DEVELOPMENT OF ELASMOBRAXCH FISHES. 35 



Still well distinguished by its darker colour; and it is sur- 

 rounded by a concentric ring of light-coloured yolk, the outer 

 border of which shades insensibly into the normal yolk. 



At the embryonic portion of the blastoderm is a slight 

 swelling, clearly shown in Plate VI. fig. A, which can easily 

 be detected in fresh and in hardened embryos. This swelling is 

 to be looked upon as a local exaggeration of a slightly raised 

 rim present around the whole circumference of the blastoderm. 

 The roof of the segmentation cavity (fig. A, s. c.) forms a second 

 swelling; and in the fresh embryo this region appears of a 

 darker colour than other parts of the blastoderm. 



It is difficult to determine the exact shape of the blasto- 

 derm, on account of the traction exercised upon it in opening 

 the egg ; and no reliance can be placed on the forms assumed 

 by hardened blastoderms. This remark also applies to the 

 sections of blastoderms of this stage. There can be no doubt 

 that the minor individual variations exhibited by almost 

 every specimen are produced in the course of manipulations 

 while the objects are fresh. These variations may affect even 

 the relative length of a particular region and certainly the 

 curvature of it. The roof of the segmentation cavity is es- 

 pecially apt to be raised into a dome-like form. 



The main internal feature of this stage is the disappearance 

 of the layer of cells which, during the first stage, formed the 

 floor of the segmentation cavity. This disappearance is never- 

 theless not absolute, and it is doubtful whether there is any 

 period in which the floor of the cavity, is quite without cells. 



Dr Schultz supposes' that the entire segmentation cavity 

 is, in the living animal, filled with a number of loose cells. 

 Though it is not in my power absolutely to deny this, the 

 point being one which cannot be satisfactorily investigated in 

 sections, yet no evidence has come under my notice which 

 would lead to the conclusion that more cells are present in the 

 segmentation cavity than are represented on PL xiii. fig. 1, of 

 my preliminary paper', an illustration which is repeated on PI. 

 III. fig. 2. 



The number of cells on . the floor of the cavity differs 

 considerably in different cases, but these cases come under the 



1 Loc. cit. 2 j^oc. cit. 



3-2 



