DEVELOPMENT OF TELEOST 



20; 



chian in character, are appearing. An embryo shortly 

 before hatching is next figured (Fig. 264) ; the head has 

 now entirely lost its flattened character ; the mouth in- 

 vagination occurs at M\ the tail, much elongated, is 

 compressed laterally, and already presents the dermal 

 embryonic fin ; the yolk sac is attached along the an- 

 terior body region, in a position more nearly that of the 

 shark than of the lung-fish. 



Of the two Ganoids, sturgeon and gar-pike, the latter, 

 as the writer has pointed out, * has the more shark-like 

 developmental features. Its segmentation is incomplete, 

 since the yolk pole of the Qgg is at no time traversed even 

 by superficial furrows. The blastoderm, or cell cap, is 

 early apparent, and is clearly marked off by a furrow from 

 the irregular marginal blastomeres (Fig. 265). It resem- 

 bles closely the segmented germ disc of an Elasmobranch, 

 and the irregular marginal blastomeres may be compared 

 to merocytes. The section of a late blastula of Fig. 266 

 does not differ widely from that of the shark of Fig. 221 ; 

 a segmentation cavity is present, whose floor is smooth, 

 and contains a well-marked zone of merocytes, M \ the 

 smaller quantity and firmer consistency, perhaps, of the 

 yolk do not, on the other hand, permit the blastula to 

 occupy the sunken position of that of the shark. In the 

 gastrula of the gar, further, a well-marked notch appears 

 at the dorsal lip (as in this stage. Fig. 223, of the shark), 

 representing the primitive blastopore. And, finally, the 

 form of the embryo rises boldly from the surface, and 

 early presents the well-marked head and tail eminences, 

 HE and T, of Fig. 268, comparable with Figs. 225 and 

 227. 



* Am. y. Morph., Vol. XI, No. I. 



