DEVELOPMENT OF ELASMOBRANX'H FISHES. 87 



certain exceptionally favourable sections. In most cases the 

 yolk spherules around them render the finer details invisible. 



There is at this stage no such obvious continuity as in the 

 succeeding stage between the epiblast and the lower layer cells; 

 and this statement holds good more especially with the best 

 conserved specimens which have been hardened in osmic acid 

 (Plate III. fig. 4). In these it is very easy to see that the 

 epiblast simply thins out at the edge of the blastoderm without 

 exhibiting the slightest tendency to become continuous with the 

 lower layer cells \ 



The lower layer cells form a mass rather than a layer, 

 and constitute the whole of the blastoderm not included in the 

 epiblast. The shape of this mass in a longitudinal section may 

 be gathered from an examination of Plate ill. figs. 3 and 4. 



It presents an especially thick portion forming the bulk of 

 the embryonic swelling, and frequently contains one or two 

 cavities, w^hich from their constancy I regard as normal and not 

 as artificial products. 



In addition to the mass forming the embryonic swelling 

 there is seen in sections another mass of lower layer cells at 

 the opposite extremity of the blastoderm, connected with the 

 former by a bridge of cells, which constitutes the roof of the 

 segmentation cavity. The lower layer cells may thus be divided 

 into three distinct parts : 



(1) The embryo swelling. 



(2) The thick rim of cells round the edge of the remainder 

 of the blastoderm. 



(3) The cells which form the roof of the segmentation 

 cavity. 



1 Prof. Haeckel (Die Gastrula u. die Eifurchimg d. Thiere, Jenaische Zeit- 

 schrift, Yol. ix.) has unfortunately copied a figure from my preliminary paper 

 (loc. cit.) (repeated now), which I had carefully avoided using for the purpose of 

 describing the formation of the layers on account of the epiblast cells in the 

 original having been much altered by the chromic acid, as a result of which the 

 whole section gives a somewhat erroneous impression of the condition of the 

 blastoderm at this stage. I take this opportunity of pointing out that the 

 colouration employed by Professor Haeckel to distinguish the layers in this 

 section is not founded on my statements, but is, on the contrary, in entire 

 opposition to them. From the section as represented by Professor Haeckel 

 it might be gathered that I considered the lower layer cells to be divided into 

 two parts, one derived from the epiblast, while the other constituted the hypo- 

 blast. Not only is no such division present at this period, but no part of the 

 lower layer cells, or the mesoblast cells into which they become converted, can 

 in any sense whatever be said to be derived from the epiblast. 



