EMBRYOLOGY 



493 



a median pit of epiblast forms the stomodtzum, which will 

 eventually break through to the enteron and become the 

 mouth. Behind the stomodaeum is a horseshoe-shaped 

 sucker ; above it a pit in each of the sense plates gives rise 

 to the olfactory organ. When these changes are complete 

 the animal hatches. This happens about a fortnight after 

 the eggs are laid. 



T^ 



B 



In the external develop- 

 ment of the 



Feature tad P° le ^ 6 ) 



of Larva. the following 

 changes take 

 place. A third pair of ex- 

 ternal gills is formed, and 

 the mouth opens and is 

 provided with a pair of 

 horny jaws, which are 

 eventually lost. Meanwhile 

 four gill-clefts open, and 

 the external gills wither, 

 being replaced by new gills m 

 on the walls of the clefts. 

 The latter represent the first 

 to fourth branchial clefts of 

 the dogfish, the external gills 

 standing on the first three 

 branchial arches. Shortly _ . .„ , 



after, the appearance of the Fia 373 - A stlU Iater embr y°- 

 clefts a fold of skin grows A < From behind £j£ t above = B > from in 

 back from each side of an< Proctoda , um - e ' a . t g m arches ; ,., 



the head SO as tO COVer sucker ; s.fi., sense plate. 



them. The folds are the 



opercula ; they meet ventrally, and presently their hinder 

 edges fuse with the body everywhere, except in one spot 

 on the left side, where an opening is left for the dis- 

 charge of the water used in breathing. The sucker now 

 begins to degenerate. Shortly afterwards rudiments of 

 the hind-limbs appear at the base of the tail, as a pair 

 of small knobs, which increase rapidly and become first 

 jointed and then divided into toes. The fore-limbs arise at 

 the same time as the hind-limbs, but as they are covered 



