250 DEVELOPMENT OF ELASMOBRANCH FISHES. 



directions. This arrangement is, however, by no means strictly 

 adhered to, and the regularity of it is exaggerated in Plate 

 7, fig. 4- 



The nuclei of the epiblast cells have the same characters as 

 those of the lower layer cells to be presently described, but their 

 intimate structure can only be successfully studied in 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 

 (PI. 7, fig. 4). In these it is very easy to see that the epiblast 

 simply thins out at the edge of the blastoderm without exhibit- 

 ing the slightest tendency to become continuous with the lower 

 layer cells 1 . 



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 7, figs. 3 and 4. 



It presents an especially thick portion forming the bulk of 

 the embryonic swelling, and frequently contains one or two 

 cavities, which 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 



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

 schrift, Vol. 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 hypoblast. 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. 



