DEVELOPMENT OF FISH-GASTRULA. 



217 



Osseous Fishes (Teleostei), the evolution of which I studied 

 at Ajaccio, in Corsica, in 1875, were of the greatest service 

 to me in this i"espect (Plate III. Fig. 18-24). I found these, 

 massed together in lumps of jelly, floating on the surface of 

 the sea ; and as the tiny eggs were quite transparent, I was 

 easily able to watch each stage in the evolution of the 

 germ.''^ These eggs, probably those of a cod-fish of the 

 Gaddoid family, but perhaps of a Cottoid, are colourless 

 globules, as transparent as glass, and of rather more than half 

 a millimetre in diameter (0'64 — 066 mm.). Within a thin, 

 structureless but firm egg-membrane (chorion, Fig. 42, c) lies 



Fig. 42. — Egg of an oceanic Osseous 

 Fish : p, protoplasm of the parent. cell ; Tc, 

 kernel of parent-cell ; n, clear albumin- 

 ous ball of nutritive yelk ; /, fat-globule 

 of the latter ; c, external egg-membrane, 

 or chorion. 



a large albuminous ball, which 

 is quite transparent and as clear 

 as water (71). At both poles of 

 the axis of this ball there is a 



groove-like indentation. In the groove at the upper pole, 

 which, in the floating egg, is turned downwards, lies a 

 simple, lentil-shaped cell, containing a kerne] (Fig. 42, p). 

 In the unfertilized egg, this is the original egg-cell ; after 

 fertilization it is the parent-cell. In the interval between 

 these two nucleated stages there is probably a non- 

 nucleated condition, representing the Monerula. At the 

 opposite pole of the egg, in the lower groove, lies a simple, 

 clear fat-globule (/). This small fat-globule and the large 

 albuminous globule together form the nutritive yelk. The 



