EXPLANATION OF PLATE X. 



Complete list of reference letters. 



ep. epiblast df. dorsal fin. sp. c. spinal cord. 



W. white matter of spinal cord. nc. neural canal. 



pr. posterior root of spinal nerve. 



ar. anterior root of spinal nerve. wz^?. muscle-plate. 



mp'. early formed band of muscles from the splanchnic laver of the 

 muscle-plates. 



Vr. vertebral rudiment. so. somatic laver of mesoblast. 



sp. splanchnic layer of mesoblast. pp. bodv-cavity, 



2K. pericardial cavity. c. connective-tissue'cells. 



sd. segmental duct. st. segmental tube. po. primitive ovum. 



ao. dorsal aorta. ca v. cardinal vein. .sv. sinus venosus. 



ht. heart. v. splanchnic vein. ch. notochord. 



X. subnotochordal rod. al. alimentary tract. sp. v. spiral valve. 



y. passage connecting the neural and alimentary canals. 



I hver. p. protoplasm from yolk in the alimentary tract. 



Fig. 1. Section from the caudal region of a Pristiurus embryo be- 

 longmg to stage H. Zeiss C. Ocul. 1. Osmic acid specimen 



It shews (1) the constriction of the subnotochordal rod (x) from the 

 summit of the ahmentary canal. (2) The formation of the body-cavity 

 m the muscle-plate and the ventral thickening of the parietal plate 



Fig. \a Portion of alimentary waU of the same embryo, shewing the 

 formation of the subnotochord rod {x). 



Fig. 2. Section through the caudal vesicle of a Pristiurus embryo be- 

 longing to stage H. Zeiss C. Ocul. 1. 



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

 of the mesoblast and hypoblast at the caudal vesicle. 



Fig. 3«. Sections from the caudal region of a Pristiurus embryo be- 

 longing to stage H. Zeiss C. Ocul. 1. Picric acid specimen. 



it shews the communication which exists posteriori v between the neural 

 and alimentary canals and also by comparis(m with ?J) it exhibits the dila- 

 tation undergone by the alimentary canal in the caudal vesicle. 



1 ig. 36. Sec-tiou from the caudal region of an embryo slightly younger 

 than 3«. Zeiss C. OcuL 1. Osmic acid specimen. o j j « 



Fig. 4. Section from the cardiac region of a Pristiurus embryo be- 

 longing to stage H. Zeiss C. Ocul. 1. Osmic acid specimen. 



It shews the formation of the heart {ht) as a cavity between the splanch- 

 nopleure and the waU of the throat. 



h^in^l^'nw.^f "^'Vf'^'^^.^'® P^!{^''^'^^ ^^^^''^^ region of a Scvllium embrvo, 

 belonging to s^tage H. Zeiss C. Ocul. 1. Osmic acid specimen. 



it slieu-s the general features of an embrvo of stage H, more especiallv 

 the bttr^l f '^^ bodj-eavitj in the parietal and vertebral poTonfS 

 l^yVrtf U^^^^^^^^^^^ '^'"^'^ ^-^ «f --^^^ (-^') - the splanchnic 



lonc^^j,?: t^o .t!tf r y"""' ^^^^^^fPbageal region of Scyllium embrvo be- 

 iongmg to stage I. Zeiss C. Ocul. 1. Chromic acid specimen. 



