380 THE PHYSIOLOGY OF REPRODUCTION 



II. The Nuteitive Importance of the Yolk-Sac 



When the blastodermic vesicle becomes adherent to, or 

 sinks into, the uterine mucosa, the wall of the yolk-sac in some 

 orders becomes intimately related to the uterine mucosa and 

 is nourished by it. Even in the non-mammahan Vertebrata 

 the latter condition has been observed. In the Lacertiha 

 the yolk-sac absorbs nutriment from the uterus through the 

 porous shell. In Mustelus Iwvis the embryos he in a fluid 

 derived from the surface secretion and a lymphoid transudate 

 of the uterine mucosa. It passes through the porous shell to 

 reach the yolk-sac (Brinkmann ^). In Seps chalcides, a reptile, 

 the insufficient supply of yolk is added ta by a uterine secretion 

 containing degenerated cells and blood derivatives, the outer 

 layer of the blastocyst being distinctly phagocytic (Giacomini ^). 

 But in the Sauropsida no union takes place between the 

 maternal tissues and the foetal membranes, and so in one 

 order of Mammals, the Ornithodelphia,^ where the young de- 

 velop outside the body. In all the other orders the wall of the 

 yolk-sac comes into relation with the uterine wall over a greater 

 or less area, depending on the extent to which the mesoblast, 

 spreading round the wall of the blastocyst, spUts into two layers. 

 In the non-mammahan Vertebrates, the mesoblast and the 

 coelom extend completely round and the yolk-sac is entirely 

 separated from the surface layer ; so in the sheep and Man. 

 In others {e.g. the rabbit) the coelom does not spread so far. 



It stiU remains to consider the path by which the nutriment 

 is conveyed to the embryo. In partial extension of the area 

 vasculosa, the wall of the yolk-sac consists of three parts, each 

 with different relations (see Fig. 77) : — (1) The non-vascular part, 

 with a two-layered wall of epiblast and hypoblast ; (2) the 

 vascular part, where the mesoblast is unspUt, e.g. in the opossum 

 — the mesoblast splits in its entire extent in the rabbit ; (3) the 

 part opposite the ccelom. In aU three parts the trophoblast is 



1 Brinkmann, " Histologie, Histogenese und Bedeutving der Mucosa Uteri 

 einiger Viviparer Haie und Rochen," Mitt. u. d. Zool. Stat. z. Neapel., 

 vol. xvi., 1903. 



' Giacomini, " Ueber die Entwicklung von Seps Chalcides," Anat. Anz., 

 vol. vi., 1891. 



^ Or Monotremata. 



