FCETAL NUTRITION: THE PLACENTA 415 



duct. While the true splanchnic stalk of the yolk-sac is becoming 

 narrow, a somatic stalk connecting the amnioii with the walls of the 

 embryo is also formed, and closely envelops the stalk both of the 

 allantois and yolk-sac. The somatic stalk, together with its contents, 

 is known as the umbilical cord " (Balfour x ). The yolk-sac atrophies 

 completely in some, but in others it is only removed at birth. 



II. THE NUTRITIVE 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-mammalian Vertebrata the latter condition has been 

 observed. In the Lacertilia the yolk-sac absorbs nutriment from the 

 uterus through the porous shell. In Mustelits Icevis the embryos lie 

 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 2 ). In Seps chalcides, a reptile, the insufficient 

 supply of yolk is added to by a uterine secretion containing 

 degenerated cells and blood derivatives, the outer layer of the 

 blastocyst being distinctly phagocytic (Giacomini 3 ). But in the 

 Sauropsida no union takes place between the maternal tissues and 

 the foatal membranes, and so in one order of Mammals, the 

 Ornithodelphia, 4 where the young develop 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, 

 splits into two layers. In the non-mammalian Vertebrates, the 

 mesoblast and the ceelom 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 still 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. 102) : (1) The non-vascular part, with a two- 

 layered wall of epiblast and hypoblast ; (2) the vascular part, where 

 the mesoblast is unsplit, e.g. in the opossum the mesoblast splits in 

 its entire extent in the rabbit ; (3) the part opposite the ceelom. In 

 all three parts the trophoblast is bathed by the uterine secretion 



1 Balfour, Comparative Embryology, London, 1881. 



2 Brinkmann, " Histologie, Histogenese und Bedeutung der Mucosa Uteri 

 einiger Viviparer Haie und Rochen," Mitt. a. d. Zool. Stat. z. NeapeL, vol. xvi., 

 1903. 



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

 vi., 1891. 



4 Or Monotremata. 



