78 GENERAL FEATURES. 



forms the anterior termination of the long axis of the body. 

 The thin roof of the fourth ventricle may in the figure be 

 noticed behind the mid-brain. The auditory sac is nearly 

 closed and its opening is not shown in the figure. In the eye 

 the lens is completely formed. 



Owing to the opacity of the embryo, the muscle-plates are 

 only indistinctly indicated, and no other features of the meso- 

 blast are to be seen. 



The mouth is now a deep pit, whose borders are almost com- 

 pletely formed by the thickening in front of the first visceral cleft, 

 which may be called the first visceral arch or mandibular arch. 



Four visceral clefts are now visible, all of which are open to 

 the exterior, but in a transparent embryo one more, not open to 

 the exterior, w^ould have been visible behind the last of these. 



L. 



This embryo is considerably older than the one last de- 

 scribed, but growth is not quite so rapid as might be gathered 

 from the fact that L is nearly twice as long as K, since the two 

 embryos belong to different genera ; and the Scyllium embryos, 

 of which L is an example, are larger than Pristiurus embryos. 

 The umbilical stalk is now quite a narrow elongated structure, 

 whose subsequent external changes are very unimportant, and 

 consist for the most part merely in an increase in its length. 



The tail has again grown greatly in length, and its terminal 

 dilatation together with the alimentary vesicle contained in it, 

 have both completely vanished. A dorsal and ventral fin are 

 now clearly visible ; they are continuous throughout their whole 

 length. The limbs have grown and are more easily seen than 

 in the previous stage. 



Great changes have been effected in the head, resulting in a 

 diminution of the cranial flexure. This diminution is never- 

 theless apparent rather than real, and is chiefly due to the rapid 

 growth of the rudiment of the cerebral hemispheres. The three 

 main divisions of the brain may still be clearly seen from the 

 surface. Posteriorly is situated the hind-brain, now consisting 

 of the medulla oblongata and cerebellum. At the anterior 

 part of the medulla is to be seen the thin roof of the fourth 

 ventricle, and anteriorly to this again the roof becomes thickened 



