216 RICHARD ASSHETON. 



band may be ascribed to the activity of tlie primary centre of 

 growth (segmentation of the ovum) ; all posterior to this due 

 in the main to the secondary centre of growth (primitive 

 streak). Referring again to fig. 8, it will be seen that this 

 again indicates the point Z. as the line of demarcation between 

 the two areas. This spot is marked by a very distinct difference 

 in the character of the neural groove, which is seen in the later 

 stages also. It is at this spot that the first protovertebrae are 

 formed (vide fig. 9). 



As development proceeds, the fore-brain, whose outline is 

 in figs. 8 and 9 concentric with that of the pericardial band, 

 becomes thrust forward (vide fig. 10), so as when viewed from 

 above to be no longer concentric. So also there is a similar 

 thrusting forward of the whole of the dorsal portion of the 

 embryo, including the protovertebrae, and the pericardial band 

 no longer serves as a landmark of the same character. 



The Shortening of the Primitive Streak. 



Although the primitive streak becomes very much shortened, 

 I am not at all sure that its area is diminished during the process. 



The hinder end of the streak is about the same width through- 

 out its existence up to such a stage as fig. 10, and somewhat 

 later, but the elongated anterior portion is extremely narrow. 

 After the shortening the anterior part is almost as wide as the 

 posterior part. So I doubt whether there is any diminution of 

 area during the contraction. 



After the cessation of tension has been effected by the close 

 attachment of the surrounding walls of the blastodermic vesicle 

 to the uterus, the result of growth must be soon to cause pres- 

 sure of the nature of a thrust in the tissues which are most 

 actively growing. This, we know well, brings about a very 

 considerable thrust in the direction of the longitudinal axis of 

 the embryo, causing the head and tail folds. 



There is also the tendency that has all along existed for the 

 secondary area of proliferation to be radially symmetrical. This, 

 together with the pressure which is known to exist in the direc- 

 tion of the longitudinal axis, seems to be quite sufficient cause to 



