232 PHYSIOLOGY CHAP. 



The upper vesicle surrounded by ectodermic cells is the 

 amniotic vesicle ; the lower one surrounded by endodermic cells 

 is the umbilical vesicle. The region of contact between the two 

 vesicles is the embryonic area, in which the cells are distinguished 

 by their cylindrical shape (Figs. 94 and 95). 



In the young human ova described by Peter, von Spee, and 

 C. Giacomini, which were from nine to twelve days old, the 

 mesoderm was already easily recognisable. It not only surrounded 

 the two vesicles (splanchnopleure), but also covered the internal 

 surface of the chorion (somatopleure) ; further, and this is of the 

 greatest importance, a cord of mesoblast connects the embryonic 

 formations with the internal surface of the chorion forming the 



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FIG. 94. FIG. 95. 



FIGS. 94 and 95. Diagrammatic longitudinal sections of two stages of development of the 

 primitive blastodermic vesicle of man and of the anthropoid ape, reconstructed by Hubrecht 

 from the drawings of Selenka and Peter, tr, trophoblast ; a, amniotic vesicle ; uv, umbilical 

 vesicle (between the two vesicles is the area of the embryonic bud) ; so.ra, sp.m, somatic and 

 splanchnic mesoblast bounding the coelomic cavity ; c, abdominal peduncle of His, in the 

 thickness of which can be distinguished a diverticulum of the umbilical vesicle which will 

 become the allantois. 



abdominal peduncle of His, as may be observed in the accompany- 

 ing diagram of Hubrecht. In these very young ova there is a 

 short diverticulum of the umbilical vesicle corresponding with 

 the attachment of the abdominal peduncle, which is almost com- 

 pletely traversed by it ; this diverticulum is the allantois. 



In the human ovum of this early period in which, according 

 to von Spee, the embryo is in the condition of the primitive 

 streak, the embryonic membranes are already in existence and 

 readily recognisable. 



The amnion, which is formed in an entirely different way 

 from that which we saw to be the case in the sauropsida and 

 lower mammals, forms the upper vesicle, which is the nearest 

 to the internal surface of the chorion. 



The umbilical vesicle, which is already entirely closed by its 



