THE YOLK-SAC. 



85 



gathered into two compact groups, symmetrically placed in the extra-embryonic 

 region, but not far from the embryo. The cells then break apart from one 

 another and gradually become separated, and migrate by unknown means, first 

 over the wall of the intestine, which has meanwhile been differentiated, then 

 over the surface of the mesentery into the anlage of the genital gland. During 

 their entire migration they are lodged in the mesothelium, and when they have 

 reached their final destination they are still in the mesothelium of the genital 

 anlage, where they remain until finally differentiated in the adult. The epithe- 

 lium, with the germ-cells in their definite position in it, is called the germinal 

 epithelium (compare page 39). The germinal 

 epithelium has been observed in all vertebrates, 

 but the origin of the germ-cells in amniota is 

 entirely unknown. The hypothesis may be ac- 

 cepted, that they arise in a manner essentially 

 similar to that known in the dog-fish. For some 

 of the theories based on the known develop- 

 ment of the germ-cells, see page 40. 



The Yolk=sac. 



General Morphology. — The yolk-sac is the 

 container of the nutritive yolk destined to be 

 assimilated by the embryo. The principal 

 factor in its morphological constitution is the 

 entoderm, which, after the differentiation of the 

 definitive germ-layers, contains nearly all of the 

 yolk material. In the primitive vertebrates, 

 as exemplified by the marsipobranchs, ganoids, 

 dipnoi, and amphibia, we find this yolk material 

 lodged in the walls of the primitive digestive 

 tract. It is situated chiefly on the ventral side of this tract, and extends from the 

 point where the heart is formed toward the tail of the embryo to the point where 

 the- allantois is formed. In other words, it is situated in a region corresponding 

 to the territory of the future abdominal cavity. In the primitive types just re- 

 ferred to, the yolk-bearing entoderm becomes divided into distinct cells which 

 form a large mass. The condition may be understood from Fig. 30, which repre- 

 sents a transverse section of the early stage of an axolotl embryo. The cavity 

 of the entodermal canal (digestive tract) is small. It is bounded on its dorsal 

 side by a single layer of cells distinctly epithelial in their development, and 

 on the ventral side by a great mass of rounded cells heavily laden with yolk 



Fig. 34. — Diagrammatic Section of 

 the Yolk of a Hen's Egg at 

 an Early Stage to show the 

 Relation of the Primitive Ento- 

 dermal Cavity, Aeh. 



Civ, Ccelom. ///, Intestinal cavity. Som, 

 Somatopleure. Spl, Splanchnopleure. 



