80 GENERAL FEATURES. 



manner not only cliaraoteristic of the Elasmobranclis but even 

 of the genus Scjllium. 



There is a tail fin, an anal fin and two dorsal fins, both 

 the latter being situated behind the posterior paired fins. 



In the head may be noticed a continuation of the rapid 

 growth of the anterior part. 



The mouth has become far more narrow and slit-like ; and 

 with many other of the organs of the period commences to 

 approach the form of the adult. 



The present and the three preceding stages show the gradual 

 changes by which the first visceral arch becomes converted into 

 the rudiments of the upper and of the lower jaw. The fact 

 of the conversion was first made known throucrh the investi- 

 gations of Messrs Parker and Gegenbaur. 



O. 



In this stage the embryo is very rapidly approaching the 

 form of the adult. 



This is especially noticeable in the fins, which project in a 

 manner quite characteristic of the adult fish. The mouth is 

 slit-like, and the openings of the nasal sacs no longer retain 

 their primitive circular outline. The external gills project 

 from all the gill-slits including the spiracle. 



P. 



The head is rapidly elongating by the growth of the snout, 

 and the divisions of the brain can no longer be seen with 

 distinctness from the exterior, and, with the exception of the 

 head and of the external gills, the embryo almost completely 

 resembles the adult. 



Q. 



The snout has grown to such an extent, that the head has 

 nearly acquired its adult shape. In the form of its mouth 

 the embryo now quite resembles the adult fish. 



This part of the subject may be conveniently supplemented 

 by a short description of the manner in which the blastoderm 

 encloses the yolk. It has been already mentioned that the 

 growth of the blastoderm is not uniform. The part of it in the 

 immediate neighbourhood of the embryo remains compara- 



