^ 



FOETAL NUTRITION: THE PLACENTA 415 



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

 narrow, a somatic stalk connecting the amnion with the walls of tin- 

 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 : ). The yolk-sac atrophies 

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



II. THE NUTRITIVE IMPORTANCE OK TIIK 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 Yertebrata the latter condition lias been 

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

 uterus through the porous shell. In Must tin* la-rix the embryos lie 

 in a fluid derived from the surface secretion and a lymphoid transudatr 

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

 the yolk-sac (Brinkmann 2 ). In Sep* clittlriilex, 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 foetal 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 blastoc\-t. 

 splits into two layers. In the non-mammalian 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 co-loni 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 an -a vasculosa. 

 the wall af the yolk-sac consists of three parts, each with diHHvnt 

 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 coelom. In 

 all three parts the trophoblast is bathed by the uterine secretion 



1 Balfour, Comparative Embryology, London, 1881. 



2 Brinkmann, "Histologie, Histogeneae und Bedeutung der Mucosa I'tcii 

 einiger Viviparer Haie und Rochen," Mitt. a. d. Zl. Stt. :. .\V,/ /( ,Y., vol. xvi.. 



1903 



3 Giacomini, "Ueberdie Entwicklung von Seps Chalcides, 4tto& .\<>... v..I. 



vi., 1891. 



4 Or Monotreinat;i. 



