234 



PHYSIOLOGY 



CHAI>. 



umbilical vessels. Later the cavity of the amnion dilates with in- 

 creasing rapidity ; the umbilical vesicle becomes smaller ; from being 

 sessile it becomes pedunculated and consequently the line of 

 attachment of the amnion becomes more restricted. The body 



S^ M' J 



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J ~~&$yr 



FIG. 96. Human ovum of fifteen to eighteen days (according to Costa). The cavity of the chorion 

 is open to show the embryo, 0'5 mm. long, and its adjuncts, ch, chorion ; v.ch, chorionic 

 villi ; a, amnion ; p, abdominal peduncle ; v.o, umbilical vesicle. 



of the embryo changes in shape as it grows ; from being almost 

 pisciform with a dorso-caudal curvature, it very soon assumes 

 the form of a C (Fig. 98). As the result of all these changes 

 the abdominal peduncle (with the allantoid canal and the umbilical 

 vessels) approaches and lies on the omphalo-mesenteric canal 



