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 



