206 EICHAED ASSHETON. 



would probably be such as I have drawn in fig. 43, on Plate 

 22, to be explained presently. I conclude, therefore, that the 

 lengthening of the primitive streak is due to the expansion in 

 an antero-posterior direction of the most anterior part (half?) 

 of the secondary area of cell-proliferation due to the conditions 

 under which the embryo is developing. 



May not the primitive groove be also due to ontogenetic 

 conditions ? 



It will be noticed that it does not exist during the early 

 stages of the formation of the primitive streak (vide figs. 1 — 

 4 a, and the sections of these). 



It will be noticed also that it is at its greatest development 

 at the time the primitive streak reaches its greatest length, and 

 from that moment it becomes shallower and very rapidly dis- 

 appears altogether (vide fig. 8 and sections). It is deepest 

 and most distinct along that part of the streak which is 

 thinnest, and gradually shallows and disajipears towards the 

 thicker posterior end of the streak. 



The sections through the stages in figs. 4, 5, and 6 show 

 that the groove commences at the same time as the spreading 

 of the mesoblast. 



The mesoblast is a reticulum connected with the area of pro- 

 liferation, and is seen to spread out from both sides of the 

 drawn-out proliferating area. 



I have explained above how this spreading out of the meso- 

 blast may be due to the general expansion of the walls of the 

 vesicle of this region by the increasing hydrostatic pressure. 

 If the proliferating area produces cells more quickly than they 

 can be removed laterally by the expanding walls of the vesicle, 

 a heap of cells will be the result at this spot, or if the prolife- 

 rating area is linear a ridge will be formed. If the cells pro- 

 duced exactly balance those removed, the surface will remain 

 flat ; there will be no ridge. 



But what will happen if the cells produced along the prolife- 

 rating area are removed more rapidly than they are produced ? 

 If the deficiency of production is very great, then probably the 

 cells — few in number — will be torn apart and removed as 



