EARLY MAMMALIAN DEVELOPMENT. 



105 



the.notocord corresponds in main outline to their behavior in the lower 

 mammals and in birds. 



The series of diagrams in Figs. 80, 81 and 82 has been constructed to give 

 the student a general idea of the changes that occur in the early stages of 

 human development. It must be recognized, however, that the diagrams 

 represent purely hypothetical stages up to the conditions shown in diagram 

 B in Fig. 81 which corresponds roughly to the Bryce-Teacher embryo (Fig. 

 73) ; even in this diagram the extent of the mesoderm is much less than in the 



JeUfSbtt 



Belly Stalk 

 AlUjtois 



D 



FIG. 82. Diagrams representing stages of development of the human embryo (to follow Fig. 81). 



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

 74 and 73). Owing to the rapid enlargement of the chorionic vesicle, the extraembryonic 

 body cavity has become much larger than in Fig. 81 , C. B,A stage (in longitudinal section) 

 corresponding to that of von Spec's embryo (Fig. 77) . 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. 



known human embryo. In Fig. 82 diagram A approximates the Peters 

 embryo (Fig. 74), diagram D the von Spee embryo (Fig. 77). The history 

 of the accessory structures which are shown in part will be considered in the 

 chapter on "Fcetal Membranes." 



