THE PRIMITIVE STREAK OF THE RABBIT. 203 



Having drawn attention to this possibility^ I must leave the 

 consideration of the mesoblast for the present. 



What are the conditions under which the blastodermic vesicle 

 exists during the lengthening of the primitive streak and the 

 formation of the deep primitive groove ? 



In the previous paper mentioned above, I have accounted 

 for various changes in shape of the blastodermic vesicle, as 

 well as the apparent growth of one layer over another, 

 by pointing out how these changes could be produced by 

 increasing hydrostatic pressure within, together with more 

 rapid cell-division in one part, and less rapid cell-division in 

 another part. 



So, I believe, the same principle holds good with regard to 

 the lengthening of the secondary area of proliferation into a 

 streak, and to the production of the median groove. 



The blastodermic vesicle is still expanding rapidly by reason 

 of the increasing hydrostatic pressure. The walls of the vesicle 

 are thin except at one point, the embryonic disc, where tlje 

 wall is thick and compact. Somewhat suddenly, a spot at the 

 posterior edge of this embryonic disc becomes extremely 

 active — the secondary area of proliferation. This gives rise, 

 as we have seen, to a mass of cells produced very rapidly at 

 one spot (fig. 3), forming a second compact disc. 



We have now two very distinct compact masses in the outer 

 layer or epiblast, one formed by the presence of the inner layer 

 of epiblast and its fusion with the outer layer, the other by the 

 very rapid proliferation at one spot caused by the appear- 

 ance of the secondary growing point or primitive streak. 

 While this is proceeding, the whole blastodermic vesicle is still 

 expanding from the hydrostatic pressure within. Now what 

 effect, if any, will this hydrostatic pressure have in regard to 

 these two compact masses ? In the first place there will be a 

 tendency for them to part. There will be a tendency to part from 

 another cause, namely, the counteracting effect of their re- 

 spective growths ; but this would tend to produce a space (i. e, 

 mass of cells) between the primary growing point and the ante- 

 rior border of the secondary growing point (i. e. anterior end 



