GERM LAYERS. 



91 



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

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

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

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

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

 from entoderm, a similar origin here seems the more reasonable. 



FIG. 90. Diagrams representing stages of development of the human embryo (to follow Fig. 89). 

 A t 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 exfraembryonic 

 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 

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

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

 the larger amnion. 



In the majority of the lower Mammals the intraembryonic mesoderm arises 

 from the entoderm and then grows out into the wall of the blastodermic vesicle. 

 In a few, however (sheep, roe, shrew), the peripheral mesoderm (p. 83) 

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

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



