J 44 EICHAT?D ARSHETON. 



than anteriorly. Although present to a slight extent at first 

 anteriorly, it never develops greatly. This means that the 

 zone of special activity is at a later stage more intense poste- 

 riorly than anteriorly. So apparently it is at an earlier stage, 

 and causes the greater bulkiness of the posterior end of the 

 blastodermic vesicle of the seventh day, and causes the appa- 

 rent throwing forward of the embryonic disc (v. " 175 hr./^ 

 fig. 42). 



Table of Measurements of Thickness of Albumen 

 Layer at Different Parts of the Vesicle in Mil- 

 limetres. 



Five Days. Six Days Four Hours. 



Embryonic area . . -0060 . . '0058 



Placental zone, A. . . -0024 . . -0022 



Placental zone, P. . . '0030 . . -0024 



Lower pole . . . '0036 . . -0034! 



To sum up the above section : 



(1) There is no growth round of the inner mass cells over 

 the surface of the outer layer cells in the sense of a migration. 

 It is only an apparent growth round produced by the more 

 rapid growth of a zone of the wall of the vesicle immediately 

 surrounding the embryonic disc, in which zone the marginal 

 cells of the inner mass lie. 



(2) The presence of such a zone accounts in a great measure 

 for the shape assumed by the blastodermic vesicle during the 

 fifth, sixth, and seventh days. 



(3) A zone of such a character undoubtedly exists during 

 the eighth, ninth, and following days, giving rise to the ecto- 

 placenta. 



(4) Such a zone of activity accounts for the varying thick- 

 ness of certain parts of the albumen layer. 



To return for a time to the question of the growth round of 

 the inner mass cells. It must be remembered that the cells 

 never completely surround the cavity of the blastodermic 

 vesicle. The lower pole is always, in the rabbit, one-layered 

 only as long as it exists. 



