ATTACHMENT OP MAMMALIAN EMBHTO TO UTERUS. 177 



here and there large and granular with round nuclei, but the 

 whole of the epiblastic layer is now composed of thickened cells^ 

 almost columnar with rounded nuclei, and for a given length 

 there are many more nuclei than there were at that region 

 during the sixth and seventh days. In fig, 9 a piece of the 

 same region a few hours later is shown, and here the epi blast 

 is seen to be so thickened as to be actually several cells 

 thick, — in fact, a proliferation of cells is taking place out- 

 wards. 



Figs. 10 and 11 are still later stages of the same area. 



I believe that to understand the first outgrowth of these 

 papillse, both the small ones scattered over the lower pole, and 

 the area nearer to the embryonic disc, the great change that 

 the embryo has now undergone in the physical and mechanical 

 conditions must be taken into careful consideration. 



During the fifth, sixth, and seventh days I described in my 

 previous paper the growth of the walls of the vesicle as being 

 due to the hydrostatic pressure within it, together with the 

 multiplication of the cells of the walls of the vesicle, support 

 being rendered to the delicate wall of the vesicle by the albumen 

 layer. The cells of the wall (the epiblast) are very much 

 flattened because of the tension produced by the hydrostatic 

 pressure. 



The hydrostatic pressure is sufficient to keep the cells always 

 taut, and any increase in size of a cell owing to growth, or any 

 aggregation of cells by multiplication, is prevented by their 

 being flattened out by the internal pressure as soon as formed. 

 Thus all the cells are uniform in thickness, and extra activity 

 of one part shows itself by extra expansion of that 

 arc of the vesicle. 



What happens, however, when the walls of the uterus, by 

 reason of the great size now attained by the blastodermic 

 vesicle, affords supports to the walls which was hitherto want- 

 ing ? It must decrease the ratio of hydrostatic pressure to the 

 rate of growth of the cell wall. 



If the amount that the cells are stretched is constant when 

 the hydrostatic pressure is to the rate of growth of the cell 



VOL. 37, PART 2. NEW SER. M 



