FORMATION OF THE LAYERS. 257 



tween the columnar cells and the typical rounded lower layer 

 cells 1 . 



In the last described embryo, although the embryonic rim 

 had attained to a considerable development, no trace of the 

 medullary groove had made its appearance. In an embryo in 

 the next stage of which I propose describing sections, this struc- 

 ture has become visible. 



A surface view of a blastoderm of this age, with the embryo, 

 is represented on PI. 8, fig. B ; and I shall, for the sake of con- 

 venience, in future speak of embryos of this age as belonging to 

 period B. 



The blastoderm is nearly circular. The embryonic rim is 

 represented by a darker shading at the edge. At one point 

 in this rim may be seen the embryo, consisting of a somewhat 

 raised area with an axial groove (mg). The head end of the 

 embryo is that which points towards the centre of the blasto- 

 derm, and its free peripheral extremity is at the edge of the 

 blastoderm. 



A longitudinal section of an embryo of the same age as the 

 one figured 2 is represented on PL 7, fig. 7. The general growth 

 has been very considerable, though as before explained, it is 

 mainly confined to that part of the blastoderm where the 

 embryonic rim is absent. 



A fresh feature of great importance is the complete dis- 

 appearance of the segmentation cavity, the place which was 

 previously occupied by it being now filled up by an irregular 

 network of cells. There can be little question that the oblite- 

 ration of the segmentation cavity is in part due to the entrance 

 into the blastoderm of fresh cells formed around the nuclei of the 

 yolk. The formation of these is now taking place with great 

 rapidity and can be very easily followed. 



Since the segmentation cavity ceases to play any further 

 part in the history of the blastoderm, it will be well shortly to 

 review the main points in its history. 



1 When writing my earlier paper I did not feel so confident about the mode of 

 formation of the hypoblast as I now do, and even doubted the possibility of determining 

 it from sections. The facts now brought forward are I hope sufficient to remove all 

 scepticism on this point. 



a Owing to the small size of the plates this section has been drawn on a con- 

 siderably smaller scale than that represented in fig. 5. 



