GENERAL FEATURES. 295 



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 shew 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 through the investigations 

 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 distinct- 

 ness 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 comparatively 

 stationary, while the growth elsewhere is very rapid. From 



