GENERAL FEATURES. 293 



opening is not shewn 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 

 completely 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, would have been visible behind the last of these. 



L. 



This embryo is considerably older than the one last described, 

 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 

 to form the rudiment of the cerebellum. In front of the hind- 

 brain lies the mid-brain, the roof of which is formed by the 



