THE PRIMITIVE STREAK OE THE RABBIT. 205 



they must have some disturbing effect upon each other; and 

 the line along which that disturbance will be most marked will 

 be along a line drawn from the centre of the one to the centre 

 of the other — from M to N. 



1 have argued before that while the increasing hydrostatic 

 pressure within the blastodermic vesicle causes its expansion, 

 yet a more rapid expansion of one part of the wall than another 

 may be brought about by the more rapid cellular growth of 

 that part of the wall. In other words, areas of weakness are 

 produced upon which the hydrostatic pressure takes more 

 effect than elsewhere. 



So at this stage must there not be a line of weakness between 

 the two points M and N in diagram 11, an area along, at any 

 rate, that portion in which the influences of both centres are 

 at work, in which there will be less resistance offered to the ex- 

 pansive force of the hydrostatic pressure than elsewhere ? It 

 seems to me that this is extremely likely, and that a figure 

 such as that represented in diagram fig. 12 must be the result. 



The anterior half or portion of the secondary area of pro- 

 liferation will be drawn out as indicated in the diagram by the 

 separation of the black dots j the posterior portion, being unin- 

 fluenced by the primary area, will not lose its radial symmetry. 



Similarly the embryonic disc will be influenced, and its 

 hinder portion drawn out as indicated by the black crosses on 

 the two diagrams, its anterior border not losing its radial sym- 

 metry. It must be remembered, however, that the secondary 

 area of proliferation is in its function of cell-production more 

 vigorous now than the primary area, and becomes still more 

 vigorous for the next two days. 



Also the visible line of the primitive streak does not even at 

 this comparatively early stage represent the whole of the effects 

 of the secondary area of proliferation. No doubt the epiblast 

 immediately surrounding it owes its origin to the energy of the 

 area in question, but to what extent it is not easy to determine. 

 At any rate it is probably not less than that part beneath which 

 primitive streak mesoblast lies. So that a more accurate 

 figure of the conditions of the two areas of activity respectively 



