MOUTH. 



5 



Coelomata ; and as in most of the latter the part of the nerve 

 rudiment behind it (in the primitive position, anterior in the 

 position which the anus secondarily acquires on the ventral 

 surface) undergoes atrophy. 



We now come to the question of the chordate mouth, a much 

 vexed question, and one about which much of a highly specu- 

 lative character has been written. We may at once concede 



Fig. 2. — Heads of young Elasmobranch embryos {ScyUium canicula) (after Sedgwick). 



A. Ventral view of liead of embryo, 7 mm. in length, with two open pharyngeal clefts. 

 The mouth is present as a longitudinal groove in the ectoderm of the buccal depression. 



B. Same view of a slightly older embryo ; tlie buccal groove has become a longitudinal 

 slit. C. Side view of head of embryo, 9 mm. in length, with three open slits. D. Side 

 view of head of embryo, 11 mm. in length ; rudiments of external gills have appearedion 

 the hyoid and on the first and second branchial arclies. E. Side view of head of embryo 

 of 16 mm. ; external gills have appeared on mandibular arch and the angle of the jaw is 

 marked. 1 mandibular arch ; 2 angle of jaw ; 3 second pharyngeal cleft ; 4 nasal pit ; 

 5 eye ; 6, midbrain ; 7, auditory sac ; 8 hyoid arch ; 9 spiracle. 



the point that the chordate mouth has never been brought 

 into developmental relation with the blastopore. Even if it be 

 allowed that the chordate blastopore really extends to the 

 front end of the nerve rudiment (medullaiy plate), which is in 

 itself a disputed point, no morphologist has ever brought to 

 light any embryological fact which is at all in favour of the 

 view that the mouth was originally within'^the nerve rudiment. 



