100 



TEXT-BOOK Of EMBRYOLOGY. 



however, with the general principles of mammalian ontogeny. The vesicle- 

 like structure of the entire developing organism is a fairly close approxima- 

 tion to the trophodermal sac of the lower forms. In both cases the rudi- 

 ment of the embryonic body is contained within the sac. In the human 

 embryo in question there are two cavities within the vesicle; the larger is 

 regarded as the amniotic cavity lined with ectoderm, and the smaller as the 

 cavity of the yolk sac lined with entoderm. The double wall between the 

 two would be the embryonic disk. The precocious development of the meso- 

 derm, which as a loosely arranged tissue fills in all the space between the 



Coagulurn 



Trophoderm 



Uterine epithelium 



Gland 



Decidua basalis 



FIG. 74. Section through very young human chorionic vesicle embedded in the 



uterine mucosa. Peters. 



The vesicle measured 2.4 x 1.8 mm., the embryo .19 mm. Peters reckoned the age as 3 or 4 days, 

 but later studies of other embryos go to show that the age is much greater; Bryce and 

 Teacher estimate it at 14 to 15 days. 



trophoderm and the two small cavities, is one of the remarkable features of 

 this embryo. The trophoderm is a most elaborate layer and has sent out 

 irregular projections into the uterine mucosa in which the whole structure 

 is already embedded. The early embedding or implantation and the elabo- 

 ration of the trophoderm are probably closely correlated. 



In a slightly older embryo described by Peters (Fig. 74) a space has ap- 



