192 EIOHAED ASSHETON. 



I have also tried to explain how the changes in shape of the 

 primitive streak are brought about. 



I begin with a short account of this area of activitv (which 

 I shall speak of as the secondary area or centre of cell- 

 production — as compared with the process known as the 

 segmentation of the ovum, which I call the primary centre of 

 cell-production) which adds certain details of interest to the 

 accounts already given of the rabbit primitive streak by other 

 authors, notably by Hensen, KoUiker, and Rabl. 



The Earliest Signs of the Formation of the Secon- 

 dary Area of Proliferation. 



Until the middle of the seventh day the embryonic disc is 

 circular in outline. After the fusion of the inner and outer 

 layers of epiblast, the outline of the embryonic disc becomes 

 very much sharper. This state continues until about the 

 middle of the seventh day, when, although the anterior 

 periphery of the embryonic disc retains this character, the arc 

 of the hindermost quadrant becomes very much less distinct, 

 and is no longer a semicircle. 



On measuring the embryonic disc very soon after this, it is 

 found to have long and short axes, the long one usually parallel 

 to the long axis of the vesicle, the short to the short axis of 

 the vesicle. The lengthening has been in the antero-posterior 

 direction. The anterior end presents much the same shape 

 and features as before, except that its periphery describes an arc 

 of a larger circle than before. It is still sharply defined. The 

 posterior end is, if anything, more ragged, and is undoubtedly 

 at this moment thinner than the rest of the embryonic area. 



The measurements are as follows : 



Blastodermic vesicle, long axis . . . 4"1 mm. 



„ „ short axis . . . 3"85 „ 



„ „ polar axis . . . 3"25 „ 



Embryonic area, long axis . . . 1'35 „ 



„ „ transverse axis . . 1*25 „ 



This ragged border I believe to be the effect of the setting 

 up of an area of increased activity of cell-production either 



