4i FORMATION OF THE LAYERS. 



In tlie 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. VI. fig. B; and I shall, for the sake of 

 convenience, in future speak of embryos of this age as belong- 

 ing 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^ is represented on PL IV. 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 w^ell shortly to 

 review the main points in its history. 



Its earliest appearance is involved in some obscurity, though 

 it probably arises as a simple cavity in the midst of the lower 

 layer cells (PL ill. fig. 1). In its second phase the floor ceases 

 to be formed of lower layer cells, and the place of these is 

 taken by the yolk, on which however a few scattered cells 



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

 considerably smaller scale than that represented in fig. 5. 



