EARLY MAMMALIAN DEVELOPMENT. 



103 





peared within the mesoderm, so that one portion remains as a lining for the 

 trophodermal wall and the remainder closely invests the yolk sac and amnion 

 and also forms a layer between ectoderm and entoderm in the embryonic 

 disk (Fig. 75). The disk is therefore composed of all three germ layers, but 

 there is still no indication of a primitive streak. It would seem that in the 

 highest primate the mesoderm develops independently of the primitive 

 streak; but whether it arises from ectoderm or entoderm it is not possible in 

 the present state of our knowledge to determine. 



D 



FIG. 80. Diagrams representing hypothetical stages in the development of the human embryo. 

 A, Morula; compare with Fig. 55, a. B, Morula with differentiated superficial cells; compare with 

 Fig- 55) b. C, Central cells have become vacuolized to form the yolk cavity, leaving a small 

 group (the inner cell mass) attached to the enveloping layer (trophoderm) ; compare with 

 Fig- 5 5 , d. D, Cells of the inner cell mass which are adjacent to the yolk cavity have become 

 differentiated and have begun to grow around the cavity, forming the entoderm; compare 

 with Fig. 59, a. 



In a somewhat older human embryo described by von Spee a dorsal view 

 of the embryonic disk shows close resemblances to conditions in the lower 

 mammals (Fig. 76). The position of the primitive streak is indicated by the 

 conspicuous primitive groove. Anterior to this the neural groove extends 

 almost the full length of the disk which has become considerably elongated. 

 The yolk sac is now suspended from the ventral side of the disk. 



