208 RTCHARD ASSHETON. 



layer has ruptured^ and the ectoplacental region is firmly 

 attached to the placental lobes. 



From this moment the upper pole of the blastodermic 

 vesicle can only be expanded by the hydrostatic pressure as 

 much as the placental lobes and the thick mesometrial wall of 

 the uterus will allow it. As is well known, this portion of the 

 uterus expands now very little and slowly ; by far the greater 

 part of the swelling of the blastodermic vesicle concerns only 

 the ab-embryonic pole of the vesicle and the obplacental lobe 

 of the uterus. 



In this way a very large part^ if not all, the tension is 

 rather suddenly removed from the actual embryonal area and 

 immediately surrounding tissues, and this is exactly coincident 

 with the attainment of the maximum length of the primitive 

 streak and with the rather sudden obliteration of the primitive 

 groove. 



No doubt to all appearances the primitive streak of a bird 

 is at the time of its greatest development extremely like the 

 primitive streak of a mammal of the type of development such 

 as we find in the rabbit. 



If we accept DuvaFs description for the bird, and also the 

 explanation I have ofi"ered above for the rabbit, as being both 

 correct, we are bound to conclude that it is really only a coin- 

 cidence that the secondary area of cell-production in each case 

 should assume a linear form. Such a coincidence is unlikely, 

 though of course not impossible. 



An Attempt to determine which Portion of the 



Embryo is derived from the Cells proliferated 



from the Primitive Streak. 



Those who have followed my description of figures will have 



noticed that, according to the account, there is one part. of the 



embryo laid down as the immediate result of the segmentation 



of the ovum, and that the most conspicuous part of this is a 



circular patch — the embryonic disc. Subsequently a renewed 



activity of cell-production takes place upon the posterior 



border of this embryonic disc, giving rise to new tissue, and 



