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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.

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 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 "Foetal Membranes".

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