260 ADAM SEDGWICK. 



EXPLANATION OF PLATE XVI, 



Illustrating Mr. Sedgwick's "Notes on Elasmobranch 

 Development." 



Fig. 1. — Embryo of Soyllium oanicula, 2| mm. in length. The hinder 

 end of the embryo is notched. The medullary groove is just beginning. The 

 tail swellings of Balfour are well marked. 



Fig. 2.— Embryo of Raja P sp., 4 mm. in length. The medullary groove is 

 closed except at the hind end. The notched embryonic part of the edge of 

 the blastoderm has grown faster than the rest, and come to project over the 

 surface of the yolk. The sides of this projection are already slightly bent 

 ventralwards. They will eventually meet and form the ventral part of the 

 caudal region of the body. 



Fig. 3.— Raja ? sp. Embryo of Stage E or F, 4^—6 mm. in length. The 

 medullary canal is still open, but the medullary folds are almost touching 

 except behind, where the medullary canal widens out in a wide medullary 

 groove, in the floor of which is placed the dorsal part of the blastopore. The 

 blastopore is slit-like, but dilated in front ; posteriorly it is continued round 

 the hind end of the body into the ventral portion. 



Fig. 4. — Raja ? sp. Stage E or F, 5 — 5^ mm. in length, a little older than 

 Fig. 3. Medullary canal closed except behind, where it widens out and 

 encloses the blastopore. The blastopore is slit-like, but the hinder end of 

 the dorsal portion is faintly marked. 



Figs. 3 and 4 are somewhat diagrammatic, but they show correctly the 

 relations of the medullary groove and dorsal part of the blastopore. I 

 hope to publish figures of the sections through them shortly. 



Fig. 5. — Ventral view of head of Soyllium canicula between Stage I 

 and K. Total length 7 — 8 mm. The two first pharyngeal clefts are open. 

 The mouth rudiment is present as a longitudinal groove in the ectoderm of 

 the buccal depression, which is fused with the endoderm. At intervals there 

 are perforations along this groove. The groove reaches into the rudiment of 

 the pituitary body. The mandibular arch is present as a backwardly directed 

 longitudinal ridge, and bounds the buccal depression externally. 



Fig. 6. — Ventral view of head of Scyllium canicula a little older than 

 the preceding. The buccal groove has become a longitudinal slit. 



Fig. 7. — Side view of head of Scyllium canicula a little younger than 

 Stage K- Total length about 9 mm. X could not distinguish any trace of 



