THE PRIMITIVE STREAK OF THE EABBIT. 207 



isolated cells. But if the deficiency is only slight, surely the 

 result will be that the meshes of the reticulum of mesoblast 

 will be drawn out into larger meshes, and the connecting 

 filaments become finer. 



It seems to me that this must mean that a tension between 

 cell and cell must be produced, and not only between cell and 

 cell of the reticulum, but between the reticulum on each side 

 of the primitive streak and the actual proliferating area itself. 

 Is it not possible for this tension to produce such a groove as 

 that shown in figs. 22 and 26 ? 



Posteriorly there is no groove. Here the supply of cells is 

 greater than the removal, and a heap is formed ; but along the 

 narrowest part of the proliferating area the conditions favour the 

 formation of a groove as I have suggested, for not only is the 

 proliferating area very attenuated, but lateral dragging of the 

 mesoblast is in the same direction along a considerable length. 



It follows that upon the above explanation of the lengthening 

 of the primitive streak and formation of the groove any increase 

 in activity of the proliferating area might lead to the oblitera- 

 tion of the groove, and even perhaps a cessation of the increase 

 in length of the streak. 



Also any modification which either prevented the increase of 

 hydrostatic pressure within the vessels, or which prevented the 

 portion of the wall containing the embryonal area from being 

 aflPected by the increase of hydrostatic pressure, would bring 

 about the obliteration of the groove and cause the cessation of 

 lengthening of the streak. In the rabbit it seems probable 

 that the latter event occurs. 



During the stages represented by figs. I to 5, that is during 

 the formation of the greater length of the primitive streak, the 

 blastodermic vesicle lies freely in the uterus. About the age 

 represented by fig. 7 the papillae are appearing by which the 

 lower pole becomes attached to the obplacental portion of the 

 uterus. The ectoplacental area is thickening, but is still quite 

 smooth and quite free to slide over the surface of the placental 

 lobes; but between that time and the time represented by 

 fig. 8, the ectoplacenta has become irregular, the albumen 



