NOTES ON ELASMOBEANOH DEVELOPMENT. 245 



while the anus is formed within the area of the blastopore, and 

 is in some Vertebrates actually a persistent part of the blasto- 

 pore, in no Vertebrate has the mouth been traced into connec- 

 tion with the blastopore. The fact that no such connection 

 has been established is not surprising when one remembers 

 how. early the anterior part of the blastopore closes in Elasmo- 

 branchs and Amphibia, and must not be taken as proving that 

 the blastopore never extended in front of the present medullary 

 plate on to the ventral surface of the head. I shall return to 

 this question in the part of this paper which deals with the 

 Vertebrate head. 



It will be seen from the above account that the behaviour of 

 the blastopore of Elasmobranchs — in its relation to the anus, 

 neurenteric canal, and growing point — resembles very closely 

 that of the frog as described in the admirable paper by 

 Assheton and Robinson in vol. xxxiiof the ' Quarterly Journal 

 of Microscopical Science.^ 



3. On the Formation of the Mouth and Gill-clefts 

 in Elasmobranchs. 



I have had a number of drawings made of the head of 

 embryos of Scyllium canicula to illustrate certain points 

 in the formation of the mouth and clefts. Some of the points 

 have been known before, and some are, I believe, recorded for 

 the first time. 



The mouth makes its first appearance in Stage I as a row of 

 dots lying in the middle line between the two mandibular 

 arches (fig. 5), and connected by a kind of shallow groove in 

 the ectoderm, along which the ectoderm and endoderm are 

 fused. These pores soon become connected (fig. 6) to form a 

 long slit, which extends from the ventral point of junction of 

 the mandibular arches forward along the depression between 

 the latter as far as the pocket of ectoderm which is destined to 

 give rise to the pituitary body. The first rudiment of the 

 mouth actually extends into the rudiment of the pitui- 

 tary body. At the front end of the buccal slit the fore-gut, 

 the notochord, the ectoderm, and the mesoderm are all con- 



