422 WALTER HEAPE. 



be seen in the secondary cavity, and in the section here figured 

 there were none present. I have, however, never found this 

 to be the case in any other specimen, and imagine they must 

 have been displaced during the process I then used of cutting 

 and mounting sections. 



The flattening process afterwards extends all round the edge 

 of the inner mass and the cavity is throughout much shallowed 

 (fig. 26), cells are present within the secondary cavity, and are 

 seen in this section becoming incorporated with the plate of 

 columnar cells. The edge of the plate, as I before mentioned, 

 is continuous with the cells of the outer layer, and at a slightly 

 later stage, when the plate is completely flattened out, it occu- 

 pies the position until then held by that portion of the outer 

 layer which overlay the inner mass. 



At this stage the two layers are indistinguishable from one 

 another, but wedge-shaped cells can be observed in the upper 

 portion of the plate (fig. 27 t c) which on account of their 

 shape, the direction of the long axis of their oval nucleus, and 

 the position they occupy appear to me without doubt to 

 have been derived from the cells of the outer layer. ^ 



Up to this point in the development the blastodermic vesicle 

 lies free within the cavity of the uterus, and can be obtained 

 therefrom without difficulty by merely slitting up the uterus 

 with scissors and transferring the ovum upon the point of a 

 scalpel to a watch-glass containing the hardening reagent. 

 This method is, however, no longer possible when the ovum 

 attains a very slightly older stage. It then becomes still 

 further enlarged and its walls project into the widely open 

 mouths of the uterine glands. I find no actual attachment 

 between the two, and have not been able to distinguish any 

 outgrowths from the zona such as Bischoff described for the 

 rabbit and dog (Nos. 6 and 7). 



The only method which I have found to enable me to obtain 

 the fresh vesicle entire, is to sink the uterus, after being cut 

 open, with the ovum in situ, slowly in a vessel of salt so- 



' In support of this view see below, an accouut of a stage iu the formation 



of the epiblast of the rabbit. 



