FORMATION OF THE LAYERS. 271 



with the yolk in the second ; the difference between the yolk in 

 the two cases arising from the fact that in the Elasmobranch 

 ovum the yolk-spherules bear a larger proportion to the proto- 

 plasm than they do in the Amphibian ovum. As I have suggested 

 elsewhere 1 , the segmentation or non-segmentation of a particular 

 part of the ovum depends solely upon the proportion borne by 

 the protoplasm to the yolk particles ; so that, when the latter 

 exceed the former in a certain fixed proportion, segmentation 

 is no longer possible ; and, as this limit is approached, seg- 

 mentation becomes slower, and the resulting segments larger 

 and larger. 



The question how far the facts in the developmental history 

 of the various vertebrate blastoderms accord with the view of 

 the nature of the yolk just propounded is one of considerable 

 interest. An answer to it has already been attempted from a 

 general point of view in my paper 2 entitled ' The Comparison of 

 the early stages of development in Vertebrates'; but the subject 

 may be conveniently treated here in a special manner for 

 Elasmobranch embryos. 



In the wood-cut, fig. i A, B, C 3 , are represented three dia- 

 grammatic longitudinal sections of an Elasmobranch embryo. 

 A nearly corresponds with the longitudinal section represented 

 on PL 7, fig. 4, and B with PL 7, fig. 7. In PL 7, fig. 7, the 

 segmentation cavity has however completely disappeared, while 

 it is still represented as present in the diagram of the same 

 period. If these diagrams, or better still, the wood-cuts fig. 

 2 A, B, C (which only differ from those of the Elasmobranch fish 

 in the smaller amount of food-yolk), be compared with the 

 corresponding ones of Bombinator, fig. 3 A, B, C, they will 

 be found to be in fundamental agreement with them. First let 

 fig. i A, or fig. 2 A, or PL 7, fig. 4, be compared with fig. 3 A. 

 In all there is present a segmentation cavity situated not centrally 

 but near the surface of the egg. The roof of the cavity is thin in 

 all, being composed in the Amphibian of epiblast alone, and in 



1 "Comparison," &c., Quart. Journ. Micr. Science, July, 1875. [This Edition, 

 No. VI.] 



2 Loc. cit. 



3 This figure, together with figs. 2 and 3, are reproduced from my paper upon the 

 comparison of the early stages of development in vertebrates. 



