EXPLANATION OF PLATE II. xix 



Germinal epithelium, At. Heart. /. Liver, mp. Muscle-plate, mp'. Early formed 

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

 p. Protoplasm from yolk in the alimentary tract. / c . Pericardial cavity, p o. Primi- 

 tive ovum. pp. Body cavity. / r. Rudiment of posterior root of spinal nerve, s d. 

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

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

 renal body. ^ t. Segmental tube, s v. Sinus venosus. u a. Umbilical artery, urn. 

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

 cleft. V r. Vertebral rudiment. W. \Vhitematterofspinalcord. x. Subnotochordal 

 rod (except in fig. 140). y. Passage connecting the neural and alimentary canals. 



PLATE 11. 



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. 3^. Section from the caudal region of an embryo slightly younger than $a. 

 Zeiss C, ocul. i. Osmic acid specimen. 



Fig. 4. Section from the cardiac region of a Pristiurus embryo belonging to stage 

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



It shews the formation of the heart (ht) as a cavity between the splanchnopleure 

 and the wall of the throat. 



Fig. 5. Section from the posterior dorsal region of a Scyllium embryo, belonging 

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



It shews the general features of an embryo of stage H, more especially the rela- 

 tions of the body-cavity in the parietal and vertebral portions of the lateral plate, and 

 the early-formed band of muscle (mp') in the splanchnic layer of the vertebral plate. 



Fig. 6. Section from the cesophageal region of Scyllium embryo belonging to 

 stage I. Zeiss C, ocul. i. Chromic acid specimen. 



It shews the formation of the rudiments of the posterior nerve-roots (pr) and of 

 the vertebral rudiments ( Vr). 



Fig. 7. Section of a Torpedo embryo belonging to stage slightly later than I. 

 Zeiss C, ocul. i, reduced \. Osmic acid specimen. 



