Picture - Cosmic Polymath

PLACENTAL AND FCETAL CIRCULATION 397
PLATE XOII (continued)
D. Permanent arterial trunks in their primitive form, the obliterated portions still shown in dotted lines. n, Aorta ;
b, branches to the lungs ; r, pulmonary artery ; d, root of thoracic aorta ; c, internal and external (/) carotids ; g, ij , axillary
arteries ; h, h', vertebral arteries ; i, common carotid artery (after Carpenter).
Fig. 5.—Formation of cerebro-spinal axis. «, Vesicle of the hemispheres ; ft, vesicle of the tubercula quadrigemina ; c, vesicle
of the medulla oblongata (after Dalton).
Fig. 6.—Brain of human embryo at twelfth week.
A. Seen from behind.
B. Side view.
0. Sectional view. The same letters apply to all the figures. », Corpora quadrigemina ; ft, ft', hemispheres ; d, cerebellum;
e, medulla oblongata ; /, optic thalamus ; g, floor of third ventricle ; I, olfactory nerve (after Carpenter).
The ovum, as stated, generally reaches the interior of the uterus about the eighth day. At the twelfth or
fourteenth day it is a rounded, flattened sac, and measures from three to five milhmetres. It is smooth unless at the
equatorial zone, where short, unbranched villi appear. At the end of the second week the embryo is discernible,
and measures about yV of an inch. The amnion and allantois also make their appearance at this period. It is
not necessary to go into the structure of the several parts forming the embryo and foetus ;
suffice it to say, that the
amnion is a delicate but tough membranous envelope which invests the embryo and contains a fluid in which the
embryo floats ; the umbilical vesicle being the precursor of the umbilical cord or navel string, and the allantois that
of the bladder. The chorion (shaggy chorion) is the outer covering or membrane of the embryo, which in its early
stage is covered with villi or hair-like processes which play an important part in nourishing the embryo. In the
later stages of gestation the chorion only retains part of its villi ;
but these are modified and differentiated, and contain
a very large number of looped, capillary blood-vessels, which with cells, cellular and other tissue, form the foetal part
of the placenta (Plate xcii.. Fig. 1, I, page 396). As the looped capillary blood-vessels of the chorion when gesta-
tion is well advanced interdigitate with the capillary blood-vessels in the mucous membrane of the uterus (maternal
part of placenta), they play a leading role in nourishing and aerating the blood of the foetus (Plate xcv., Figs. 1 and 3,
page 407). Of course the aeration of the blood of the foetus is primarily effected in the lungs of the mother. In
like manner the nourishment of the foetus is primarily traceable to the operations of the stomach and alimentary
canal of the mother. It is, however, through the thin, osmotic walls of the capillary blood-vessels of the foetal and
maternal portions of the placenta that the actual interchange of aerated and specially enriched blood on the part
of the mother, and of more or less impoverished, impure blood on the part of the foetus, takes place. The placenta
at once acts as a lung and a stomach to the foetus. The changes which occur in it are vito-meohanioal and chemical
in their nature. The blood of the foetus parts with carbonic acid and obtains oxygen ; a very necessary interchange,
as foetal blood is, at best, a mixed, impure blood. The hearts of both mother and foetus are at work in carrying
pure and impure blood to and from the placenta, and the deUcate, thin walls of the innumerable capillary blood-
vessels of the maternal and foetal portions of the placenta act as osmotic media for the constant interchange of fluids
and gases alike. All these arrangements are in no way due to irritability of the parts concerned, or to extraneous
stimulation of any kind. They are one and all natural and necessary processes, and take part according to original
design in the great and pre-arranged scheme of reproduction. The vito -mechanical arrangements, and the compU-
cated and extensive series of reactions, are all necessary to bring about the desired result, and nothing is left to chance.
The placenta is the indirect means of communication between the mother and foetus, and it is through this
temporary bond of union that both are influenced. The placenta is an indirect mode of communication, because
the blood of the mother in its entirety does not pass to the foetus or that of the foetus to the mother ;
the interchange
being partial, and confined to the gases and finer fluid parts of the blood in either case. The interchange, as explained,
is osmotic in its nature, there being no open vessels either in the maternal or foetal portions of the placenta.
The impregnated human ovum very soon begins the work of differentiation. At the end of the second week
it is about the size of a small pea ; the entire ovum measuring about i of an inch, and the embryo ^\ of an inch.
The amnion, allantois, and first rudiments of the embryo appear. The chorion and its vilh are also indicated. At
the end of the third week the structures referred to are more developed, the embryo as a whole measuring ^ of an
inch. At this period (third week) the shaggy chorion (chorion plus vilh) is in evidence, and traces of the primary
division of the brain, the cephahc visceral arches, the primitive ocular and auditory vesicles, the primary circu-
lation and a straight aUmentary canal with a pentagonal-shaped mouth can be traced.
At the end of the fourth week the embryo with its coverings is about the size of a pigeon's egg ; the embryo
itself measuring i of an inch in length. At this period (fourth week) the whole chorion is covered with vilh, the
umbiUcai vesicle fully developed, and the body curved upon itself almost in a circle. The segmentation of the trunk
is also indicated by visceral arches and by square-shaped proto-vertebrae ; the future spmal column terminatmg in