358 DEVELOPMENT OF ELASMOBRANCH FISHES. 



of Pristiurus, is the absence of any signs of nuclei or pro- 

 toplasm passing to the periphery. Around the exterior of the 

 cord there is early found in Torpedo a special investment of 

 mesoblastic cells. 



EXPLANATION OF PLATES n AND 12. 



COMPLETE LIST OF REFERENCE LETTERS. 



al. Alimentary tract, an. Point where anus will be formed, ao. Dorsal aorta. 

 ar. Rudiment of anterior root of spinal nerve, b. Anterior fin. c. Connective-tissue 

 cells, ca v. Cardinal vein. ch. Notochord. df Dorsal fin. ep. Epiblast. gc. 

 Germinal epithelium, ht. Heart. /. Liver. ;;//. Muscle-plate, nip'. Early formed 

 band of muscles from the splanchnic layer of the muscle-plates, nc. Neural canal. 

 p. Protoplasm from yolk in the alimentary tract, pc. Pericardial cavity, po. Primi- 

 tive ovum. pp. Body cavity, pr. Rudiment of posterior root of spinal nerve, sd. 

 Segmental duct. sh. Cuticular sheath of notochord. so. Somatic layer of mesoblast. 

 sp. Splanchnic layer of mesoblast. spc. Spinal cord. sp.v. Spiral valve, sr. Inter- 

 renal body. st. Segmental tube. sz>. Sinus venosus. ua. Umbilical artery, tint. 

 Umbilical cord. uv. Umbilical vein. v. Splanchnic vein. v. Blood-vessel, vc. Visceral 

 cleft. Vr. Vertebral rudiment. IV. White matter of spinal cord. x. Subnotochordal 

 rod (except in fig. 14 a), y. Passage connecting the neural and alimentary canals. 



PLATE 1 1 . 



Fig. i. Section from the caudal region of a Pristiurus embryo belonging to stage 

 H. Zeiss C, ocul. i. Osmic acid specimen. 



It shews (i) the constriction of the subnotochordal rod (x) from the summit of the 

 alimentary canal. (2) The formation of the body-cavity in the muscle-plate and the 

 ventral thickening of the parietal plate. 



Fig. i a. Portion of alimentary wall of the same embryo, shewing the formation 

 of the subnotochord rod (x) . 



Fig. 2. Section through the caudal vesicle of a Pristiurus embryo belonging to 

 stage H. Zeiss C, ocul. i. 



It shews the bilobed condition of the alimentary vesicle and the fusion of the 

 mesoblast and hypoblast at the caudal vesicle. 



Fig- 3 a. Sections from the caudal region of a Pristiurus embryo belonging to 

 stage H. Zeiss C, ocul. i. Picric acid specimen. 



It shews the communication which exists posteriorly between the neural and 

 alimentary canals, and also by comparison with 3 b it exhibits the dilatation undergone 

 by the alimentary canal in the caudal vesicle. 



Fig. 33. Section from the caudal region of an embryo slightly younger than 3 a. 

 Zeiss C, ocul. i. Osmic acid specimen. 



