DEVELOPMENT OF THE ELASMOBRANCH FISHES. 6l 



seen at present, the differences between the various species in 

 early embryonic life are of no importance. 



Without further preface I will pass on to my investigations. 



The Egg-shell. 



In the eggs of all the species of Dog-fishes which I have ex- 

 amined the yolk lies nearest that end of the quadrilateral shell 

 which has the shortest pair of strings for attachment. This is 

 probably due to the shape of the cavity of the shell, and is 

 certainly not due to the presence of any structures similar to 

 chalazae. 



The Yolk. 



The yolk is not enclosed in any membrane comparable to 

 the vitelline membrane of Birds, but lies freely in a viscid albu- 

 men which fills up the egg-capsule. It possesses considerable 

 consistency, so that it can be removed into a basin, in spite of 

 the absence of a vitelline membrane, without falling to pieces. 

 This consistency is not merely a property of the yolk-sphere as 

 a whole, but is shared by every individual part of it. 



With the exception of some finely granular matter around 

 the blastoderm, the yolk consists of rather small, elliptical, highly 

 refracting bodies, whose shape is very characteristic and renders 

 them easily recognizable. A number of striae like those of 

 muscle are generally visible on most of the spherules, which give 

 them the appearance of being in the act of breaking up into a 

 series of discs; but whether these striae are normal, or produced 

 by the action of water I have not determined. 



Position of the Blastoderm. . 



The blastoderm is always situated, immediately after impreg- 

 nation, near the pole of the yolk which lies close to the end of 

 the egg-capsule. Its position varies a little in the different 

 species and is not quite constant in different eggs of the same 

 species. But this general situation is quite invariable. It is of 

 about the same proportional size as the blastoderm of a bird. 



Segmentation. 



In a fresh specimen, in which segmentation has only just 

 commenced, the blastoderm or germinal disc appears as a circu- 



