
coalesced to form the brain vesicle, and consequently the neural
groove is open throughout its entire length. The posterior ends
of the neural folds embrace the much reduced primitive streak.
The embryos are bounded laterally by an area pellucida, which is
rapidly being invaded by the blood cords.

In sharp contrast to this embryo is the individual from the other
pair showing seven somites (fig. 31), and unless one were from the
first aware that they were members of the same set of embryos,
one would not so classify them. There are really only six and one-
half somites in this embryo, for the most anterior or cephalic
pair is connected witn the head mesoderm and is somewhat smal-
ler than the succeeding pairs (fig. 15). There is a slight indica-
tion of an eighth pair being cut off from the anterior end of the
unsegmented paraxial mesoblast.

The amnion has undergone several marked changes, chief among
which are (1) its enlargement in the cephalic region of the embryo
and (2) its reduction in width at the level of the distal part of the
belly-stalk. In this stage the neural folds have risen up and coal-
esced to form a portion of the neural tube. The point where the
fusion first occurs is at the level of the micUbrain region, and from
this place it progresses both backwards and forwards. The anter-
ior progress of the union, however, takes place rather slowly and
the final closing on the under side of the fore-brain to form the
neuropore does not occur until a period much later than this.

At the posterior end of the diverging folds the reduced primi-
tive streak is seen as a broad plate, which in the mid-ventral
region is slightly concave, and by transmitted light appears to be
decidedly thicker than the lateral portions. The notochord is
seen to arise from the anterior end of the primitive streak and
to extend forward between the folds. At the point of origin
of the notochord the primitive streak is unusually thick, forming
a distinct primitive knot, just back of which is the suggestion of
a primitive pit. At the posterior end of the primitive streak the
entodermal allantois is faintly visible. It extends backward
lying beneath the floor of the posterior amniotic process, and falls
far short of reaching the tip of the latter.

382 H. H. Newman and J. T. Patterson.

The belly-stalk now shows a tendency to form into" two bands
at the proximal or attached end. Each band later carries an umbil-
ical artery and vein from the placental disc to the embryo, that is,
they form the attachment of the umbilical cord to the wall of
the vesicle. The anterior margins of the bands are turned up to
form scroll-like structures beyond which the scale-like villi of
the Trager are beginning to extend out over the yolk-sac (fig.

15 8. V.).

There is yet to be considered the yolk sac circulation. This
consists of a net work of anastomosing mesodermal cords, which
in section are seen to be composed of a central mass of incipient
blood cells, surrounded on the upper side by an attenuated layer
of mesoderm and on the lower by the entoderm (fig. 8, b. c.).
These cords do not become hollowed out even at a much later
period than this. Indeed it is doubtful whether they ever become
functional blood vessels.

In considering the details of structure we shall confine our ac-
counts to a brief description of a series of transverse sections of
the five somite embryo, and to the median longitudinal section
of a seven somite embryo.

In the region of the neural fold the neural groove has become
greatly deepened to form the first rudiment of the brain vesicle
(fig. 26, n. g.), and the lateral margins of the medullary plate
have become tucked in beneath, thus forming a bay on each side
that is at once recognized as the lateral extensions of the head-
fold ( h. /.). In consequence of this folding the extreme lateral
portions of the amniotic cavity have had the marginal parts of
the medullary plate withdrawn from them, with the result that
the walls of the amnion have more or less collapsed, obliterating
the cavity. In all probability the obliteration is an artifact, due
to the rapture of the amniotic canals and the consequent escape
of the amniotic fluid.

In the central region the entoderm has undergone a transfor-
mation to produce the notochord ( n. ch.) which consists of a row
of columnar cells. Already the entoderm shows signs of beginning
to grow beneath the notochord, so that this structure will soon
be cut off from the archenteron. The primordia of the pharyn-

Development of the Nine-Banded Armadillo. 383

geal pouches ( ph. p.) are seen as bays of entoderm lying on each
side of the neural tube.

The mesoderm in this region is in two rather distinct forms;
the outer portion is epithelial in character and conforms to the
general contour of the entire surface of the section; and the other
part is composed of mesenchyme and lies to each side of the im-
perfectly formed brain vesicle, and consists of scattering stellate
cells.

The medullary plate gradually grows narrower as one passes
backward until the region of the somites is reached, where its
width is about one-third that of the entire embryo. The margins
of the entoderm have almost grown together beneath the noto-
chord. The mesoblastic somites are partly constricted off from
the lateral plates, which are undergoing the process of splitting
into the somatic and splanchnic layers, between which is the weak-
ly developed coelome.

In the region of the proximal part of the allantois (fig. 28) the
belly-stalk bands are very much folded, having their outer margins
turned up to form the scrolls that were noted in fig. 15. The
umbilical blood vessels in the bands are well organized and are
lined with an endothelium. The only other structure worthy
of special mention is the posterior amniotic process which is re-
duced to a small flat tube.

The final section of this series to be considered here is one taken
through the posterior end of the amnion (fig. 29). The amnion
and median posterior portions of the belly-stalk bands are con-
nected by a rather slender stalk with the Trager (ms. co.). The
exact nature of the Trager will be considered in another section,
and it remains here merely to point out that the original primitive
knots are being rapidly transformed into villi.

The longitudinal section of the seven somite embryo (fig. 25)
should be compared with that of the primitive streak stage,in
order to bring out the most significant changes occuring in devel-
opment. The notochord lies exposed throughout the greater
part of its length, but at each end it is covered beneath with the
entoderm. At the posterior end, where the notochord is covered
over, the entoderm is seen to turn back on itself for a short dis-

384 H. H. Newman and J. T. Patterson.

tance (fig. 25, en'). This is doubtless only an expression of the
same process noted in the study of cross section, in which it was
seen that the entoderm was growing in beneath the notochord.
The primitive streak has become greatly reduced, due to its
transformation into the embryo. The final change to which we
would call attention is seen in the great reduction in the length
of the allantoic entoderm (al) . It is now not more than one-half
of its former length, and is soon destined completely to disappear.

V. HISTORY OF THE PLACENTA

Certain isolated stages in the development of the placenta have
been described for at least three species of armadillo.

Kolliker (76), Milne-Edwards (78), and Duges (79-'80),
successively described the placental conditions seen in rather
advanced vesicles of the South American nine-banded armadillo.
Of these accounts that of Milne-Edwards appears to be themost
detailed. The embryonic vesicle is described as being a pear-
shaped body covered with a chorion, the proximal and distal
parts of which were thin and membranous, while the middle part
formed a thick, vascular, four-scalloped ring, composed of four
fused placentae.

A stage similar to that just cited was recently described in
somewhat greater detail by the present writers, (;09), and illus-
trated with two diagrammatic figures. This description of the
North American variety of the species seems to agree closely with
that of the South American variety as given by the authors just
referred to. No doubt we have essentially the same species on
both continents.

The only other reference to the placentation of Tatu novem-
cinctum is that of Lane ('09), who described in some detail the
afterbirth of a specimen sent to him from central Texas.

A more comprehensive account of placental conditions is found
for Tatu hybridum. Von Ihering states with reference to an
advanced stage of placentation, that there is a zonary placenta
which has nothing in common with that of the carnivora, but must
be considered as a "placenta annularis composita." Each of the

Development of the Nine-Banded Armadillo. 385

eight discoid placentae is pressed against the margins of the two
contiguous ones so that the whole set forms a ring or zone
encircling the vesicle at right angles to the long axis of the uterus.

The most detailed account of the armadillo placenta yet pub-
lished is that of Fernandez, who describes several important early
stages of this structure in connection with his account of the early
development of the Mulita.

Chapman (;01), gives a detailed description of the after-birth
of a single specimen of Dasypus sexcinctus. Excellent figures of
all structures involved accompany the text. As seen from the
foetal side the placenta appears to be truly discoidal in form,
but on the maternal side the distribution of the villi is decidedly
different from that usually found on that type of placenta. The
markedly arborescent villi are arranged in a broad, somewhat
lobose ring around the margin of the disc, leaving the centre of
the latter free of villi, a condition strongly reminding one of a much
earlier stage in the development of the placenta of Tatu novem-
cinctum, when the original saucer-shaped Trager has begun to
produce villi along the free overgrowing margin, but has a com-
paratively non-villous central area. The forked connection of
the umbilicus with the placenta is almost identical with that found
in our species. In view of these striking similarities in the placen-
tal details of the two species one is led to conjecture that the con-
ditions found in six-banded armadillo closely approximate the
ancestral conditions of the more highly specialized armadillos,
of which Tatu hybridum seems to be the most pronounced exam-
ple and T. novemcinctum the next.

In view of the fact that there has yet appeared no complete
and consecutive account of the history of the placenta of any
species of armadillo it seems worth while to devote a special chap-
ter to a description of the conditions seen in our species.

For the earliest condition it will be necessary once more to call
attention to the youngest embryonic vesicle of Fernandez. Here
we find surrounding the true embryonic layers the trophoblast,
which is attached to the uterine mucosa by means of a thickened
disc or plug of trophoblast tissue, called the Trager. This attach-
ment disc is to be considered as the primary placenta. As the

JOURNAL OF MORPHOLOGY, VOL. 21, NO. 3.

386 H. H. Newman and J. T. Patterson.

vesicle develops the Trager assumes a saucer-shaped form, as
seen in vesicles 10 and 18 (figs. 12 and 14).

It will have been noted that, owing to the inversion of germ
layers, the whole yolk-sac region of the vesicle is covered exter-
nally with entoderm, and that the trophoblast layer of this region,
which in species with a diffuse placenta ultimately forms the outer
lining of the villi, has practically disappeared. In the Trager
region, however, the original trophoblastic epithelium persists
in a somewhat modified form. This region of the vesicle consists
of an inner layer of mesoderm, at this time rather thin and free of
blood vessels, and an outer trophoblastic layer of true epithelial
character, from the surface of which protrude branching and anas-
tomosing cords of trophoblast tissue, which give to the Trager a
characteristic rough or ridged appearance (fig. 12). These cords
of cells appear to function at first as adhesive pads in that they
no doubt serve to give the vesicle a firmer grip upon the uterine
wall.

In the primitive streak stage these Trager cords, when exam-
ined histologically, show themselves to be composed of solid
masses of cells with large nuclei and deeply staining cytoplasm,
surrounded by a rather flattened layer of epithelium continuous
with that covering the general surface of the Trager. Mitotic
figures are of frequent occurrence among the cord cells, showing
rapid cell proliferation. In spme respects the appearance of the
tissue suggest a glandular function, and it may well be that from
it a secretion is given off which subsequently facilitates the pene-
tration of the villi into the uterine mucosa. That these cords
of cells are of trophoblastic origin seems certain, for the meso-
derm, the only other layer in this region of the vesicle, is a thin
membrane entirely separate from the trophoblast, which at this
period it has not begun to invade. The Trager cords then must
be formed by a process of rapid local cell proliferation which causes
masses to protrude from the surface and frequently to overgrow
it to such an extent that they appear to be almost completely
constricted off (fig. 9).

Taking the primitive streak stage as the last phase of the prim-
itive placentation, we may note thac the Trager occupies roughly

Development of the Nine-Banded Armadillo. 387

one-third of the area of the embryonic vesicle (the remainder
consisting of the yolk-sac region), that the embryos are attached
to the Trager by paired bands of mesoderm, equivalent to the
belly-stalk of the primates, and that the central area of the Trager
is freer from thickenings than the periphery.

The function of the Trager or primary placenta appears to be
not so much nutritive as merely adhesive, since tfiere are at this
time no blood-vessels in it by means of which nutriment might
be conducted to the embryos. It is highly probable that whatever
nutriment reaches the embryos comes to them by a process of
osmosis through the thin wall of the yolk-sac region of the
vesicle.

The formation of the secondary placenta occurs entirely within
the confines of the Trager and involves at tne beginning practi-
cally its whole area. A very instructive stage in the development
of the placenta is seen in vesicle 18, (figs. 14 and 15). Here the
Trager epithelium has been pushed out into short scaly villi,
which show a tendency to overlap one another as well as the mar-
gin of the yolk sac region. These protuberances have been in-
vaded by a stroma-like mesencbyme, which has arisen from the
original thin mesodermal epithelium lining both Trager and yolk-
sac regions of the vesicle. The free ends of the scale-like villi
are tipped with masses of solid gland-like tissue derived by the
breaking up of the branching cords of earlier stages into numerous
knots which are carried out to the extremities of the individual
villi. Although the general Trager epithelium which surrounds
the villi has persisted in the form of a rather thick syncytial layer
the knots are bare of covering except for the presence of an ex-
tremely thin layer of much flattened and scattered cells. The
knot cells therefore are in a position to come into most intimate
contact with the uterine tissues and probably serve as organs
of penetration, softening the maternal tissues by means of a
secretion and forcing open a path for the villi, in much the same
way as the diamond tips of drills cut away the harder materials
and open up a path for the shaft. These Trager knots forming the
tips of the villi appear to persist throughout almost the entire
foetal life in a form practically identical with that just described.

388 H. H. Newman and J. T. Patterson.

The tip of one of the branches of an arborescent villus is shown
in fig. 11. The terminal knot of cells is seen to be practically
naked, while farther down in the villus are shown blood vessels
containing nucleated blood cells.

Although the formation of villi occurs at first over almost the
entire area of the Trager, somewhat more advanced stages clearly
show the beginning of a tendency for them to become restricted
into four distinct patches near the boundary line between the
Trager and yolk-sac and around the umbilicus of each embryo.
The villi of other regions cease to grow and remain short, as in
fig. 3, even flattening down into small rounded prominences
which probably serve no nutritive function. Small patches of
these flattened villi are scattered over the central area of the Tra-
ger as well as between the newly formed placental discs of the
various embryos.

Diiring this period the Trager area of the vesicle has been grow-
ing more rapidly than the yolk-sac region, the boundary between
the two remaining at all times definitely marked. In fig. 3 is
shown semidiagrammatically the conditions in vesicle 11 in which
four discoid placentae are clearly marked off from the surrounding
areas of scattering flat villi. At this stage the placentation is
obviously discoid for each embryo.

In vesicle 14, (fig. 4) a decided change is in evidence. The four
formerly quite separate discs have undergone a considerable in-
crease in diameter and have come into very intimate contact
along contiguous margins. This fusion is more complete between
the placentae of embryos I and II and between III and IV than
between II and III or I and IV. The significance of this is dis-
cussed later. A further change is seen in that the villous margin
of the Trager region has overgrown the yolk-sac region (not fusing
at this time with the latter) and has extended the placental area
of the vesicle along the sides of the cervix cavity as far as the os
uteri. Judging by the size and abundance of the arborescent villi
in this placental annex it seems obvious that it plays the princi-
pal nutritive role at this period. One might compare this over-
growing fringe of branching villi to the cricoid placenta of Dasy-
pus sexcinctus.

Development of the Nine-Banded Armadillo. 389

IV

FIG. 3. A semi -diagrammatic representation of a vesicle seen from the dorsal
side. II, III, and IV are the placental discs of the embryos so numbered. Note
that those belonging to the paired embryos III and IV are closer together than
II and III. f.v., flattened villi of the Trager; h., horn of the yolk sac. X 2.

FIG. 4. A semi-diagrammatic drawing of the dorsal view of a vesicle slightly
older than that seen in fig. 3. This shows the fusion of the placental discs I, II, III,
and IV into a zone. Note that the fusion between the discs of III and IV (of paired
embryos) is more intimate than between II and III. In the cervix region of the
vesicle the dorsal part of the overgrowing placental ring, p.r., has been removed
to show the smooth yolk-sac lying within (y.a.}. The ring was fused with the wall
of the cervix at " z" . The dotted line lying just above the discs represents the line
along which the upper part of the ring was cut. X 2.

The yolk-sac region of the vesicle is from this period on cut off
from all contact with the uterine wall except at the mouth of the
uterus where a small circular area remains uncovered by any
outer layer. This condition persists until birth except that the
overgrowing ring of arborescent villi undergoes a gradual degene-
ration, as the placental discs increase in functional prominence
until the long, branched villi become mere flattened prominences,
which serve only to slightly roughen the membraneous area at
the cervix end of the vesicle.

390 H. H. Newman and J. T. Patterson.

The fundus end of bhe vesicle is still villous to some extent, but
the villi are so small and scattered as to interfere only slightly with
the transparency of the membrane. One can readily view the em-
bryos in situ through this end . Subsequently the villi of this region
disappear entirely with the exception of occasional small tufts
that might readily be overlooked. In several vesicles (nos. 116
and 117) this region was seen to be four-lobed owing to the pre-
sence of two thickened bands of tissue crossing each other at
right angles (figs. 37). These may indicate a demarkationof the
several embryonic primordia earlier than that seen in the differ
entiation of the embryos themselves.

Stages intermediate between that shown in fig. 4 and the defini-
tive condition can best be shown by a series of photographs.

Fig. 34 shows a somewhat older vesicle, in which the area at
the fundus end is seen to be smooth and almost free of villi.
The lobing of the composite zonary placenta is only slightly
marked.

In fig. 35 is shown the cervix end of a stage slightly more ad-
vanced than the preceding one. The heavy coating of arbores-
cent villi is seen to cover the entire cervix end of the vesicle with
the exception of the small area that lies across the mouth of the
uterus.

The dorsal surface of another vesicle, approximately of the same
age as the last, is seen in fig. 36. The vesicle is attached to the
shrunken cervix of the uterus. Here is evidenced the tendency
on the part of the composite zonary placenta to divide into two
double lateral discs. The deep notch occurs between the placental
areas of embryos II and III. The small lobe (d. 6.) is destined to
persist as a bridge between the two lateral discs.

Two farther steps in the development of the definitive placenta
are seen in figs. 38 and 39. The vesicle has grown to be several
times the size of that shown in fig. 34. Coincident with this great
increase in surface the villi in the composite zone have increased
in functional importance while those that previously over-
grew the yolk-sac region of the vesicle have degenerated, leaving
a membraneous area at the cervix pole, which in time becomes as
large or even larger than that at the other end of the vesicle.

Development of the Nine-Banded Armadillo. 391

In fig. 40 is seen a condition slightly more advanced than that
described in detail in our preliminary paper. There is now at
each pole of the oval vesicle a star-shaped clear area, with a
broad, deeply notched placental zone between, which still shows
distinct signs of its origin from four discoid placentae . The notches
are more deeply cut along the dorsal and ventral lines than along
the lateral, where the placentae of the paired embryos I and II
and likewise III and IV are so intimately fused as barely to show
the points of union.

Shortly after the condition just described the placenta takes on
what appears to be approximately the definitive condition. The
tendency to form two well defined lateral discs is carried still
farther, but in no case have we observed the complete separation
of the two placental areas. As a rule the bridge between the two
main discs is narrower on the dorsal side than on the ventral, but
its narrowness is compensated for by the presence of a heavier
coating of villi and by that of rather large placental blood-vessels
which serve to connect one main disc with the other. It seems
to be almost invariably the case that the division into the two dou-
ble lateral discs strikes only approximately along the boundary
lines of the original discoid areas, for colored injections forced
into the placental vessels of individual foetuses run across the
narrow placental bridges and invade more or less extensive and
clearly marked villous areas of the other main disc. Such a
condition is well shown in fig. 41.

The umbilical cords which may be from 18 to 20 centimeters
long are attached rather near the fundus margin of the placentae
except in rare -cases where five foetuses occur and involve the
crowding of one or more unbilical cords away from the margin.

Although a litter of young armadillos was born in the laboratory
we were not fortunate enough to secure the after-birth and there-
fore cannot describe this final stage of the placenta. A comparison
of the size and degree of development1^ the new-born young with
the oldest foetuses in our possession convinces us that the condi-
tions just described stand as definitive. Yet Lane in his recon-
struction of the after-birth of the single individual under observa-
tion fails to find connecting bridges between the main discs. He

392 H. H. Newman and J. T. Patterson.

may have observed a rare case in which the line of separation into
lateral discs passes exactly between the placental areas of the two
dorsal and the two ventral embryos. Moreover we find no such
clearly marked non-villous areas at the two poles as he describes.
The smooth area at the cervix end is in all of our specimens very
small and circular in outline, while that at the fundus end is
only vaguely outlined and frequently shows patches of flat
villi.

Any attempt to classify a placenta with the above history
meets with grave difficulties, as one might conjecture from the
multiplicity of terms applied to it by different writers. Kolliker
in his original description of the conditions of the embryonic
membranes of T. novemcinctum refers to the placenta as disc-
oidal and deciduafce. Milne-Edwards considers it to be com-
pound zonary in structure. Beddard describes it as dome-shaped
and deciduate; while Lane suggests the term " zono-discoidalis
indistincta," subdividing StrahFs class " zono-discoidalis " into
two varieties, "distincta and "indistincta."

Somewhat similar placental conditions, as found inT. hybridum,
are designated by von Jhering as indications of a "placenta annu-
laris composita." Chapman's use of the term " deciduate cricoid"
appears to be apt for the placenta of the six-banded armadillo.

Of all these terms the one that appeals most strongly as des-
criptive of a certain rather persistent phase in the development
of this multiformed structure is that used by von Thering, " plac-
enta annularis composita," but one must not forget that at first
it is simply discoidal, then cricoid, then tetra-discoidal, later
annularis composita, and finally incompletely doubly discoidal.

If animals are to be classified according to the form of their
placentae, a method 0f classification that is fortunately falling into
disrepute, it would be very difficult to classify the nine-banded
armadillo, unless we arbitrarily decide to select some particular
developmental phase of the placenta as a criterion for classifi-
cation. In such cases one would be led to chose either the primary
or the definitive condition and would thus call the placenta either
"simply discoidal" or "incompletely doubly discoidal." Other
terms scarcely find a rational basis.

Development of the Nine-Banded Armadillo. 393

The conjecture that the compound placenta of T.novemcinctum
has been derived without any fusion of "four embryonic vesicles
from a condition similar to that described by Chapman for Dasy-
pus sexcinctus, is very tempting in view of the evident close rela-
tionship of the two species and the striking resemblance that exists
between them in the details of the placenta, umbilicus and other
structures. This if true would furnish one of the most cogent
proofs of polyembryony, since we find in the more highly special-
ized species a quadruple placenta, which at a rather early period
closely resembles the definitive placenta of a more primitive
species that gives birth to single young or to twins.3

VI. HISTORY OF THE AMNION

From Fernandez's description of his earliest stage it is clear that
the common amniotic cavity is at first the hollow of the ectoder-
mic vesicle, which, through the inversion of germ layers, has come
to lie within an envelope of entoderm. Regional differentiation
of this ectodermic vesicle produces the ectodermal portions of the
embryonic primordia, which are at first contained within a single
vesicular amniotic cavity. Subsequently the individual embryos
sink into pockets in the floor of the common amnion, which has
evidently become fused to the walls of the yolk-sac at the cervix
pole of the embryonic vesicle. The posterior end of each embryo
has become fixed by means of the primordium of the belly-stalk
to the margin of the Trager, and consequently, as the yolk sac
gradually increases in size, the embryos are drawn away from the
common amnion, retaining connection with it only by means of
slender tubes, the amniotic connecting canals (figs. 12 and 14).
It has been shown that each pair of embryos withdraws from the

3 We are informed by Mr. Robert D. Carson, superintendent of the Philadelphia
Zoological Garden, that a female six-banded armadillo in captivity gave birth to:

1. A single male, on May 10, 1901.

2. Twin males, on April 6, 1902.

3. Twins (male and female), on July 19, 1902.

394 H. H. Newman and J. T. Patterson.

common amnion into a single pocket and leaves for a short dis-
tance a single connecting canal. Later each member of these pairs
loses its connection with its partner and acquires its own canal.
This secondary separation of the pairs produces a forking of each
of the original two connecting canals, a condition that persists
for a long time.

After the embryos have left the common amnion the latter
probably becomes functionless and ceases to grow. Fortunately
however it persists with all of its connections through a consider-
able developmental period, furnishing evidences of polyembryony
and of embryonic pairing. In fig. 44 it is shown still typical in
form with its connecting canals entire but with their lumens in-
terrupted with plugs of tissue. The regions between the plugs
have become distended through local secretion of ammo tic fluid,
so that the canals as a whole present a decidedly moniliform appear-
ance. In fig. 45 a somewhat more advanced stage of degenera-
tion in these structures is seen. The common amnion can no
longer be recognized but the canals are still clearly defined.
Each of these shows a number of pronounced bead-like swellings,
one of which may represent the remains of the common amnion.
These canals may persist until stages as advanced as that shown
in fig. 33, but are seldom to be detected in later stages.

The posterior amniotic processes, which in early stages were
seen to be closely associated with the development of the allan-
tois, do not persist in so marked a form in our species as in the
Mulita. Only in rare cases does one see any traces of these
structures at a period later than the five to seven somite condition
(fig. 15). In vesicle 17, however, one of the embryonic amnia
is connected by means of an amniotic canal with a sac as large or
larger than the common amnion but lying at the opposite pole
of the vesicle. This condition is no doubt exceptional and may be
accounted for on the supposition that the posterior amniotic
process of one of the embryos, on account of its unusual length,
protruded far down into the Trager region, came into contact
with and united with it, and subsequently swelled into an amniotic
sac at the point where its terminal bulb fused with the Trager
wall.

Development of the Nine-Banded Armadillo. 395

Another exceptional condition is that seen in fig. 46, where
branching from a typical amniotic canal of one of the embryos,
is an accessory canal running to an empty amniotic sac at the
center of the Trager. Such a condition is doubtless due, as was
stated in another place, to the presence of the remains of a degen-
erated fifth embryo. Teratological amniotic structures similar to
those just described were observed in a number of other cases.
In most instances there seems to be no doubt that they represent
the retarded or degenerate remains of supernumerary embryos.
The frequent occurrence of similar rudimentary embryos in
Tatu hybridum and in our own species seems in itself a strong
piece of internal evidence of specific polyembryony, for, on the
basis of the origin of the several embryos from separate eggs, it
would be difficult to understand why some should develop into
complete embryos, and others, in the same vesicle and under
practically identical conditions, should meet with so little success.

After the closure of the lumens of the various amniotic canals
all communication between the four or more amnia is cut off;
and henceforth each embryo has its own separate amnion in as
true a sense as in those mammals that produce several entirely
independent young. The developmental history of these envel-
opes is moreover in no important way different from that of
other mammals except that in late stages a gradual fusion occurs,
first of all with the wall of the chorionic vesicle and later with one
another, where, through the pressure of growth their walls have
come, into contact.

Various representative stages in the later history of the amnion
are seen in the photographs herewith presented. In fig. 44
the amnia may be seen to lie rather closely applied to the bodies
of the embryos. In fig. 33 the cavities of the individual amnia
have increased greatly in size and the sacs have assumed an o\oid
form with the narrower end directed toward the cervix pole of
the vesicle. In fig. 34, an external view of the fundus end of a
somewhat older vesicle, the amnia are seen pressed against the
membraneous area of the Trager, producing at points of contact
an added transparency, reminding one of windows through which
the embryos can clearly be viewed.

396 H. H. Newman and J. T. Patterson.

Even after the embryos have reached a length of 4 cm. the am-
niotic sacs are still quite free from one another, but a little later
they begin to fuse along contiguous surfaces. Not until about a
month before birth however do they become inseparably bound
together. After the fusion is complete the amnia occupy the entire
cavity of the vesicle and divide it into (normally) four quadrants
of equal size, each running from pole to pole. This nearly defini-
tive condition was described in detail in our preliminary account
and needs no further attention here. In fig. 46 the edges of the
amniotic partitions separating adjacent embryos may be seen at
"a." The umbilical cords are always attached just to the left of
the partitions.

VII. HISTORY OF THE ALLANTOIS AND THE UMBILICUS

The early history of the allantois was shown to be very inti-
mately bound up with that of the belly-stalk or primitive umbilicus
This intimate connection persists as long as the allantois retains
a distinguishable structure. In stages of the degree of advance-
ment shown in vesicle 17 and 11 (figs. 1 and 44) the entodermal
allantois is seen as a slender cord of cells more or less closely fused
with the umbilicus and showing here and there traces of a former
lumen. The outlines of the mesodermal allantois, however, are
no longer distinguishable from the tissues of the belly-stalk. The
allantois of the armadillos seems then to be entirely vestigeal
in later stages of development.

The umbilicus arises directly from the primitive belly-stalk,
which was shown in the description of vesicles 10 and 18 to con-
sist of paired flat bands of mesoderm uniting the posterior end
of the embryo to the margin of the Trager or primitive placenta.
That the mesodermal allantois contributes some tissue to the defi-
nitive umbilicus has already been intimated, but at no time do
allantoic blood vessels function. The placental circulation is
carried on exclusively by the umbilical vessels, paired arteries and
veins. Each artery arises along the inner margin of a belly-stalk
band, while each vein forms in the scroll-like outer margin. In
later stages the two bands fuse at a short distance from the vesicle

Development of the Nine-Banded Armadillo. 397

and continue to the body of the embryo as a single somewhat
flattened cord. The forked connection between the cord and the
vesicle is maintained as a characteristic feature of the placenta-
tion. In the definitive condition the umbilicus measures from 18
to 20 cm. in length and about 1 cm. in greatest diameter. The
veins are longer than the arteries and take an open spiral course
along the flattened edges of the cord.

VIII. PAIRING OF THE EMBRYOS

In our preliminary paper attention was called, in treating of the
nearly complete identity of the four embryos, to indications of
a still closer resemblance between the individuals of the right
and left hand pairs. In attempting to derive the four embryos
from the blastomeres of the four-cell stage the following suggestion
was offered: "This possible interpretation receives a striking
confirmation in the fact that the four embryos can be arranged
into two pairs, the individuals of which approach almost complete
identity; and these identicals are not only adjacent to each other
but are also attached to placental discs that are closely united.
If all four embryos are derived from a single egg, this is exactly
what we should expect to find; for surely the individuals derived
from one of the blastomeres of the two-cell stage ought to be
more nearly similar to each other than to the individuals of the
other blastomere."

The subsequent acquisition of a large amount of additional
data has served only to strengthen our conviction concerning this
strong tendency toward pairing among the four embryos: a
tendency that expresses itself in the method of separation of the
embryos from the common amnion ; in the fusion of the four discoid
place'ntal areas into two double lateral discs ; in the different rates
of development seen in the embryos of a single vesicle; and in the
closer resemblance, as a rule, between the paired embryos of
one double placental disc than between the embryos in general.

The forked arrangement of the amniotic canals, as was pointed
out in connection with vesicles 10 and 18, shows that the embryos
retreat from the common amnion in pairs and that only when at

398 H. H. Newman and J. T. Patterson.

some distance from the latter do the individuals of a pair sever
their intimate connection and acquire separate amnia. Subse-
quently these embryos show their pairing in their mode of attach-
ment to the definitive placental discs, embryos I and II being
attached to the right hand disc and III and IV to the left.

Fernandez calls attention in the case of the Mulita to the exact
identity in stage of development among the embryos of a set.
That this is not always fche case in our species is well brought out
by a comparison of figs. 30 and 31, two embryos from vesicle 18.

Fig. 30 represents embryo III, and IV was identical with it.
Fig. 31 was taken from embryo II but would serve equally well
as a figure of I. The difference in degree of development between
the two pairs is well marked not only in the number of somites
(5 in III and IV and 7 in I and II), but in the conditions in the
head region and in other parts.

It is not likely that a difference in rate of development between
the two pairs is of common occurrence, but the clear case of it
just presented seems worth recording not only on account of its
rarity but because it serves to emphasize the tendency of the indivi-
duals of a pair to be alike, but somewhat different from the equally
identical opposite pair.

Although of very common occurrence the pairing of embryos on
the basis of resemblances in the total number of scutes in the nine
bands of armor, is not without exception. In many cases the
pairing is so marked as to be startling, as for example in one case
where I and II each has 555 plates and III and IV each has 548 ;
or in another case where I and II have respectively 551 and 552
and III and IV have respectively 560 and' 559. In many other
cases the pairing is obvious but not so clean cut.

There are on the other hand two cases where there was a close
resemblance between three embryos, but one was strikingjy
different, as for example where II, III, IV have respectively
544, 545, 543 and I has 549; or again where I, II, III have
respectively 562, 565, 564 and IV has 573. Finally two cases
occurred in which, if any pairing at all exists it appears to be
between I and III and between II and IV, as for example where
I and III have respectively 544 and 546 while II and IV have
550 and 548.

Development of the Nine-Banded Armadillo. 399

On the whole however, in spite of these exceptions, the general
rule holds good, that the closest resemblances occurs between
paired embryos.

In this connection it should be mentioned that even where
there is exact resemblance between the individuals of a pair in
the total number of scutes in the nine bands of armor, there is
no perfect correspondence with respect to individual rows. The
resemblance in total numbers of scutes is however, a matter of
more importance than the exact manner of their arrangement
into rows, which is a secondary process. Each primary scute
is the equivalent of a well defined hair group atid these groups,
as can be seen in other regions of the body, are quite definite units,
although subject to more or less shifting before reaching their
final arrangement into rows. In a subsequent paper we expect to
make a special study of variation and heredity in the elements
of the armor and shall in this place refrain from any more detailed
reference to the subject.

Another source of data, however, which furnishes striking evi-
dence of pairing is seen in connection with a fairly common ten-
dency for regional fusion of adjacent bands of armor, or for the
occurrence of interrupted and of incomplete bands in definite
regions. Such atypical conditions occur in from three to four
per cent of all cases, a fact that we have established from an exam-
ination of considerably over a thousand shells. This comparative
rarity of occurrence, while it renders the collection of data on
pairing and identity difficult, gives to such data an added value,
in that chance resemblances are very unlikely to occur.

Only four cases of strikingly atypical armor arrangements have
so far been discovered in the collection of foetuses now in our
possession'. In one case in embryos I and II there occurred a
remarkably atypical scute arrangement in the first band of
armor, while III and IV were quite normal. In a second case
I and II showed a slight fusion between the first two rows at the
right hand margin, while III and IV showed a much more esten-
sive fusion in exactly the same region. The pairing in this case
was only a matter of degree of fusion, but there was a decided differ-
ence in extent of the region of fusion in the two pairs. In a third

400 H. H. Newman and J. T. Patterson.

case III and IV exhibit almost precisely the same atypical condi-
tion, a short interruption in the first band a little to the left of the
median line; II has an interruption in the same band, involving
considerably more than half of the total length of the band, while
I, although appearing to be perfectly normal, seems to have carried
the tendency toward the suppression of a band to the extreme in
that the whole band is lacking. In a fourth case one of the four
embryos shows a short fusion between the first two rows on the
left hand side, while the other three are perfectly normal.

Three out of four cases, then, furnish strong evidence of pairing,
while the fourth case, which is after all atypical only to a mini-
mum extent, affords an exception, whose weight can scarcely
be sufficient to discredit the evidence of the other cases.

Although the pairing of embryos is not always perfectly obvi-
ous the cumulative evidence in favor of its general occurrence
is convincing and must have some fundamental significance, an
understanding of which is undoubtedly closely bound up with the
early developmental mechanics as we shall attempt to show.

It has occurred to us that the division of the four-scalloped
placental band into right and left lateral discs migh t be dependent
upon the fact that the blood supply of the uterus comes from the
paired ovarian blood vessels that enter the uterus laterally. It
is true that the paired embryos, with very few exceptions are
located on the same side of the uterus, but that the pairing is in
any way caasally related to the fact of their location near the en-
trance of a single maternal blood vessel is highly improbable,
because the maternal blood does not reach the embryos.

It has also been suggested that the close resemblance between
the individuals of a pair might be due to admixture of foetal blood,
but we have demonstrated by the use of colored injections that
the placental area of each embryo is sharply circumscribed and
that no blood passes from one embryo to another. A common
blood environment then cannot be held accountable for the near
approach to identity seen in the pairs. Moreover it has been
shown that long before there was any sign of the definitive placen-
tation, and hence before there was any circulation of blood, pair-
ing of embryos was evident in the relationship of the amriiotic

Development of the Nine-Banded Armadillo. 401

connecting canals and in one case, in the degree of development
of the embryos.

These observations force us to the conviction that the orienta-
tion of the vesicle in the uterus and the pairing of the embryos are
expressions of the cleavage polarity and symmetry of the ovum.
The cell products of the first two blastomeres would occupy the
right and left halves of the early blastocyst and the daughter
cells derived from the first two blastomeres would normally
hold their relative positions as quadrants of such a blastocyst, so
that, although it may not be possible to note any definite demarka-
tion of embryonic primordia until a much later stage, they may be
well defined from the first. When however pairing seems to exist
between diagonally opposed embryos it might conceivably be due
to a shifting of blastomeres in the four-cell stage, which could
readily occur in such loose cell aggregates as prevail in early mam-
malian cleavage stages. A shifting upwards of two diagonally
placed blastomeres and a consequent shifting downward of the
other two would bring about a recombination of blastomeres
into two new pairs without interfering with the hereditary ten-
dencies of the individual units. Such an appeal to the imagina-
tion of the reader would scarcely be justified were it not the logical
outcome of a failure to explain the conditions on any other basis.
We are much inclined, in spite of Fernandez' failure to note any
indication of a demarkation of separate embryonic areas in his
earliest vesicles, to believe that such areas exist from the beginning
and express themselves as separate primordia only on the differ-
entiation of embryos. This view is in direct opposition to that of
Fernandez who holds that up to the time when the separate em-
bryos are distinguishable, the vesicle is a single embryo.

IX. CONDITIONS IN VESICLES CONTAINING FIVE FOETUSES

Out of a total of seventy embryonic vesicles there occurred
three in which there were five foetuses. In all of these the sex
could be determined and, curiously enough, they were all males.
Whether or not this condition is universal could not be determined.
If however it should prove that all five-embryo sets are males it

JOURNAL OP MORPHOLOGY, VOL. 21, NO. 3.

402 H. H. Newman and J. T. Patterson.

would mean that sex is determined by certain conditions in the
egg. With only three cases in hand a discussion of the matter
would be unprofitable.

In two cases out of three it was possible to enumerate the scutes
in the nine bands of armor and on that basis to determine the
varying degrees of resemblance among the embryos.

The occurrence of five embryos involves a decided asymmetry
of the placental and amniotic elements and an atypical arrange-
ment of the embryos. In each case the condition of two main
lateral discs was maintained, but one of these discs, the one to
which three embryos were attached, was considerably larger than
the other. An examination of the larger disc shows that in each
case it is composed of only two, not three, primary discs. One
of the primary discs, on the side where three embryos are attached,
is twice the normal size and to it are attached in symmetrical
fashion the umbilical cords of two embryos. Apparently there is
no regularity about the position of the double disc. In one case
the double disc is ventral, and in the other two right lateral in
position. Believing that the two embryos attached to a single
primary disc are the equivalent of one typical embryo, we shall
give them the same number, as for example, I and I'.

The following conditions are found in vesicle 91, the relative
positions of the embryos being indicated in the diagram of the
placenta, represented as cut open along the narrow dorsal bridge
and laid out flat (fig. 5) . The number of scutes in the nine bands
of armor are indicated on the figure. It will be noted that there is
distinct pairing on the normal side of the vesicle, between em-
bryos III and IV; that the resemblance between the two embryos
on the large disc (I and I') is equally close; but that there is a
wide difference bewteen these two embryos and the single embryo
on the same side (no. II).

In vesicle 108 somewhat similar conditions exist, but the vesi-
cle is laid open along the ventral bridge (fig. 6). Embryos II and
II', having a common primary placental disc, are identical in the
number of scutes but widely different from embryo I, which is
attached to the other primary disc on the same side of the vesicle.
Embryos III and IV are quite different from those on the other
side, but are fairly similar to each other.

Development of the Nine-Banded Armadillo. 403

6

FIG. 5. Diagram of the placenta of vesicle no. 91, showing the placentation and
the numbers of scutes in the nine bands of each embryo. Cut open along the dor-
sal notch.

•

FIG. 6. The same scheme for vesicle 108. Cut open along ventral notch.

404 H. H. Newman and J. T. Patterson.

In the case of the third five-embryo vesicle a satisfactory enum-
eration of the scutes was .not found possible, but the position of
he large disc was the same as in 108.

In all three cases the amnia of the three embryos occurring on
the same side are irregularly arranged. Instead of occupying
whole quadrants of the subspherical vesicle the amnion of one or
more embryos is forced away from one end and crowded past the
opposite end, thus causing the amniotic partitions to run diago-
nally across the placental discs instead of taking a meridional
course from pole to pole as in typical cases. The relative positions
of the embryos is of course correspondingly irregular so that one
is immediately struck by it when the vesicle is first exposed to
view.

The high degree of mal-adjustment seen in these vesicles would
seem to indicate that the occurrence of more than four embryos
is the expression of a coenogenetic tendency to carry polyembryony
a step farther by a doubling of the present typical number of
embryos. In the Mulita this condition has been attained and there
exists a strong tendency to double again, as seen in the frequency
of vesicles containing nine or more embryos. It appears probable
to us in view of the occurrence of one case of twins in our collection,
that in T. novemcinctum specific polyembryony had its origin in
the acquisition of a habit of producing identical twins in a fashion
similar to that seen in other mammals, that the inversion of germ
layers made it easy for this tendency to express itself still more
fully in the habitual production of four embryos. The production
of more than four embryos in our species seems to involve so
great a disturbance of a very accurate adjustment of embryos
and embryonic membranes that it seems highly improbable that
a larger number of embryos will ever become typical.

It would be interesting to find out whether there is in T. hybri-
dum any tedency of the embryos to arrange themselves into two
groups corresponding to the right and left sides of the vesicle. A
study of Fernandez' photographs (figs. 1 and 2) would seem to in-
dicate that such is the case. It is hoped that this matter will
receive some attention and that the degree of resemblances
among the embryos of the various sets will be determined.

Development of the Nine-Banded Armadillo. 405
X. THE QUESTION OF IDENTITY OF EMBRYOS

In the case of identical or monochorial twins the question of
close resemblance has been much discussed and the impression
seems to prevail that the individuals of a pair show such marked
similarity in their finer details of structure as to be practically
identical.

In our earlier contribution to this subject we were inclined to
look for the resemblances between the embryos of a litter and to
understimate the value of the points of difference. Now however
that we feel that the question of specific polyembryony has been
established, the differences among embryos interest us more
than the resemblances, because they indicate a rather marked
degree of versatility in the hereditary possibilities of a single
fertilized germ cell.

The only point of unfailing identity among the individuals of
a litter is that of sex. In 38 cases where the sex was definitely
determined there was no exception to the rule that all embryos
in a vesicle are of the same sex.

So far as dimensional differences go there is again practical iden-
tity, although in a few cases» there seemed to be a slight difference
in the size of the two pairs. In comparing one individual with
another we were forced to admit that they differed only in the
minutest details, such as the number of scutes in the armor. A
comparison on this basis is just about as searching as would be a
comparison of the number of feathers in a given feather tract of
two birds, or of the hairs in a given hair area of two mammals.
We have for the present limited our comparison to the total num-
ber of large scutes (with corresponding underlying bony plates),
in the nine moveable bands of armor. The extreme range of
variability in the total number of these plates (in all of the indi-
viduals so far examined) is rather wide, running from 511 to 620,
a range of 109. In a number of cases the individuals of a litter
exhibit a range of only five or six scutes, but as a rule the range
is wider, averaging in all cases studied twelve, or less than one-
ninth of the total range of our sample of the species. Whether
or not this represents a closer esemblance than exists between

406 H. H. Newman and J. T. Patterson.

the individuals in a litter of rats or other mammals cannot • at
present be determined.

Although the difference between the two pairs of a litter may
on the average be rather marked, that between the individuals
of a pair seldom exceeds three scutes and averages in all cases
observed less than three, while cases of absolute identity in the
total number of scutes is of frequent occurrence.

It will be remembered also that in our discussion of pairing a
considerable mass of evidence was adduced to show that even in
atypical scute arrangements a high degree of identity existed
between pairs, while in most cases the pairs differed greatly from
each other. All of these observations go to show that the identity
between the individuals of a pair is a very real thing but that the
there is nothing approaching true identity between the pairs.
The condition may well be described as a case of double identical
twins.

XI. SPECIFIC POLYEMBRYONY AND THE DETERMINATION

OF SEX

The first clue to the existence of polyembryony in the armadillos
was furnished by the discovery that all of the individuals of a litter
are of the same sex. This together with his observation of a com-
mon chorion, led von Jhering to surmise that all of the embryos
of a vesicle arise from a single fertilized egg. That this flash
of insight foreshadowed the discovery of a truth has been suffi-
ciently demonstrated, we believe, by Fernandez for Tatu hybri-
dum and by us for T. novemcinctum.

Identity of sex then is in some way closely bound up with the
phenomenon of polyembryony. Presumably all of the individuals
of a litter are of the same sex because they have been derived
from a single fertilized ovum; but this presumption involves the
corollary that sex is determined in the germ before any demarka-
tion of embryonic rudiments has occurred. The only alternative
is that similarity of environmental conditions during the devel-
opmental period has the effect of producing offspring all of the
same sex, an alternative with no factual basis, as is shown by the

Development of the Nine-Banded Armadillo. 407

following observations : that at a very early period each embryo
is surrounded by its own amnion; that a little later each draws
maternal nutriment from a separate area of the uterine wall; and
that there is no admixture of foetal blood. We are therefore
driven to the conclusion that sex is determined before there
occurs any splitting of the single germ into separate embryonic
primordia.

Opinions differ as to the exact period at which this splitting
takes place. Fernandez maintains, on the basis of his studies of
the early blastocyst of the mulita, that there is no trace of poly-
embryony until after the two primary germ layers have been
laid down. What he probably means is that previous to this time
there is no visible demarcation of the germ layers into isolated
blastodermic areas. That the real separation of embryonic rudi-
ments occurs at a much earlier period, even during the early cleav-
age stages (in our species at the four-cell stage), seems probable
in view of the discovery of pairing among the embryos, a phenome-
non for which no other explanation offers itself; and by the obser-
vations of Marchal, ('04), and Silvestri ('06), on the parasitic
hymenoptera, where each embryo in a set takes its rise from a
single cell of a rather advanced cleavage stage.

It seems highly probable then that the tissues involved in each
of the four quadrants of an embryonic vesicle, whether or not
they may show a demarcation, do really arise as the lineal descend-
ants of one of the first four blastomeres. In this sense the four
embryos are delimited at the four cell stage. It is hardly to be ex-
pected that any demarcation would be visible before the beginning
of the period when the separate embryonic shields are differen-
tiated.

The question as to the exact period of separation of the several
embryonic rudiments is one that cannot at present be definitely
settled. Even if one should be fortunate enough to obtain the
early cleavage stages it is improbable that he would be able to
observe any essential departure from the usual plan of mammalian
cleavage, for a blastomere of the four cell stage would have the
same appearance whether it were destined to produce a whole
or only a quarter of an embryo.

408 H. H. Newman and J. T. Patterson.

It seems probable from our studies of the ovaries that the ten-
dency to polyembryony is inherent in the unfertilized egg, which
is the seat of a developmental vigor somewhat more intense than
that exhibited in the ova of other mammals. This extra expresses
itself sometimes by parthenogenetic divisions and at other times
in the formation of fairly regular morulae within the confines of
the Graafian follicles. That polyembryony is simply a more nor-
mal expression of the same superabundant energy in the female
germ cells seems highly probable, and we would offer this as a
tentative explanation of the physiology of polyembryony, pending
an exhaustive study of a large collection of ovaries.

Taking it for granted then that sex is determined in the undi-
vided oosperm, the question naturally arises as to which of the
two germinal elements is the sex determiner. Cytological exam-
ination of the ovaries reveals no dimorphism of the ova. They all
have 32 chromosomes and are equally alike in other respects.
The possibility that sex might depend on which of the two ovaries
produced the egg that became fertilized as suggested by the work
of Dawson ('09) . This writer maintains on observational grounds
that in the human being the male producing ova come from the
right and the female producing ova from the left ovary. The cor-
pus luteum served to indicate which ovary functioned in any given
pregnancy. In the armadillo we have an exceptional opportunity
to put Dawson's theory to a test, for the corpus luteum of this
species is a very prominent feature of the ovary that has func-
tioned. A study of our data reveals the fact that the corpus
luteum is found with almost equal frequency in right and left ova-
ries, which coincides with the exact equality of male and female
litters. Unfortunately for the theory, however, there is no cor-
relation between the sex of the embryos and the dextrality or
sinistrality of the functional ovary. Out of twenty ^cases in which
the right ovary contained the corpus luteum, the sex of the em-
bryos was male in seven and female in thirteen; while out of
thirteen cases in which the left ovary held the corpus luteum,
the sex was male eight times and female five. Evidently then
the position of the functional ovary has no determining influence
on sex.

Development of the Nine-Banded Armadillo. 409

There is on the other hand excellent evidence that the male cell
may act as a sex determiner. Studies of the spermatogenesis
of our species show that the spermatogonial number of chromo-
somes is in all probability 31, one less than the oogonial. There
is moreover in the reduction division a very definite and obvious
odd chromosome, which precedes the other chromosomes to the
pole of the spindle and serves to institute a dimorphism of the
spermatids. That the odd chromosome is concerned with the
determination of sex is as probable for the armadillo as for the
insects and other forms in which it has been described. Both
rest on the same observations. Since it is our intention to make
a detailed study of the cytology of the germ cells in this species,
it must suffice for the present to have indicated the sort of
external, evidence of polyembryony and of sex determination we
have at our command.

The discovery of so clear a case of an accessory chromosome
in a mammal is in itself worthy of mention, since it brings us a
few steps nearer to the discovery of the physiology of sex deter-
mination in man. In addition to the intrinsic value of this dis-
covery, however, we are afforded another strong proof of specific
polyembryony, in that it is highly improbable, on the basis of
the origin of the embryos of a vesicle from several fertilized eggs
that each of these eggs would be fertilized by the same kind of
•spermatozoon. Such a possibility could be realized only through
the instrumentality of selective fertilization, the occurrence of
of which has never been successfully demonstrated.

XII. SUMMARY OF EVIDENCE FOR SPECIFIC POLYEMBRYONY

1. The uterus is simple, resembling that of the primates, which
give birth typically to one offspring at a time.

2. There is never more than one large corpus luteum in the
ovaries of a pregnant female.

3. In over 90 per cent of vesicles the number of normal embryos
is four, a number that suggests their origin from the blastomeres
of the four-cell stage. It is also unlikely that this number of ova
would so often be given off at the same time.

410 H. H. Newman and J. T. Patterson.

4. The fact that all of the embryos of a set are invariably of the
same sex strongly suggests their origin from a single fertilized egg.

5. The definite orientation of the embryos in the vesicle, and
of the vesicle in the uterus, precludes the possibility of their origin
from several eggs, even though these might conceivably be simul-
taneously given off from the ovary.

6. The inversion of germ layers presents a condition in both
Tatu hybridum and in T. novemcinctum, which could not be at-
tained by the union of several eggs to form a single vesicle. This
is the strongest piece of evidence for specific polyembryony that
has been advanced, and, to our minds, is practically conclusive.

7 . The Trager or primitive placenta, common to all four embryos,
is the morphological equivalent of that seen in the monembryonic
vesicles of certain rodents.

8. The overgrowing fringe of arborescent villi seen in middle
stages of gestation reminds one strongly of the cricoid placenta
seen in the monembryonic vesicle of the six-banded armadillo,
figured by Chapman.

9. The existence of partial or rudimentary embryos is evidence
against the idea that the several embryos have been derived from
separate eggs, for it is difficult to understand why some should
develop perfectly, while others, under the same environmental
conditions, should have so little success.

10. The pairing of embryos points to the origin of each pair
from one of the first two blastomeres.

11. The presence of an accessory chromosome in the male germ
cells suggests that the spermatozoon is the sex determiner. On
this basis the fertilization of several eggs always by the same kind
of spermatozoa seems highly improbable.

Development of the Nine-Banded Armadillo. 411

BIBLIOGRAPHY

BAILEY, Vernon. Biological Survey of Texas. North Amer. Fauna, no. 25.

1905.
BEDDARD, F. E. Mammalia. Cambridge Natural History, vol. 10.

1902.
CHAPMAN, H. C. Observations upon the placenta and youngof Dasypus sexcinctus

1901. Proc. Acad. Nat. Sci. Philadelphia, pp. 1-4.

CUENOT, L. L'ovaire des Tatous et 1'origine des jinneaux. C. R. Soc. Biolog-

1903. T. 60, pp. 1391-1392.
DAWSON, E. R. The causation of sex. London, H. K. Lewis Co. pp. 1-190.

1909.
DUGES. Annales des Sciences Naturelles, Sixieme Ser. Zool. 9. p.l.

1879.
FERNANDEZ, MIGUEL. Beitrage zur Embryologie der Giirteltiere, 1. Zur Keim-

1909. Matter-inversion und spezifischen Polyembryonie der Mulita (Ta-

tusia hybrida Desm.). Morpholog. Jahrb. Bd. 39, pp. 302-333.
HUBRECHT, A. A. W. Early ontogenetic phenomena in mammals and their bear-

1908. ing on our interpretation of phylogeny of the vertebrates. Q. J. M.
5., vol. 53, pp. 1-181.

JHERING, H. von. Ueber die Fortpflanzung der Giirteltiere. Sitzungsberichte der

1885. konigl. preuss. Akademie der Wissenchaften. Heft 47, S. 105.

1886. Ueber Generationswechsel bei Saugetieren. Archiv /. Anatomie

und Physiologie, Physik. Abteilung., s. 442-450.

Nachtrag zur Entwicklung von Praopus. Ebenda. s. 541-542.
JENKINSON, J. W. A reinvestigation of the early stages of the development of

1900. the mouse. Q. J. M. S., vol. 43, pp. 61-82.
KOLLIKER, A. Lehrbuch der Entwicklungsgeschichte des Menschen. p. 362.

1876.

LANE, H. H. Placentation of an armadillo. Science, N. S. vol. 29, p. 715.
1909.

1909 . Some observations on the habits and placentation of Tatu Novem-

cinctum. Bull. State Univ. of Oklahoma, no. 1. pp. 1-li.
1909. A suggested classification of edentates, idem, no. 2. pp. 21-27.
MARSCHAL, P. Recherches sur la biologic et le developpement des Hynenopteres
1904. parasites. I. La polyembryonie specifique ou germinogonie.

Arch. Zool. Exper., Series 4, vol. 2, pp. 257-335.
MELLISSINOS, KONST. Die Entwicklung des Eies der Maus. Archiv /. mikr.

1907. Anat. Bd. 70 pp. 587-628.

MILNE-EDWARDS, A. Sur la conformation des placenta chez le Tamandua,
1872. Ann. des Sci. Nat., 15.

1878. Recherches sur les enveloppes fcetales du Tatou a neuf bandes.

Ann. Nat., Ser. 6. Zool. T. 8.

NEWMAN H. H. AND PATTERSON, J. T. A case of normal identical quadruplets
1909. in the nine-banded armadillo, and its bearing on the problems of
identical twins and of sex determination. Biol. Bull., vol. 17,
no. 3, pp. 181-187.

412

H. H. Newman and J. T. Patterson.

ROBINSON, ARTHUR. Observations upon the development of the segmentation

1892. cavity, the artfhenteron, the geiminal layers, and the amnion in

mammals. Q. J. M. S., vol. 43, pp. 369-456.
ROSNER, M. A. Sur la genese de la grossesse gemellaire monochoriale. Bull.

1901. Acad. Sc. de Cracovie.
SELENKA, EMIL. Die Blatterumkehrung im Ei der Nagethiere. Wiesbaden,

1884. pp. 67-99.
SILVESTRI, FILIPPO. Contribuzioni alia conoscenza biologica degli Imenotteri

1906. parassiti. I. Annali R. Scuola Sup. d" Agricoltura. Portici. vol.
6. 15 Gennaio, 1906.

REFERENCE LETTERS

a. a., amniotic attachment to wall of

vesicle.
al., allantois.

al.en., allantoic entoderm.
al.ms., allantoic mesoderm.
am., amnion.
am.c., amniotic cavity.
am.c.c., amniotic connecting canal.
a.v., aborescent villi.
b.b., belly-stalk bands.
b.c., blood cords.
b.s., belly-stalk.
b.v., blood vessel.
c., cervix of uterus.
c.a., clear area of Trager.
c.am.c., canal of the common amnion.
c.e., canal enlargement.
c.L, corpus luteum.
co.} coelome.
d.b., dorsal bridge.
d.n., dorsal notch.
ec., entoderm.
en., entoderm.
e.v., extra chorionic vesicle.
ex.c., extra embryonic body cavity.
f.g., fore-gut.

f.n.t., floor of the neural tube.
f.t.t Fallopian tube.
f.u., fundus end of uterus.
/.»., flattened villi of Trager.
h.f., head fold.
h.ms., head mesoderm.
h.p., head process.
i., infundibulum of the Fallopian tube.

i.L, intestinal loop.
l.s., lymph sinus.
ms., mesoderm.

ms.co., mesodermal connection.
m.p., medullary plate.
n.ch., notochord.
n.g., neural groove.
n.l.L, notch of the left lateral lobe.
o., ovary.

p. am., posterior amniotic process.
p. am.c., posterior amniotic cavity.
p.p.h., protochordal plate of Hubrecht.
p.p., primitive pit.
p.r., placental ring.
p.s., primitive streak,
s., somite.

s.am.c.c., supernumerary connecting can-
al of amnion.
s.t., sinus terminalis.
s.v., scale-like villi.
t.m.p., tip of the medullary plate.
tr., Trager.
tr.c., Trager cavity.
tr.e., Trager epithelium.
tr.k., Trager knots.
u.m., uterine mucosa.
v., villi.
vg., vagina.
y.s.j yolk-sac.
y.s.en., yolk-sac entoderm.
y.s.w., yolk-sac wall.
I, II, III, and IV, refer respectively to
the ventral, right lateral, dorsal, and
left lateral embryos.

Development of the Nine-Banded Armadillo. 413

c. i o

-*5**' C

— vg

he.

8

FIG. 7. The genitalia of an adult virgin female as seen from the dorsal aspect.
b.L, broad ligament; c., cervix of uterus; c.L, coprus luteum in left ovary; f.u.
fundus end of uterus ;/.<., fallopian tube; i., infundibulum; o., ovary. X 3.

FIG. 8. Portion of the yoik-sac wall in the region of the area vasculosa (see fig.
15). It shows three blood cords in section. These are made up of a central core of
solidly packed cells, b.c., which are surrounded by the mesodermal epithelium, ms.
X 215.

FJG. 9. Cross section of the Trager of our youngest vesicle (fig. 12), showing
three adjacent Trager cords or knots (tr.k) ; tr.e., Trager epithelium. X 265.

414

H. H. Newman and J. T. Patterson.

11

FIG. 10. Cross section of scale-like villi, v., of the vesicle in fig. 14. The Trager
knots are still covered with a thin epithelium. The epithelium of the villi has
become a syncytium. The mesoderm has proliferated cells which have invaded
the villi, but as yet blood formation has not taken place. X 265.

FIG. 11. The tip of a villus from a more advanced stage, showing, in addition to
the features described in preceding. figure, the well developed blood vessels, b.v.
X 265.

Development of the Nine-Banded Armadillo. 415

-- c.am

12

FIG. 12. A detailed drawing of vesicle no. 10 as seen from the ventral side as a
semi-transparent object. The embryos in white (I and IV) are on the upper side
of the vesicle, and since there is an inversion of germ layers, these are seen from
their ventral aspects. Embryos II and III are shaded, and lie on the far side of the
vesicle. For a fuller description see text. X 9.

416

H. H. Newman and J. T. Patterson.

anrac.

-- am

13

JIIG. 13. A detailed drawing of embryo I (fig. 12) as seen from the dorsal aspect,
that is, as viewed from the inside of the vesicle, am., amnion; a.mc.c., connecting
canals of the amnion; p.am., posterior amniotic process; b.s., belly-stalk; tr.,
Trager. X 25.

Development of the Nine-Banded Armadillo. 417

14

FIG. 14. Ventral view of vesicle No. 18, seen as a semitransparent object. The
Trager is covered with scale like villi, which overlap the lower margin of the yolk-
sac. This vesicle should be compared with that shown in fig. 12, in which the let-
tering is the same. X 5.

JOURNAL OF MORPHOLOGf, VOL. 21 NO. 3.

418

H. H. Newman and J. T. Patterson.

amxic.

p.am.

15

FIG. 15. A detailed drawing of embryo I (fig. 14) as seen from the dorsal side. A
photograph of this embryo is shown in fig. 31. al., allantois; am. ex., connecting
canal of amnion; 6.6., belly-stalk band, note that the band is much more distinct
on the left side than on the right; p.s., primitive streak; s.v., scale-like villi; tr.,
Trager region, which shows the villi as seen from their under sides. For a fuller
description see text. X 21.

Development of the Nine-Banded Armadillo. 419

NOTE — Figs. 16-23 represent a series of transverse sections taken through various
regions of an embryo from the same vesicle as that shown in fig. 13.

FIG. 16. A section taken through the anterior end of the medullary plate. The
most important feature of this section is the thickening of the entoderm to form
the "protochordal plate" of Hubrecht, p.p.h. X 130.

FIG. 17. A section taken through the medullary plates at a point lying half way
between the fore end of the head process and the anterior tip of the embryo. The
entoderm is distinct from the mesoderm, which is scarcely more than one cell thick.
X 130

420

H. H. Newman and J. T. Patterson.

FIG. 18. A section taken through the middle of the head process. In this region
the entoderm is very intimately associated with the mesoderm, especially in the
central part of the section. X 130.

FIG. 19. A section taken through the primitive pit. It shows the primitive
streak proliferating mesoderm in the characteristic manner. X 130.

Development of the Nine-Banded Armadillo. 421

FIG. 20. A section taken through the connecting canal, which is seen to be com-
posed of two layers, ectoderm on the inside and mesoderm on the outside, and is
loosely connected with the mesoderm of the yolk-sac wall. X 143.

FIG. 21. A section taken through the tip of the medullary plate. This is the first
section that shows the anterior end of the protochordal plate of Hubrecht. Note
that there are a few scattering mesoderm cells (ms.) that have wandered in between
the plate and the ectoderm. X 143.

FIG. 22. A section taken through the belly-stalk at the level of the mouth of the
allantois (aZ.)- The cavity of the posterior amnotic process (p.am.c.) does not cover
more than one-half the width of the section. The mesoderm of the belly-stalk
extends laterally to form wing-like processes. These are the belly-stalk
bands (6.6.) through which the umbilical blood vessels pass to the Trager. X 143.

422

H. H. Newman and J. T. Patterson.

FIG. 23. A section taken through the belly-stalk near the posterior tip of the al-
lantoic entoderm (al. en.}. The mesoderm is indistinctly divided into two por-
tions: (1) that forming the belly-stalk bands, and (2) that part immediately sur-
rounding the allantoic tube — this may be called the allantoic mesoderm (al.ms.}.
The belly stalk is here separated from the wall of the yolk-sac by a space (ex.c .),
which is only a part of the general extra-embryonic cavity. X 143.

Development of the Nine Banded Armadillo.

423

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PLATE I

EXPLANATION OF FIGURES

NOTE, figs. 26-29 represent a series of transverse sections of a five-somite embryo.

26. A section through the region of the head-fold. The brain vesicle is in the
process of formation, and the neural groove (n.g.) has become very deep. The no-
tochord (n.ch.) is represented by a row of cells, and to each side of it the entoderm
is bayed to form the pharyngeal pouches (ph.g.}. X 68.

. 27. A section through the somite region. The somite shows a distinct cavity,
and the coelomic cavity is forming. The entoderm is beginning to close in beneath
the notochord. X 68.

28. A section through the proximal part of the allantoic tube. The bands of
the belly-stalk have become much folded, and contain a number of umbilical blood
vessels. The posterior amniotic process has become reduced to a very small tube.
X 68.

29. A section through the posterior mesodermal connection (ras.co.) of the
belly-stalk. The Trager shows the villi in the process of formation. X 30.

DEVELOPMENT OF THE NINE-BANDED ARMADILLO.
H. H. NEWMAN AND J. T. PATTERSON

PLATE I.

26

n.ch.

en a 1. en. alms.

28

29

JOURNAL OF MORPHOLOGY, VOL. 21, NO. 3.

PLATE II

EXPLANATION OF FIGURES

30. One of the five somite embryos (III) of vesicle No. 18 (see figs. 14 and 32) .
Note how the embryo is attached to the Trager by means of the belly-stalk (6.s.).
The area vasculosa, like that of the chick, does not extend in to the embryo, but
is. separated from it by a clear space which corresponds to the area pellucida. On
the right is seen the compound sinus terminalis (s.t.) lying between the vascular
areas of the two contiguous embryos. The posterior prolongation of the amnion
is not clearly seen, but its extreme tip is indicated by the leader, p. am. X 16.

31. A seven somite embryo (I) of this same vesicle. For a description of this
embryo see the detailed drawing shown in fig. 15. X 16.

32. The dorsal view of the vesicle reconstructed in detail in fig. 14. The cervix
end is slightly torn and is turned under, consequently the common amnion and its
canals are not shown in the photograph. The turning under of the torn piece also
makes the vesicle appear shorter than it really is. At o.m. may be seen the scale-
like villi beginning to overgrow the lower portion of the yolk-sac. X 2.15.

33. A vesicle cut open along the mid-ventral line and spread apart to show the
pairing of the embryos. It will be noted that the embryos are arranged so that the
right-hand pair (III and IV) is the mirrored image of the left-hand pair (I and II).
At this stage the amnia are still distinct, and in shape are oval with the broad end
directed toward the fundus. X |

DEVELOPMENT OF THE NINE-BANDED ARMADILLO.
H. H. NEWMAN AND J. T. PATTERSON.

PLATE II.

30

31

33

JOURNAL OF MORPHOLOGY, VOL. 21, NO.

PLATE III

.

EXPLANATION OF FIGURES

34. A view of the fundus end of a vesicle which contained embryos measuring
31 mm. head rump length.4 In the portion of the vesicle lying within the margin
of the placenta are seen four window-like spots. These are the areas where the am-
nia come in contact with the wall of the vesicle. The fundus end is now practically
free of villi. X \

35. A view of the cervix end of a vesicle in which the embryos measured 31
mm. The clear yolk-sac is seen through the opening in the rather thick placental
overgrowth. The margin of this opening represents the place where the placenta
is attached to the uterine mucosa at the cervix end of the uterus. X f

36. The dorsal view of a vesicle which is still attached to the cervix of the con-
tracted uterus. This vesicle shows a distinct placental bridge (p.6.) connecting
the lateral placentae, and also a number of blood vessels at the fundus end. Em-
bryos 32 mm. in length. X f

37. A view of the fundus end of a vesicle which contained embryos measuring
33 mm. This view shows two points worthy of especial note: (1) the four-lobed
appearance of the fundus membrane, due to constrictions occurring between the
fundus areas of the individual embryos (seen more clearly before fixation) ; (2)
the persistence of a few villi, which in the photograph appear as scattering black
specks. X f

38. A view of the ventral side of vesicle, with embryos measuring 36 mm. The
cervix end of the yolk-sac is clearly visible, and blood vessels are seen at the fundus
end. The placental bridge although present is not clearly brought out in the
photograph. X I

39. A view of the ventral side of a vesicle which contains embryos measuring
53 mm. The division of the zone-like placenta into right and left halves is clearly
brought out. The fundus end of the vesicle is now practically free of both villi
and blood vessels, and the membranous area at the cervix is much larger than in
the preceding figure. X §

4Unless otherwise stated, the length of the embryo will mean the head-rump
measurement.

DEVELOPMENT OF THE NINE-BANDED ARMADILLO.
H. H. NEWMAN AND J. T. PATTERSON.

PLATE III.

34

35

37

JOITRNAL OF MORPHOLOGY, VOL. 21, NO. 3.

PLATE IV

EXPLANATION OF FIGURES

40. The dorsal view of a vesicle in a rather advanced stage of development.
The embryos measure 155 mm. from tip to tip. The dorsal notch, d.n., although
extending down to near the meddle of the vesicle, does not completely separate
the lateral placental discs. X f

41. Dorsal view of a vesicle showing the difinitive condition of the placenta.
The placenta is divided into two lateral discs, each of which is distinctly bilobed.
The notch between the two lobes of the left lateral (on right) disc is clearly shown
in the photograph (n.l.l.). The discs are united to each other both on the dorsal
and ventral side by placental bridges, the one on the dorsal side (d.b.) being the
narrower. The original arborescent villi at the cervix end have greatly degen-
erated, and have become reduced to flat, blunt knobs. The embryos in this vesicle
are about 210 mm. from tip to tip. X \.

42. Right lateral view of a uterus showing a dorso-ventral bilobing. Embryos
are 48 mm. long. X |r.

43. Ventral view of a pear-shaped uterus, which contained embryos measuring
52 mm. This and the preceding uterus show two of the several forms that have
been observed. X \

DEVELOPMENT OF THE NINE-BANDED ARMADILLO.
H. H. NEWMAN AND J. T. PATTERSON.

PLATE IV.

n.l.L

42

43

JOURNAL OF MORPHOLOGY, VOL. 21, NO. 3.

PLATE V

EXPLANATION OF FIGURES

44. A vesicle split open to show the internal relationships of the different parts.
The amniotic connecting canals are seen to pass from the anterior ends of the amnia
to the spot occupied by the common amnion. This vesicle also shows a supernu-
merary canal (s.aw.c.c.) extending from a small vesicle in the Trager wall to the
canal belonging to the lower, right-hand embryo. In the entire condition the vesi-
cle measured 24 mm. wide by 29 mm. long, (see fig. 3 for a diagram of the placenta.)
Very slightly enlarged.

45. A vesicle laid open in a manner similar to the preceding. At the distal end
of each canal is shown a series of bead-like enlargements (c.e.). The origin of the
canal from the anterior tip of the amnion is shown with especial clearness in the
embryo lying nearest the foot of the plate. In the entire condition the vesicle
measured 24 mm. wide and 30 mm. long. Very slightly enlarged.

DEVELOPMENT OF THE NINE-BANDED ARMADILLO.
H. H. NEWMAN AND J. T. PATTERSON.

PLATE V.

44

JOURNAL OF MORPHOLOGY, VOL. 21, NO. 3.

PLATE VI

EXPLANATION OF FIGURES

46. A vesicle cut open along the mid-ventral line to show the relationship of
the embryos to each other and to the wall of the vesicle. Each of the four amniotic
partitions (a), which have been cut off close to the chorionic wall, lies just to the
left of the umbilical cord. These are attached to the wall near the tips of the
placental lobes at the f undus end. The left lateral placental disc is indistinctly
seen thiough the chorionic wall, and the notch separating it from the right
lateral disc is marked with the larger "n", while that indicating its division into
the two lobes is designated by the smaller "n," X |.

47. A photograph of* vesicle no. 108, which contained five embryos. This
vesicle was cut open along the mid-ventral line. Embryos nos. I, II, and II, are
attached to the large, right lateral placental disc, and embryos III and IV to the
smaller, left lateral. (See text for a fuller description and significance.) X £.

DEVELOPMENT OF THE NINE-BANDED ARMADILLO.
H. H. NEWMAN AND J. T. PATTERSON.

PLATE VI.

ill

IV

JOURNAL OF MORPHOLOGY, VOL. 21, NO. 3.

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