204 RICHARD ASSHETON. 



of primitive streak). This takes place, no doubt, and is a very 

 conspicuous feature at a later stage, as described later, but it 

 does not account for a lengthening of, and within, 

 the secondary area of proliferation itself. 



The lengthening of the, at first, circular patch into a streak, 

 i. e. the change from fig. 11 to fig. 12, I believe to be due almost 

 entirely to the expansion of the blastodermic vesicle by the 

 hydrostatic pressure within. 



The expansion of the vesicle is quite sufficient to allow of 

 this. It is not easy to make very satisfactory measurements 

 in support of or against this view. If we take a specimen 

 about the stage shown in fig. 3, we find the secondary area to 

 measure not less than '38 mm. antero-posteriorly, and the 

 diameter of the whole vesicle about 4*5 mm. to 5 mm. 



If we take another specimen in which the secondary area has 

 attained its maximum elongation, we find the area to measure 

 about 1*08, which is rather less than three times the length it 

 was in stage fig. 3. The diameter of such a blastodermic vesicle 

 is about 10 mm. to 12 mm., which is from rather under two 

 and a half to rather over two and a half times the length it was 

 in stage fig. 3. 



I may, perhaps, make my meaning clearer by reference to 

 figs. 11, 12, PI. 20. 



In fig. 11 the point M is the centre of the primary area of 

 cell-production, i. e. the centre of the embryonic disc. The 

 point iV is the centre of the secondary area of cell- production. 

 The grey represents the circular embryonic disc; the white 

 represents the proliferating area of the primitive streak. 



Really this diagram represents no actual stage. Fig. 3 is 

 very near it. If my account of the course of events is correct, 

 there never could be a stage which would be exactly repre- 

 sented by this diagram, although it theoretically represents the 

 condition. 



Now the tendency of each of these two areas considered 

 separately would be to expand equally in all directions, from 

 the centre M in the one case, and N in the other. But these 

 two areas are not separate, they even overlap, and therefore 



