GANOIDEI. 



At the time of hatching there is a continuous dorso-ventral 

 fin, which, by atrophy in some parts, and hypertrophy in other 

 parts, gives rise to all the unpaired fins of the adult, except the 

 first dorsal and the abdominal. The caudal part of the fin is at 

 first symmetrical, and the heterocercal tail is produced by the 

 special growth of the ventral part of the fin. 



Of the internal features of development in the Sturgeon the most 

 important concern the relation of the yolk to the alimentary tract. In 

 most Vertebrata the yolk-cells form a protuberance of the part of the 

 alimentary canal, immediately behind the duodenum. The yolk may 

 either, as in the lamprey or frog, form a simple thickening of the alimentary 

 wall in this region, or it may constitute a well-developed yolk-sack as in 

 Elasmobranchii and the Amniota. In either case the liver is placed in 

 front of the yolk. In the Sturgeon on the contrary the yolk is placed 

 almost entirely in front of the liver, and the Sturgeon appears to be also 

 peculiar in that the yolk, instead of constituting an appendage of the 



sp.c 



FIG. 56. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE ANTERIOR 

 PART OF THE TRUNK OF A LARVA OF ACIPENSER TO SHEW THE POSITION OCCUPIED 

 BY THE YOLK. 



in. intestine; st. stomach filled with yolk; a's. oesophagus; /. liver; ///. heart; 

 ch. notochord; sp.c. spinal cord. 



alimentary tract, is completely enclosed in a dilated portion of the tract 

 which becomes the stomach (figs. 56 and 57). It dilates this portion 

 to such extent that it might be supposed to form a true external yolk-sack. 

 In the stages before hatching the glandular hypoblast, which was estab- 

 lished on the dorsal side of the primitive mesenteron, envelops the yolk- 

 cells, which fuse together into a yolk-mass, and lose all trace of their 

 original cellular structure. 



The peculiar flattening out of the embryo over the yolk (vide p. 105) 

 is no doubt connected with the mode in which the yolk becomes enveloped 

 by the hypoblast. 



