A critical period in the development of the horse

HARVARD: UNIVERSITY:

LIBRARY

OF THE

MUSEUM OF COMPARATIVE ZOOLOGY
ro K&S

GIFT OF

A IO ter

7,083
APR. ‘7 y9ng

CRITICAL PERIOD

IN THE

DEVELOPMENT OF THE HORSE

BY

JC WARY. M.D. BES:,

REGIUS PROFESSOR OF NATURAL HISTORY, EDINBURGH.

LONDON:
ADAM AND CHARLES BLACK.
VY 1897.

A

CRITICAL PERIOD

IN THE

DEVELOPMENT OF THE HORSE.

OTRO ab, te [pI OUD)

IN THE

DEVELOPMENT OF THE HORSE.

BY

Jo Oh TO WAI IE, WME ID), Tis loi,

REGIUS PROFESSOR OF NATURAL HISTORY, EDINBURGH.

LONDON:
ADAM AND CHARLES BLACK.
eS ONe

APR 7 4992

A ORITICAL PERIOD IN THE DEVELOPMENT
OF THE HORSE.

INTRODUCTORY.

Avr a meeting! of the Central Veterinary Medical Society of
London, at which I exhibited a number of young horse embryos,
I had barely time to allude to what may prove a subject of some
practical importance, viz., the apparatus provided for nourish-
ing the embryos during the early weeks of development. This
apparatus (which consists mainly of what are known as the feetal
appendages) I was led to consider from a breeder’s point of view
by Lord Arthur Cecil asking me, about a year ago, if I could
account for so many mares breaking service from the sixth to the
ninth week. At the time I could only answer that I agreed
with his lordship in believing it was in many cases due to the
detachment and escape of an embryo. I have frequently found
mares all right, as far as I could judge, at the end of the sixth
week, and all wrong some weeks later. This seems to be not an
uncommon experience, more especially at the beginning of the
breeding season, hence it is always urged by experts that mares
should be “ tried” frequently during at least the first two months.
If holding at the end of the ninth week no further trouble is, as
a rule, anticipated. But no one has, as far as I am aware,
explained why the breeder experiences so many difficulties during
the earlier weeks. According to the evidence obtained by the
Royal Commission on Horse Breeding, it appears that about 40
per cent. of the mares selected for breeding fail to produce
offspring during any given year. This is a very high percentage

1 4th March (see the Wield, 13th March 1897).

6 A CRITICAL PERIOD IN THE

of failure, but from reports recently received it seems to be still
higher in certain districts in India! Believing that some of
my mares have proved unfruitful for the time being because of
their frequently breaking service, and believing further that not
a few of the disappointments that fall to the lot of breeders
result from the same cause, I decided to examine material
collected for another purpose, with a view to shedding fresh light
on this difficult but important practical question. To admit of
the conditions which obtain in the brood mare being more easily
understood, I shall at the outset refer to the apparatus provided
for the nourishment of the chick, and to the corresponding
structures in one of the simpler mammals.

THE CHICK’S Fa@TaL APPENDAGES.2

Every one who has examined a hen’s egg at, say, the end of the
ninth day of incubation, knows that the already well-formed
chick is, as it were, mounted on a large, well-filled forage bag—
the yolk sac—from which it hour by hour draws its nourishment.
But in addition to the familiar yolk sac there is a second sac, in
its way quite as important, though empty of food, viz., the thin
compressed sac lying in contact with the shell, which plays the
part of a breathing organ or lung. This second sac is known as
the allantois. The position and relations of the yolk sae and
allantois are indicated in fig 1. Through the walls of both
saes blood is constantly passing to and from the chick, collecting
particles of altered yolk from the one and fresh supplies of
oxygen (direct from the air which penetrates the shell) by means
of the other. Fig. 1 shows that the chick is connected to the
yolk sae by a short thick stalk, amd to the allantois by a long
slender stalk, and that it is invested by a special water-tight robe

1 Mr Pease, in his work Horse-Breeding for Farmers, says :—‘‘ Nine out of ten
mares that miss are barren simply from want of care in catching the mare at the
right time, or from subsequent neglect ;” that ‘‘ half the mares that are geld are
so because the mare, having once or twice refused the horse, she is dismissed from
all further consideration,” and further, that ‘‘many mares will refuse the horse
for several successive periods, and then come in season again perhaps without
any great show of their condition.”

2 The chick has been selected not because mammals are related to birds, but

that any one desiring to see the foetal structures dealt with may readily gratify
the wish by examining hen’s eggs at the ninth or tenth day of incubation,

DEVELOPMENT OF THE HORSE. is

or inner tunic known as the amnion. The fluid lying between
the chick and the amnion (in the dark space in fig. 1) 1s most
useful in preventing jars when the egg is moved, and the thin
but tough skin forming the wall of the amnion prevents the
chick injuring the yolk sae and allantois. The figure also
indicates the position of the air space invariably found, and a
portion of the albumen (the white of the egg) which is gradually
absorbed as additional nourishment, partly by the yolk sac and
partly by the allantois.

THE Opossum’s FuTaL APPENDAGES.

The young mammal, like the chick, is invested by a fairly tight-
fitting tunic, the amnion (am., fig. 2); and, like the chick, it is
connected by one stalk to the yolk sac (y.s., fig. 2), and by a second
to an allantois (aJl., fig. 2). But, unlike the chick, the yolk sac in
the mammal never contains yolk,! and the allantois never plays
the part of a simple breathing organ—never, as in the bird,
collects oxygen direct from the air. If the yolk sac contains no
yolk, and the allantois is unable to expose the foetal blood to the
air, how does the embryo mammal live and breathe? In the
opossum, and, in fact, in all mammals except the duckmole and
echidna, nourishment, at least to start with, is absorbed directly
from the uterus into the yolkless yolk sac? This nourishment,
which is partly secreted by uterine glands, is in a sense com-
parable to the albumen in the bird’s egg, and is probably an excel-
lent substitute for the crude material stored up in the yolk sac
of birds and lizards. But the embryo mammal not only requires
nourishment, it requires to be anchored for a longer or shorter
period to the inner wall or lining of the uterus. This fixing is
secured in, ¢.g., the opossum, by means of the thin outer shell-less
coat or tunic, which incloses the embryo and all its appendages.

Fig. 2 represents in a diagrammatic way a young American
opossum. The embryo, invested by a thin inner tunic, the

! The duckmole and echidna are exceptions to this rule, but I agree with
Professor Hubrecht of Utrecht in believing that the monotremes are off the main
mammalian line.

2 The uterus, in which development takes place, being able to provide nourish-
ment, it has been found unnecessary to store up food in a large yolk sac, as in
the bird, which for the sake of lightness is almost of necessity oviparous.

8 A CRITICAL PERIOD IN THE

amnion (am.), has suspended from it a large yolk (y.s.) sac and a
relatively small allantois (a//.). Surrounding the embryo and its
appendages is an outer tunic, which may be known as the outer
embryonic sac (a, 0, c, d). With part of this embryonic sac the
yolk sac blends (from a@ to ¢), and it is through this area (a, 0, ¢)
the nourishment—the uterine milk—enters, or is absorbed into,
the yolk sac. ‘The nutriment is at once conveyed to the embryo
by the blood-vessels, which plentifully ramify in that part of the
sac which has not fused with the outer tunic. In other words,
through the bloodless area (a, 6, c) nourishment diffuses from the
uterus into the yolk sac, thence into the yolk sac vessels, which
carry it to the embryo. The embryonic sac does not simply le
in the uterus, it is fixed or cemented, and retained in one definite
position. This fixing is mainly secured by cells of the outer
tunic in the absorbing area adhering to cells of the uterine lining
or mucous membrane. In the opossum and kangaroo and the
vast majority of the marsupials these adhesions, at the best very
feeble, are in a few weeks broken down, the embryonic sac
ruptures, and the embryo, still in a very immature and helpless
condition, escapes, and is eventually suspended to a teat, which
may or may not project into a pouch or marsupium. It is
especially worthy of note that in the opossum the allantois takes
ho part in securing nourishment from, or in fixing the embryonic
sac to, the uterine wall.

THE FaraL APPENDAGES IN THE HORSE.

While opossums and kangaroos and many other ancient and
primitive creatures appear on the scene in an immature and
helpless condition, some of the higher mammals, more especially
some of the members of the hoofed or ungulate tribe, are, as
every one knows, at birth so perfect in all their parts that they
can at once join and keep pace with the herd to which they by
birth belong. Mr Hudson, in speaking of the sheep on the
Pampas, says:—‘“ I have often seen a lamb dropped on the frosty
ground in bitterly cold windy weather in midwinter, and in less
than five seconds struggle to its feet, and seem as vigorous as any
day-old lamb of other breeds. The dam, impatient at the short

DEVELOPMENT OF THE HORSE. 9

delay, and not waiting to give it suck, has then started off at a
brisk trot after the flock, with the lamb, scarcely a minute in the
world, running freely at her side.” ! One of the most renowned
Arabian mares was apparently equally precocious. In Gleanings
from the Desert of Arabia? occurs the following passage :—
“ During a short interval of rest, when on a long and rapid
journey—her master being pursued—a mare gave birth to a filly
foal. He abandoned the foal, and pursued his course on his
mare, the dam; when he again halted, he was surprised to find
the foal shortly make her appearance.” This foal was placed in
charge of an old woman, and was afterwards known as the
Keheilet Ajuz, ze. the mare of the old woman. I have else-
where? mentioned that my zebra hybrid “ Romulus” was within
a minute after birth “rushing about as if he were a young zebra
whose existence depended on his at once joining the troop of
which his dam was a member.” Even the young hippopotamus
is said to be most energetic. On one occasion it is stated a hip-
popotamus calf, only just born, escaped from its pursuers, and at
once made the best of its way to the river.

Are sheep and antelopes and horses throughout their embryonic
life nourished in the same way as the young opossum, or is some
more effective plan adopted during at least part of the develop-
mental process? I shall now proceed to show that, while at the’
outset the horse embryo has the same simple apparatus as the
opossum, a stage is soon reached when more elaborate and more
permanent nutritive appliances are provided. Further, I shall
point out that when the new apparatus is being substituted for
the old,—when the opossum plan is coming to an end, and the
more permanent appliances are barely in working order,—that at
this critical period the horse embryo may readily drag its anchors
and escape—behave as if it were a’ young American opossum
or an Australian kangaroo.

I shall best accomplish this by describing shortly, and in as
sunple terms as I can command, the foetal appendages of my five
youngest horse embryos.t

1 The Naturalist in La Plata. * Upton. * Veterinarian, November 1896.
+ Since the above was written, I have succeeded in getting a three-weeks’
embryo; but for the purposes of this paper, a four-weeks’ one is sufliciently
young.
B

10 A CRITICAL PERIOD IN THE

The Four-Weeks Horse Embryo.—Fig, 3 represents in a sem1-
diagrammatic way, but as near the actual size as possible, a four-
weeks’ horse embryo and its various appendages and tunics. The
embryo, which was found coiled with the tail overlapping the
head region, measured just under three-eighths of an inch in
length. This embryo, it will be observed, like the chick and
opossum, is closely invested by an inner tunic, the amnion (am.),
and connected by a short stalk with a large yolk sac (y.s.). It is
also connected by a second stalk (not shown in the figure) with
a still relatively small allantois (a/l.). Surrounding and protecting
the embryo and its special appendages, is the thin outer tunic
(a, 6, ¢, d). In the wall of the allantois there are many
blood-vessels, and already this sac, by bringing the foetal flood
sufficiently near the blood coursing through the uterus, may
admit of an exchange of gases taking place—it may, in fact,
already play the part of a breathing organ, the oxygen being
derived from the maternal blood in very much the same way as
in the case of the fish it is derived from the water. As in the
opossum, part of the yolk sac (from @ to ¢) has blended with the
outer tunic. Through this bloodless area nutritive material
passes into the yolkless sac. With the exception of this bloodless
part of the yolk sac, its walls are crowded with blood-vessels.
One large vessel which carries blood from the embryo forms a
complete circle around the absorbing area; from this circular
vessel numerous branches radiate upwards, forming on the way
an elaborate network. The blood is eventually collected from
this network into two large vessels (veins), which convey it back
again to the embryo. As the blood courses through the wall of
the yolk sac, some of the nutriment which has entered through the
absorbing area (a, b, ¢) is added to it. The yolk sac is thus a
reservoir into one end of which nutritive material filters from
the uterus, and then diffuses into the vessels of the sac, in
order to find its way to and supply the ever-increasing wants of
the embryo. Asin the opossum, the outer tunic is anchored to
the lining of the uterus by some of the cells occupying the absorb-
ing area (a, 0, c); but, as in the opossum, the conncctions are
easily broken down. This is in striking contrast with what
occurs in certain other mammals. In some cases a special uterine
pouch is prepared beforehand for each embryo, from which pre-

DEVELOPMENT OF THE HORSE. Th

mature escape is all but impossible. Should one of these
embryos succumb in the struggle for life (which often begins
before birth), it still remains in its pouch, and is bit by bit
absorbed. From first to last in the case of the mare the con-
nections between the embryonic sac and the uterus are, compared
even with other ungulates, easily broken down. As I have on
several occasions observed, the moment an incision is carried
upwards from the neck into the horn of the still contractile
uterus, the embryonic sac begins to protrude, and, still entire,
the sac may be at once forced, by the contractions of the uncut
upper part of the horn, into the body of the uterus. Hence, if
the neck of the double horned uterine sac is in a flaccid and
unhealthy condition, or if strong wave-like contractions are set up
in the uterine horns, there is little chance of a horse embryo only
afew weeks old long retaining its position, even if well cemented
to the uterine mucous membrane. The need of supplementary
anchorages is early recognised in the case of the horse. Even
at the fourth week a ring (az., fig. 3), consisting of delicate
nearly parallel ridges, exists around the absorbing area. These
ridges not only dip into depressions in the lining of the uterus,
but in addition many of the cells covering them blend with the
adjacent uterine cells. Further, some of the cells forming the
outer tunic or embryonic sac increase in length, so as to form an
indistinct belt or girdle, nearly on a level with the embryo. This
sirdle occupies in a section the position indicated by the letters
(jh, TO hey oe

The Five- Weeks Horse Embryo.—li we turn from the four to the
five-weeks’ embryo we find various important changes have taken
place. The embryo, now five-eighths of an inch in length, has all
but lost its gill slits, while a rudiment of the ear has appeared in
connection with the remnant of the first cleft; the tail is nearly
straight, and, compared with the limbs, relatively shorter than at
four weeks, and the limbs seem to contain at their tips rudiments
of three separate digits; at four weeks the limbs are quite un-
differentiated. In fig. 4, which represents the actual size of the
various structures, it will be noticed (1) that while the yolk sac
(y.s.) is larger, the absorbing area (a, b, ¢) is smaller than at four
weeks ; (2) that the capacity of the allantois (a/l.) has greatly in-
creased; and (3) that the girdle (ty.) is more pronounced and

12 A CRITICAL PERIOD IN THE

nearly equatorial in position. The embryo, doubtless, still obtains
its nourishment by means of the yolk sac, but it may be assumed
the allantois is indirectly a fairly effective breathing organ. Com-
pared with the chick the development has been slow ;! but although
no great size has been reached, by the end of the fifth week the
foundations of the more important organs have been laid. Com-
pared both with earlier and somewhat later periods, the five-weeks’
embryonic sac is well fixed to the uterus; for in addition to the
adhesions in the region of the absorbing area, and the hold ob-
tained by the plicated ring around this area, there is a very com-
plex and nearly equatorially placed girdle. As the embryonic sac
which contained my five-weeks’ embryo escaped from the uterus,
the girdle appeared as a whitish band nearly a quarter of an inch
in width. On section this girdle is seen to consist not merely of
elongated cells, as at the end of the fourth week, but of numerous
delicate folds separated from each other by deep furrows. That
it 1s concerned in fixing the embryo is evident from the adhesion
of uterine cells to its surface.?

When we take into consideration the size, structure, and posi-
tion of the girdle just described, and of the ring around the
absorbing area, it may, I think, be assumed that the horse embryo
is more firmly fixed to the uterus at the end of the fifth week
than at any subsequent period prior to the development of the
numerous processes (villi) which during the eighth week sprout
out like so many delicate rootlets from the outer surface of the
allantois.

The Sia-Weeks Horse Embryo.—Although the six-weeks’ embryo
is little more than an eighth of an inch longer than the five-weeks’
one,’ it is quite twice as heavy; and though it appears to be

1 There is not the same urgency to hurry on the development in the mammal
as in the chick—an embryo carefully hidden and guarded in a uterus is perfectly
safe.

2 This girdle (not hitherto found in any mammal), in addition to fixing the
embryonic sac, may be the means of absorbing additional nourishment, which
may reach the embryo either through the yolk sac or the allantois.

° During the first seven weeks the amount of nourishment is likely to vary
considerably, and the extent of the absorbing area is also likely to vary. Hence
in collections of four or five or six weeks’ embryos considerable differences in size
and extent of development would in all probability be met with. I should not be
surprised to find a six-weeks’ embryo considerably larger than the one in my
possession, or a five-weeks’ one somewhat smaller. JTurther, during the earlier

DEVELOPMENT OF THE HORSE. 13

tri-dactylous, there can no longer be any doubt as toits belonging
to the great horse family. The limbs are better formed, and,
compared with the tail, relatively longer than at the end of the
fifth week. The embryonic sac (a, 0, ¢, d), still ovoid, measures
three inches in the one direction, and over two in the other,
and the allantois (a//.) has fused with the greater extent of its
inner surface. The yolk sac (y.s.) has still about the same
capacity as at the end of the fourth week, but the absorbing area
(a, b,c) is only about half the size. The girdle (4.), equatorial
in position at the end of the fifth week, now lies near the pole
occupied by the absorbing area! From the absorbing area being
comparatively small, and the special fixing structures occupying
one pole, I am inclined to think that, about the end of the sixth
week, the whole of the embryonic sac might be easily detached.
Were it an opossum embryo, preparations would soon be made
for its birth. The horse may not have quite forgotten this
ancestral habit. About the end of the sixth, as at the end of the
third week, the whole reproductive system is in all probability
in a somewhat excitable condition. All the physiological
changes which occur during cstrum are likely to supervene
in a more or less pronounced form about the end of the third
and again at the end of the sixth week. In other words, the
habit which the nervous and other systems have of becoming
periodically excited is not apparently quite thrown off for some
weeks after a successful service. In some mammals the nearly-
ripe eges found in the ovary after development has started are
said to be absorbed; but in the mare one or more eggs may be
matured and discharged several weeks after she has settled. There
is a case on record of a mare bringing forth twins, a foal and a
mule. She was presented to a jackass fifteen days after being
served by a horse. The escape of ova (ovulation) is accompanied
with an extra rush of blood to the ovaries and to the uterus, which
implies an excited condition of the nervous apparatus of these
organs, increased secretion in the uterine glands, and more or

weeks, embryos from small mares are not likely to differ much, if at all, in
size, from embryos taken from large mares. My five smallest embryos (four to
eight weeks) are from mares from 14:2 to 15 hands high.

1 In the only six-weeks’ specimen I have seen, this girdle was narrow and
broken up into short links or segments, as if in process of disintegration.

14 A CRITICAL PERIOD IN THE

less powerful contractions of the numerous uterine muscular
fibres. The periodic disturbance is likely to be greater in mares
which have not previously bred. In mares which have had several
foals, or have recently foaled, the uterine vessels are readily
enlarged, and from the first tend to deflect blood which might
otherwise rush to the ovaries. Ata time when the connections
between the embryonic sac and the uterus are at their weak-
est, this extra excitement of the reproductive system may very
readily lead to an arrest in the developmental process, and
eventually to the embryo being discharged. When this hap-
pens, evidence will soon be forthcoming that the mare has
broken service.

The Seven-Weeks Horse Embryo.—lf the storms which natu-
rally set in about the end of the sixth week are successfully
weathered, a period of calm sets in, not likely to be seriously
interrupted at the end of the ninth week; for by that time the
uterine vessels will have sufficiently increased to carry off all the
extra blood that finds its way to the reproductive organs. The
ovaries will thus be but little excited, and the nervous system, as
a whole, will all but maintain its normal calm. There is, how-
ever, another danger ahead, for at the end of the seventh week
the supply of nourishment by means of the yolk sac has all but
come to an end, and the arrangements for providing for the
wants of the embryo, by means of the allantois, have not yet
been completed. To put it another way: at or about the end of
the seventh week the remote marsupial ancestors of the horse
were, in all probability, in the habit of leaving the uterus for the
shelter of the pouch or marsupium, already capable of imbibing
ordinary milk from teats, instead of absorbing uterine milk by
means of a yolk sac.! The seven-weeks’ embryo (with its append-
ages and its inner and outer investing tunics) is represented in
fiz. 6. This embryo, which is nearly half an inch longer than the
six-weeks’ one, is at a most interesting stage in its development.
Had it been an opossum, it would have already been glued to a
teat in the marsupium,—not yet able to suck, but sufficiently

1 Some would even go the length of saying that, in obedience to the law of
heredity, there is an attempt on the part of the horse embryo, at or about the
end of the seventh week, to slip its moorings and escape from the uterus,—to
make an attempt to repeat this particular episode in the ancestral history.

DEVELOPMENT OF THE HORSE. 1)

advanced to admit of milk being pumped from the mammary
glands into its gullet or cesophagus. The figure indicates the
exact size (just under 1} inches) of the embryo. The open
mouth and well-formed lips and tongue seem to indicate that it
is quite ready (should a huge reversion or throw-back take
place) to receive and hold on to a teat. There is, in fact, in the
seven-weeks’ embryo not a little in favour of the view that all
animals climb their own ancestral tree,—that there is a more or
less accurate recapitulation by each individual of the entire
ancestral history.1 I have no doubt the remote ancestors of the
horse were born ere this stage was reached. If the rate of pro-
gress was the same in the remote past as it is now, the birth
would originally occur on the forty-seventh or forty-eighth day.
As the figure shows, the head is less flexed than at six weeks, and
the nose, eyes, and ears are better formed, and that it is beyond
doubt a mammalian embryo is proved by the presence of rudi-
ments of hair on the snout and eyelids. But any embryologist
would not only at once place this embryo in the mammal group,
he would have no hesitation in asserting it belonged to the horse
family. The fore and hind limbs clearly point to this, for already
the frog is partly modelled, and the fetlock is quite distinct ; and
while the position of the knee and elbow are evident in the fore
limb, the position of the hock and stifle are equally evident in
the hind ; and, further, the possibility of having more than one
complete useful digit for each foot no longer exists. Moreover,
even without the aid of a lens, two teats can be detected, and
hence it would be possible, without knowing anything of the
history of this embryo, to state that it had been obtained from
some member of the horse family, and that it would probably
have developed into a filly foal.

On turning to the appendages, it will be observed that though
the yolk sac has lost little in capacity, its connection with the
embryonic sac has been considerably reduced. The absorbing area
is now very small (a-c, fig. 6), so small that it is difficult to
understand how sufficient nourishment has been obtained to

! That there is not an exact, or anything approaching an exact, recapitulation,
a glance at the seven-weeks’ embryo will at once show. The remote ancestors
had five complete digits, but even in the seven-weeks’ horse there is no external
indication of more than a single digit for each foot.

16 A CRITICAL PERIOD IN THE

admit of so much growth during the seventh week.! But while
there has been little change in the yolk sac, the allantoic sac
(a/l.) has now about three times the capacity it had at six weeks.
Whether the blood, as it circulates through the allantois, collects
nutritive material as well as fresh supplies of oxygen, I am not
yet able to say. Thechanges in the outer cells of the embryonic
sac are more interesting and suggestive than those in the yolk
sac and allantois. The ring (am.) around the absorbing area,
though flattened, still consists of delicate ridges. The girdle, still
circular at six weeks, has been folded so as to assume an extremely
irregular form (7.7, fig. 6). The outer margin of the girdle con-
sists of complex prominent ridges, which fit into corresponding
grooves in the uterine mucous membrane. These ridges extend
some distance along the main trunks of the vessels passing to and
from the allantois. Part of the space within the folded girdle is
occupied by numerous delicate ridges, which also doubtless help
in retaining the embryonic sac in its place. Up to this time the
embryo is suspended by means of the yolk sac from the upper
wall of the uterus. Hence all these ridges and processes fit into
erooves and depressions in the mucous membrane lining the roof
“of one of the uterine horns.?

Tn the six-weeks’ embryo the cells of the outer tunic beyond
the girdle are unaltered, but at the end of the seventh week this
tunic presents a countless number of minute dots. These dots (7.2.),
which are due to the elongation of small groups of cells, are the
first indication of the coming nutritive processes or villi. Later
a villus sprouts out in the position of each dot, the cells of the
tunic forming a covering for the allantoic outgrowth in very
much the same way as the finger of a glove covers a finger.
Whether these minute patches of elongated cells at the end of
the seventh week assist in taking in nourishment, and also in
fixing the embryonic sac, further investigations will doubtless
reveal. The interesting point to notice is that the rudiments of

' Although the absorbing area is small at the end of the seventh week, the
yolk sac contains countless numbers of minute granules, which are doubtless of
nutritive value.

? Usually the embryo, to start with, in the mare occupies the right horn, with
its head nearest the body of the uterus. Later, part of the embryo may lie in
the enlarged body of the uterus; but the hind limbs remain to the last in the
right horn,

DEVELOPMENT OF THE HORSE. Ny

the more permanent structures (the villi) concerned with the
nourishment of the embryo have appeared before the end of the
seventh week. Much must depend on what happens during the
beginning of the eighth week. If the embryo has been develop-
ing in a normal fashion, and is sufficiently vigorous, villi will
sprout out from every part of the now extensive allantoic sac,
and each villus as it grows will receive a covering of cells from
the outer tunic. If the lining of the uterus is in a healthy and
sufficiently vigorous condition, it will provide pits for the villi
as they develop.

When the villi actually begin to grow is not yet known; but
though small and simple, they are well formed, and present in
countless numbers at the end of the eighth week. Once estab-
lished, they increase in size and complexity (7°, 7°), which im-
plies they become more and more capable of obtaining a plentiful
supply of nourishment, and of fixing the ever expanding sac con-
taining the embryo to the lining of the uterus.

It is hardly necessary to explain that when an embryo is at
last provided with a countless number of allantoic villi, it is in
an infinitely better position than when its only means of obtain- -
ing supplies was a small porous area through which nutriment
filtered from the cavity of the uterus. Each villus may be com-
pared to a delicate branching rootlet, only it has the advantage (1)
of having blood rapidly circulating through its various branches,
by which the nutriment collected is conveyed at once to the
embryo, and (2) of having a relatively enormous amount of a
highly nutritive fluid—the maternal blood—brought sufficiently
near to admit of the materials required for building up the bones
and muscles and other parts of the embryo being readily absorbed.
But the villi may be said to play the part of leaves as well as
roots, for they admit of an exchange of gases. Through them the
foetal blood gets rid of its poisonous carbonic acid, and at the
same time secures a fresh supply of oxygen, without which tissues
can neither be built up nor maintained. When the villi begin
to act is not yet known. Though still very small, they are quite
visible to the naked eye at the end of the eighth week. If we
suppose they are fairly efficient by the middle of the eighth
week, it will be evident, if the seven-weeks’ is compared with
the eight-weeks’ embryo (fig. 7), that they are infinitely more

18 A CRITICAL PERIOD IN THE

efficient than the primitive apparatus in operation during the
earlier weeks.

The Hight-Weeks Horse Embryo.—This embryo is twice the
length and four times the weight of the seven-weeks’ embryo.
As fig. 7 shows, at eight weeks we have a miniature horse. The
mouth is now closed, the eyes are provided with eyelids, the ear
projects some distance from the head, and the tail, compared with
the legs, is relatively short. The amnion (am.) still forms a com-
plete robe or mantle, and, by means of the fluid it contains, to the
last is of the utmost use both for the protection of the foetus and
the dam. The two stalks—yolk and allantois—have already
blended for some distance (sta.), but the yolk sae (y.s.), though
now of little or no use, still persists! The absorbing area (a-c)
is very small, but still surrounded by a prominent ring (az.),
external to which the girdle (¢g.) may still be seen. The
allantoic sac is considerably larger than at seven weeks, and there
projects from it a countless number of villi. Some of these villi
are represented, but on too large a scale, in fig. 7 (all. villt).
The majority of them are still simple processes, but in a few
branching has already set in. As they increase in size they form
miniature vascular trees (7°, 7°), which occupy equally complex
pits in the lining membrane of the uterus? That the new mode
of catering for the wants of the embryo is an efficient one will
be still more evident when I mention that a four-months’ em-
bryo, «.e., an embryo twice the age of the one represented in fig:
7, may be over ten inches in length.

Having shortly described the foetal appendages of four horse
embryos, I trust those interested in the development of the horse
will now be better able to realise the conditions under which
the embryo lives during the earlier weeks, the provisional and
more permanent arrangements for its nourishment, and especially
that a critical stage is reached as the yolk sac ceases to convey
nutriment, and the allantoic villi come into action. As I have
found breeders and others practically interested in our domestic
animals only too glad to learn all that science has to say on

1 At six months the yolk sac has about the same capacity as at six weeks, but
it is folded longitudinally, and completely concealed in the umbilical cord.

2 The villi in the mare are never more than an eighth of an inch in length,
and at birth they are simply withdrawn from their pits.

DEVELOPMENT OF THE HORSE. 19

problems which have long been puzzling them, and sometimes
urgently pressing for solution, there is no apology needed for
having treated this subject on strictly scientific lines.

SUMMARY.

I have endeavoured to show that the horse embryo, like the
young opossum, is inclosed by a special inner tunic (the amnion),
and provided with a pair of appendages (the yolk sae and the
allantois), and, further, that a thin outer cellular tunic (the
embryonic sac) completely envelops the embryo and all its
belongings. The fluid which fills up the space between the
embryo and the amnion forms a sort of water-jacket, which is
doubtless most useful alike for the protection of the embryo and
its dam. ‘The yolk sac, unlike the corresponding structure in the
chick, is empty, for the simple reason that there is plenty
nourishment available in the uterus or in the blood abundantly
coursing through its walls. The more readily to secure this
nourishment, part of the yolk sac fuses with the outer cellular
tunic to form a sort of filter, through which the nutriment enters
the originally empty sac. The nutritive material, after it may
be undergoing some chemical change, finds its way from the
cavity of the yolk sac into the blood-vessels which ramify in its
wall as far as the edge of the filter or absorbing area.

In the opossum, and the vast majority of the marsupials, the
young are born in a very immature condition—as soon as they
are able to seize and hang on to the teats. These ancient forms
have not discovered that, by utilising the allantois (originally a
breathing organ), they might considerably prolong the protection
afforded by the uterus ;1 that when the yolk sac fails, by throwing
out root-like processes from the allantois, they might tap an
almost inexhaustible food supply. But all the higher forms (the
Eutheria) utilise the allantois, and some of them are already so
perfect at birth that the time may come when the milk glands
(very old-fashioned organs), so essential to the marsupials, may
in some cases be entirely, or almost entirely, dispensed with.

' The Bandicoot is the only marsupial in which the allantois has been shown
to take part in nourishing the embryo,

20 A CRITICAL PERIOD IN THE

In the case of the horse, the yolk sac ceases to provide a suffi-
cient supply about the end of the seventh week ; but the embryo,
instead of being born at this period, has the new and more
efficient nutritive structures provided—the allantoic villi.

During the first seven weeks the embryo is fixed to the lining
of the uterus by means of the embryonic sac. In the region of
the absorbing area some of the foetal cells blend with the adjacent
uterine cells. Around the absorbing area a circular adhesive
ring is formed. This ring shrinks as the absorbing area
diminishes. A supplementary grappling apparatus appears in
the form of a girdle, which becomes, up to the seventh week,
more and more complex, and travels towards the ring just men-
tioned as the allantois increases in size.

The allantois, a simple breathing organ in the chick and young
reptile, is probably concerned in the aeration of the blood from
an early period—the third or fourth week—in the horse. But
by the end of the seventh week minute patches of enlarged cells
belonging to the outer tunic indicate that allantoic outgrowths
will soon appear, and by the end of the eighth week thousands of
villi have sprouted out from its surface, and are already lodged
in minute pits specially formed in the lining of the uterus.
These villi not only procure nourishment and fresh supplies of
oxygen, they further fix more or less firmly the embryonic sac to
the wall of the uterus.

At the end of the third week of gestation, when the reproduc-
tive system passes through one of its periods of general excite-
ment, about one-fourth of the embryonic sac probably adheres to
the uterus; but at the end of the sixth week, when another wave
of disturbance arrives, all the grappling structures are at one
pole. Hence there is probably more chance of the embryo
“slipping” at the end of the sixth than at the end of the third
week. About the end of the seventh week the supply of nourish-
ment by means of the yolk sac is coming to an end, and there
is perhaps still about this time a hereditary tendency for the
embryo to escape. Unless the new and more permanent nutri-
tive apparatus is provided, unless a countless number of villi

rapidly sprout out from the allantois, the embryo will die from
starvation during the eighth week, and in a few days be
discharged.

DEVELOPMENT OF THE HORSE. My il

It may therefore be taken for granted that there is a certain
amount of danger at the end of the third and sixth weeks, but
that the most critical period is about the end of the seventh or
beginning of the eighth week ; for unless the villi appear in time
and succeed in coming into sufficiently intimate relation with
the uterine vessels, the developmental process is of necessity for
ever arrested.

CONCLUDING OBSERVATIONS.

Evidently, from what I have already stated, breaking service
may be due either to abnormal development of the embryo or to
its surroundings being unsuitable.

Nature may be said to have provided against what we call
breaking service. When one thinks of the millions of eggs pro-
duced in a single season by the cod and certain other fishes, one
-is apt to suppose that there is no relation between the number of
ova and the wants of the species. On the other hand, the solan
goose lays but one egg a year, and the elephant may not mature
a score of egos in a hundred years. I believe a mare may ripen
and shed from ten to twenty eggs during the year. If this is the
ease, bearing in mind that the period of gestation is eleven
months, evidently in the mare provision has been made against
accidents during the earlier weeks of the breeding season. Con-
sidering first how the embryo may be concerned with breaking
service, I may point out that if the egg or the sperm has been in-
sufficiently nourished, or, it may be, over nourished, the segmen-
tation of the egg may be abnormal; or, if this is successfully
accomplished, a feeble embryo may be the result—an embryo
with, it may be, a small allantois, or with an imperfect yolk
sac circulation, or an embryo incapable of anchoring itself to
the lining of the uterus. Or the embryo may reach the seventh
week, but either because the allantois is insufliciently developed,
or because it lacks the energy required to throw out the all-
important villi, the more permanent relations may never be

1Tn discussing this subject it will be best to take for granted that the mare
has settled, and held at least beyond the third week. It would be useless
to include mares which haye never settled when dealing with this subject.

iD) A CRITICAL PERIOD IN THE .

established, with the result that 1t is sooner or later discharged.
Hence the necessity of having vigorous germinal cells to start
with—in other words, of the breeding stock being in a healthy
and vigorous condition ; for whether it is or is not possible to
transmit acquired characters, it is certainly possible, by means of
unsuitable surroundings and injudicious feeding, to diminish the
vitality of both the eggs and sperms.

With regard to the immediate surroundings of the embryo,
probably the conditions are oftenest rendered unsuitable for the
development of the embryo by an unhealthy state of the uterine
mucous membrane. The uterine secretions may be acid instead
of alkaline (they are said to be acid in mares that have fre-
quently broken service ; acid secretions destroy the sperms as
they pass upwards through the uterus), excessive or the opposite.
They may, on the one hand, be unsuitable for the due nourish-
ment of the embryo, or they may prevent it being properly fixed.
At a later period, by accumulating between the basis of the villi,
the secretions may actually dislodge them from their pits. But
even should the mucous lining of the uterus be normal, neither
too congested nor too anzemic, nor yet too irritable, the muscular
fibres which enter so abundantly into the uterine wall may be
out of order, or the nerves supplying them in a state of unrest.
The fibres in the neck and body of the uterus may be relaxed or
wanting in tone, or the fibres of the horns, or it may be of the
whole organ, may be subject to frequent spasmodic contrac-
tions. Whether such contractions result from local irritation or
general nervous excitement, if excessive and long continued, the
chances of the embryo being long retained will be extremely
small.

The question now arises, can anything be done to prevent
mares breaking service? In dealing with the horse, the fact
must never be lost sight of that he is an extremely high-strung
animal, liable in a panic to completely lose the little self-control
he has inherited from his wolf-worried ancestors. This nervousness,
which was his salvation when in a wild state, has in some respects
been increased rather than diminished by the unnatural life it is
now his lot to lead. The horse has strong likes and dislikes, and
frets often when separated from his companions. This being the
case, it will be easily understood that sudden changes—changes

DEVELOPMENT OF THE HORSE. 23

of temperature, of food, of companions, changes of his surroundings
or environment generally, will greatly influence mares, more
especially at the breeding season.

It is well known that changes influence more or less markedly
all kinds of animals. A change from one country or one district
to another may, ¢.g., lead to increased fertility, or it may result in
complete sterility. This is doubtless because the reproductive
system is extremely sensitive to changes of every kind. In
several cases mares which I received from a distance required
several months to adapt themselves to their new conditions, and in
all cases the reproductive system was the last to assume its
normal state. But what bearing has this on the subject in hand ?
It all points to a very careful and thoughtful treatment of mares
at the beginning of the breeding season. For example, now
(April) that the grass is coming on, I might have several
mares mated with my zebra—mares which have been under
cover and generously fed during the winter,—and then turned
out to grass in the same field. The change from the loose-
boxes to the field would be nearly as great as from the South
of England to the Pentlands, and if the reproductive system
is most sensitive to changes, the chances would be strongly
against their settling. But even if proof against the effects of
substituting an outdoor for an indoor life, turning them one by
one immediately after mating would be most unwise. Some
mares, even when in foal, “ tease” other mares as persistently as a
horse. In this way, the excitability which should be allayed, or
at least allowed to subside, might be kept up if not exaggerated.
At the beginning of the breeding season changes of food, of tem-
perature, and of the surroundings generally should be made as
carefully and judiciously as possible ; and during at least the first
two months after service the mares should neither be over-
excited nor over-exerted, neither chilled nor over-heated, neither
over nor under-fed, and, in fact, all extremes should be carefully
avoided ; and it should be remembered that drugs readily reach,
and may profoundly influence, even young embryos.

If in this case being forewarned is to be forearmed, some
good may result from the conditions under which the horse
embryo exists during the early weeks of development having
been shortly explained. The practical application of the facts

24 A ORITICAL PERIOD IN THE DEVELOPMENT OF THE HORSE.

established may well be left to breeders and veterinarians. I
may, however, in conclusion, make the following suggestions :—

1. That mares which have been indoors during the winter, and
which are to run at grass during the summer, should be, as it
were, acclimatised before they are served, ae., they should be
allowed to run out night and day for two or three weeks, in order
to have time to adapt themselves to the change of food, the some-
what marked variations of temperature, and to their new sur-
roundings generally.

2. That mares, more especially excitable ones, should be served
in the evening, and shut up apart from other mares or geldings
during the night. They should then, until the periodic disturb-
ance has subsided, be kept in a paddock as far removed as possible
from mares or geldings likely to tease them.?

3. That when any signs of cestrum are detected in a mare
(whether she has been previously presented to the horse or not),
she should be removed from mares believed to be already in foal.

4. That each mare should be carefully watched from week to
week, and periodically—every ninth or tenth day—tried until
the critical period has been successfully passed.

5, That mares backward and out of condition in the spring
should be allowed for some weeks oats at least once a day; for
unless they are in a healthy and vigorous state, ova may not be
discharged until the summer is well advanced; or, if ovulation
takes place, the eggs may be so impoverished that the embryo
may fail to survive the critical period, or, if it succeeds in this,
develop into a small, weedy foal. It is all but impossible,
apparently, for a mare to produce strong, vigorous twins capable
of eventually reaching the size of their parents. This is evidently
a matter of nourishment. Hence, if the dam is out of condition,
or if the foetal circulation is weak, or the allantoic villi in any
way unsatisfactory, the foal, if born alive, is not likely to be a
source of credit or profit to the breeder.

| This year a very healthy, well-bred, cream-coloured mare, though repeatedly
served by the zebra during the months of March, April, and May, did not settle.
She was again served by the zebra on the 28rd and 25th of June, and then placed
in the hands of a breaker, so that she might, if necessary, be sold. I have every
reason to believe she is now (July 1897) holding. Probably we were at last suc-
cessful, because this mare was taken in from grass and kept regularly at work.

. DESCRIPTION OF THE FIGURES, 25

DESCRIPTION OF THE FIGURES.

Fras. 3-7. Natural size; the appendages are represented in section,
and are semi-diagrammatic.

d, Outer embryonic sac or tunic. | part of the allantois carries
a, b,c, Absorbing area (yolk pla- | non-vascular villi (c.v.).

centa), consisting of cells) a//. villi, Allantoic villi, each with

(trophoblastic) of the em- a covering derived from
bryonic sac, and of the | the embryonic sac.

inner (hypoblastic) cells of | c.v., Villi (coelomic) lying between

the yolk sac. | the allantois and yolk sac.

an., Section of ring (annulus) sur- | These internal villi ‘are

rounding absorbing area. | naked (not invested like

t.g., Girdle (trophoblastic), formed the external villi), and are

entirely by a modification non-vascular ; they have

of the outer cells of the not hitherto been found in

any mammal, and their

embryonic sac.
function is unknown.

am., Amnion—the amnion eventu-

ally blends with the allan- sta., Part of the umbilical cord
tois. within the amnion formed
all,, Allantois—the stalk connect- by the junction of the allan-
ing the allantois with the toic and yolk stalks, and
embryo is only shown in containing the main trunks
figs. 1 and 2. | of the allantoic and yolk
all.,1 Allantois completely fused sac vessels.
with the embryonic sac, v., These dots indicate that the
which provides a covering greater part of the wall of
for the allantoic villi. the yolk sac is vascular.
all.,? Allantois in connection with v.,! These dots indicate that the
the amnion. allantoisis vascularthrough-
all.*-all.,* The part of the allantois in | out its whole extent.
connection with the yolk | v.,- Allantoie vessels, from which
sac—in the fifth to the branches pass into the villi,
eighth week embryos this y.s., Yolk sac.

Fic. I ‘The hen’s ege at the ninth day of incubation, The embryo
is connected by a short stalk to the yol/ sac, which contains
nearly all the food required during the developmental pro-
cess. The additional nourishment is provided by the
white or albumen which surrounds the yolk. The allantois
at an early stage grows out from the intestinal tract, be-
comes extremely vascular, and plays the part of a breathing
organ. Numerous vessels are also formed in the wall of
the yolk sac; the dots are intended to indicate that the
yolk sac and allantois are vascular, The shell is lined with
a thin membrane. At one end this membrane is separated
from the shell, leaving a space—the air chamber. After
Milnes Marshall.

Vic. 2. Represents a young opossum and its foetal appendages. The
wall of the yolk sac (y.s.) is vascular as far as the circular
blood vessel (s.¢., sinus terminalis). In the area a, 0, ¢, the
yolk sac blends with the outer embryonic sac, Through

26 DESCRIPTION OF THE FIGURES.

this area the cells (trophoblastic) of the outer sac are modi-
fied so as to assist in taking up nourishment—the uterine
milk—from the uterus, and in fixing the embryo during its
uterine development. The allantois (a//.) never reaches
the outer sac. It is vascular, and serves only as a breathing
organ. After Osborn and Selenka.

Fic. 5. A semi-diagrammatic representation of a four-weeks’ (28
days) horse embryo and its foetal appendages. The embryo,
which is curved so that the tail lies under the head,
measures nearly three-eighths of an inch in length. The
limbs are represented by lobes entirely composed of cells,
7.e., the rudiments of the iimb skeleton have not yet
appeared. Behind the head are three arches and three
clefts, but the clefts do not appear to open, as in fishes, into
the pharynx. The amnion (am.) surrounds the embryo,
and two stalks proceed from the under surface. The
stalk proceeding to the left side connects the embryo with
the yolk sac (y.s.), the stalk passing to the right contains
the vessels of the allantois. The allantois (a/l.) is already
in contact with the embryonic sae (d), and with the
amnion, and it has many vessels (v') in its wall. The
yolk sac is vascular (v.), as far as the circular blood-vessel
(s..), and crowded with granules which have entered
by the absorbing area (a, 6, ¢). The cells of the outer
tunic at ¢.g. (on a level with the growing point of
the allantois) have undergone considerable elongation, while
the cells in the area (a, 0, ¢) have given rise to a number
of irregular ridges and processes.

Fria, 4, Represents a five-weeks’ (35 days) embryo. Note that the
absorbing area (a, 6, c) is smaller than in the 28 days’
embryo, while the capacity of the allantois is greater.
Delicate villi (¢.v.) now project from the allantois towards
the yolk sac, and the girdle (¢.g.) now consists of distinct
nearly parallel ridges, with furrows between—these ridges,
as well as the ridges of the ring (am.) around the absorbing
area, have been exaggerated in the drawings.

Fia. 5. The 42 days’ embryo and its appendages. Note especially
that the girdle (7.7.) lies near the absorbing area (a, 8, ¢),
and that the allantois is now a relatively large sac in con-
tact with the greater extent of the outer tunic, and sur-
rounding the greater part of the yolk sac.

Via. 6. The seven-weeks’ (49 days) horse embryo. Contrast this
with the other embryos, more especially with the five and

a7

DESCRIPTION OF THE FIGURES.

eight weeks’ embryos. Note that while the yolk sac has
remained almost stationary, the absorbing area has dimin-
ished, while the capacity of the allantois (al/.) has greatly
increased. The villi (¢.v.) extending from the allantois
towards the yolk sac are numerous, long and slender, but
still devoid of blood-vessels, Indications of the coming
external vascular villi occur in the form of minute dots
(t.t.) over the surface of the embryonic sac. The yolk-
sac and allantoic stalks have already united at their inner
or proximal ends.

Tia. 7. Represents an eight-weeks’ (56 days) horse embryo sus-
pended in the amniotic cavity, and bathed by the amniotic
fluid. The embryo is connected with the yolk sac (y.s.)
and the allantois (only part of which is shown) by rela-
tively large blood-vessels. These vessels and their investing
tissues form the proximal part of the umbilical cord (s¢a.).
The yolk sac, though having still as great a capacity as
formerly, is folded longitudinally, so as to occupy a com-
paratively small space. Note that the absorbing area is
now very small, and especially that numerous simple villi
now project from the surface of the embryonic sac. Each
villus consists of a vascular allantoic core, and a thin capsule
derived from the embryonic sac. The villiare represented on
too large a scale, but the embryo and yolk sac are natural size.

*. Represents three villi from an eight weeks’ embryo.

» A single villus from a sixteen-weeks’ embryo, and

7°. A single villus from a twenty-four-weeks’ embryo —all thirty

times natural size.

The villi fit into pits or moulds in the lining of the uterus.
As the foetal blood circulates through the villi it comes
sufficiently near the maternal blood circulating through the
wall of the complex pits or moulds to admit of an exchange
of fluids and gases—the foetal blood transfers its excess of
carbonic acid to the maternal blood, while the maternal
transfers to the foetal blood some of its oxygen, and also
fluids containing all the ingredients required for the develop-
ment and growth of the embryo. ‘There is, however, no
actual mixing of the maternal and foetal blood. At birth
the foetal villi are simply withdrawn from the uterine pits.
The villi and pits together are generally spoken of as the
Placenta.

ba |

I

NEILL AND COMPANY, PRINTERS, EDINBURGH.

J.C. Ewart, del

Fig. 5.
(6 weeks |

M¢ Lagan & Comrung, Lith’ Kein’

“ Ta =
a
Tt ry. SS ee ore cee ead

na = ——S.-__ 8 __ er
SS a

tr “nr +t r r + : ie a
ED mera lm A ULUEUALL) Sp, ueSerT 3M “Jp FEM TOP

‘SYBAM 42 YO
SUES

)

a:

S2770A

———— ee

NA

4 110 361