GERM LAYERS. 



95 



^arable with conditions in the bat (Fig. 52). In the next stage (Fig. 89, B) 

 he mesoderm is present all the way around between the trophoderm and ento- 

 lerm, in the roof of the amniotic cavity, and between the ectoderm and entoderm 

 n the embryonic disk. It is possible that the mesoderm arises in situ as a deriv- 

 itive of the entoderm or trophoderm. Since in the lower Mammals it arises 

 :rom entoderm, a similar origin here seems the more reasonable. 



Jetty Stalk 



5-talk 



OLS 



D 



FIG. 90. Diagrams representing stages of development of the human embryo (to follow Fig go) 

 I, A stage that corresponds approximately to those of Peters' and Bryce-Teacher's embryos (Figs 

 83 and 107). Owing to the rapid enlargement of the chorionic vesicle, the extraembryonic 

 body cavity has become much larger than in Fig. 89, C. B, A stage (in longitudinal section) 

 corresponding to that of von Spec's embryo (Fig. 85). Only a part of the chorion is shown- 

 the embryonic disk is slightly constricted from the yolk sac; note the belly stalk, comparing 

 A C Transverse section, same stage as B. D, Lonigtudinal section, stage somewhat 

 later than B. Note the greater degree of constriction between the embryo and yolk sac, and 

 the larger ammon. 



In the majority of the lower Mammals the intraembryonic mesoderm arises 

 rom the entoderm and then grows out into the wall of the blastodermic vesicle, 

 n a few, however (sheep, roe, shrew), the peripheral mesoderm (p. 87) 

 -rises outside of the embryonic disk and unites with the intraembryonic meso- 

 lerm secondarily. It might be suggested that the formation of peripheral 



