THE NUTRITION OF THE EMBRYO. 847 



wall is frequently spoken of as the decidua serotina. As the ovum develops 

 into the foetus with its membranes, the decidua reflexa becomes pushed 

 against the decidua vera ; about the end of the third month, in the human 

 subject, the two come into complete contact all over, and ultimately the dis- 

 tinction between them is lost. In the region of the decidua serotina, the 

 allautoic vessels of the fcetus develop a placenta. 



788. In the earliest stages of the development of the placenta, the 

 delicate villous processes of the chorion insinuate themselves into the hyper- 

 trophied follicles of the decidua serotina. The villi then undergo a rapid 

 increase in size and vascularity, becoming branched into secondary and 

 tertiary ramifications ; while, at the same time, corresponding changes are 

 taking place in the follicles, by which they become greatly increased in size 

 and vascularity, and at the same time forming diverticula in which are im- 

 bedded the ramifications of the villi. The villi and follicles thus grow 

 simultaneously, and finally become blended with each other and are no 

 longer separate structures. The follicular bloodvessels first form capillary 

 plexuses ; these vessels, however, become enlarged, forming frequent anasto- 

 moses, and finally coalescing to form venous sinuses (Fig. 229), in which are 

 bathed the fcetal villi. There is no continuity established between the 

 maternal and fcetal blood ; the interchange of nutritive material necessary 

 for the growth and development of the foetus takes place through the delicate 

 walls of the villi.] 



For further account of the various changes by which these events are 

 brought about, as well as of the history of the embryo itself, we must refer 

 the reader to anatomical treatises. 



CHAPTER IV. . 



THE NUTRITION OF THE EMBRYO. 



789. During the development of the chick within the hen's egg the 

 nutritive material needed for the growth' first of the blastoderm, and subse- 

 quently of the embryo, is supplied by the yolk, while the oxygen of the air, 

 passing freely through the porous shell, gains access to all the tissues both of 

 the embryo and the yolk, either directly or by the intervention of the allan- 

 toic vessels. The mammalian embryo, during the period which precedes the 

 extension of the allantoic vessels into the cavities of the uterine wall to 

 form the placenta, must be nourished by direct diffusion, first from the con- 

 tents of the Fallopian tube and subsequently from the decidua ; and its 

 supply of oxygen must come from the same sources. All analogy would 

 lead us to suppose that, from the very first, oxidation is going on in the 

 blastodermic and embryonic structures ; but the amount of oxygen actually 

 withdrawn from without is probably exceedingly small in the early stages, 

 seeing that nearly the whole energy of the metabolism going on is directed 

 to the building up of structures, the expenditure of energy in the form of 

 either heat or external work being extremely small. The marked increase 

 of bulk which takes place during the conversion of the mulberry mass into 

 the blastodermic vesicle shows that at this epoch a, relatively speaking, large 

 quantity of water at least, and probably of nutritive matter, must pass from 

 without into the ovum ; and subsequently, though the blastoderm and em- 



