DEVELOPMENT OF ELASMOBRAXCH FISHES. 79 



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 

 optic lobes, which are still situated at the front end of the long 

 axis of the embr3^o. 



Beyond the mid-brain is placed the fore-brain, whose growth 

 is rapidly rendering the cranial flexure imperceptible. 



The rudiments of the nasal sacs are now clearly visible as a 

 pair of small pits. The pits are widely open to the exterior, 

 and are situated one on each side, near the front end of the 

 cerebral hemispheres. Five visceral clefts are open to the 

 exterior, and in them the external gills have commenced to 

 appear (L'). 



The first cleft is no longer similar to the rest, but has com- 

 menced to be metamorphosed into the spiracle. 



Accompanying the change in position of the first cleft, the 

 mandibular arch has begun to bend round and enclose the front 

 as well as the side of the mouth. By this change in the mandi- 

 bular arch the mouth becomes narrowed in an antero-posterior 

 direction. 



M. 



Of this embryo the head alone has been represented. Two 

 views of it are given, one (M) from the side and the other (M') 

 from the under surface. The grow^th of the front part of the 

 head has considerably diminished the prominence of the cranial 

 flexure. The full complement of visceral clefts is now present — 

 six in all. But the first has already atrophied considerably, and 

 may easily be recognised as the Sj^iracle. In Scyllium, there 

 are present at no period more than six visceral clefts. The first 

 visceral arch on each side has become bent still further round, 

 to form the front border of the mouth. The opening of the 

 mouth has in consequence become still more narrowed in an 

 antero-posterior direction. The width of the mouth in this 

 direction, serves for the present and for some of the subsequent 

 stages as a very convenient indication of age. 



N. 

 The limbs, or paired fins, have now acquired the general 

 features and form which they possess in the adult. 



The unpaired fins have now also become divided in a 



