TELEOSTEL 



61 



In the just hatched larva of an undetermined fresh-water fish with a 

 very small yolk-sack I found that the yolk extended along the ventral 

 side of the embryo from almost the mouth to the end of the gut. The gut 

 had, except in the hinder part, the form of a solid cord resting in a con- 

 cavity of the yolk. In the hinder part of the gut a lumen was present, 

 and below this part the amount of yolk was small and the yolk nuclei 

 numerous. Near the limit of its posterior extension the yolk broke up into 

 a mass of cells, and the gut ended behind by falling into this mass. These 

 incomplete observations appear to shew that the solid gut owes its origin 

 in a large measure to nuclei derived from the yolk. 



When the yolk has become completely enveloped a postanal 

 section of gut undoubtedly becomes formed ; and although, owing 

 to the solid condition of the central nervous system, a communication 

 between the neural and alimentary canals cannot at first take place, 

 yet the terminal vesicle of the postaual gut of Elasmobranchii is repre- 

 sented by a large vesicle, originally discovered by Kupffer (No. 68), 

 which can easily be seen in the embryos of most Teleostei, but the 

 relations of which have not' been satisfactorily worked out (vide fig. 

 34, hyv). As the tail end of the embryo becomes separated off from 

 the yolk the postanal vesicle atrophies. 



General development of the Embryo. Attention has already 

 been called to the fact that the embryo first appears as a thickening 

 of the edge of the blastoderm which soon assumes a somewhat shield- 

 like form (fig. 33 A). The hinder end of the embryo, which is placed 

 at the edge of the blastoderm, is somewhat prominent, and forms the 

 caudal swelling (fc). The axis of the embryo is marked by a shallow 



groove. 



The body now rapidly elongates, and at the same time becomes 



> r <V? 



FIG. 33. THREE STAGES? IN THE DEVELOPMENT OF THE SALMON. (After His.) 

 ts. tail-swelling; a u.v. auditory vesicle; oc. optic vesicle; ce. cerebral rudiment ; 

 m.fc. mid-brain; cb. cerebellum; md. medulla oblongata; MI .so. mesoblastic somite. 



