EAELY STAGES OF DEVELOPMENT OF THE EABBIT. 143 



therefore less growth at the embryonic pole than at the lower 

 pole. 



It may be said that this is due to the fact of the cellular 

 wall at this point being practically several layers thick. But 

 it must be remembered that the cells of the inner cell-mass 

 are very loosely arranged (figs. 28^ 30, 35) at this time, and 

 certainly do not give me the impression of being under great 

 tension, as are the outer cells. 



As far as the tension is concerned, I believe the outer layer 

 has to bear it very nearly all, even in the embryonic disc. 

 Sections show how very thin it is here. 



Granted that they are only very equally stretched, it follows, 

 I think, since the albumen shows not the same amount of 

 stretching as elsewhere, that the multiplication of cells must 

 have been going on more slowly in the embryonic region. 

 Now from measurements of the albumen layer it will be seen 

 that the thinnest place is not usually at the lower pole, but 

 somewhere between the equator and the upper pole. This, 

 then, marks out as an area a zone of more rapid multiplication 

 of cells, that area which lies just outside of the embryonic 

 disc. 



Now we know that upon the eighth and many subsequent 

 days there is a very great activity evinced by the cells of a 

 zone immediately surrounding the embryonic disc ; it is the 

 zone called by Duval the eetoplacental area. If it is an area 

 of very great activity upon the tenth day, of great activity upon 

 the ninth and eighth days, when does it begin to be an area of 

 activity ? Why not upon the seventh or sixth, or even on the 

 fifth day? 



I believe that it does begin as early as the fifth day, and that 

 it is to the presence of this area of more active cell division 

 round the embryonic disc that the shape of the blastodermic 

 vesicle is as I have described it to be (v. fig. 42), and that it is 

 to this that the apparent growth round of the hypoblast cells 

 is due. 



The eetoplacental area is, as is well known, much more 

 strongly developed all round the posterior end of the embryo 



