CHAPTER III. 



THE NUTRITION OF THE EMBRYO. 



DURING the development of the chick within the hen's egg the 

 nutritive material needed for the growth first of the blastoderm, 

 and subsequently 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 yolk, either directly 

 or by the intervention of the allantoic vessels. The mammalian 

 embryo, during the period which precedes the extension of the 

 allantoic vessels into the cavities of the uterine walls 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, oxida- 

 tion 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 the amount of oxygen actually withdrawn from without is probably 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, shews 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 embryo may for some time draw the 

 material for their continued construction at first hand from the 

 yolk-sac or umbilical vesicle, both this and they continue probably 

 until the allantois is formed to receive fresh material from the 

 mother by direct diffusion. 



As the thin-walled allantoic vessels come into closer and fuller 



