EXPLAXATION OF PLATE XI. 

 Complete list of reference letters. 



ep. epiblast. sp. c. spiual canal. 

 pr. rudiment of posterior root of spinal nerve. 

 ar. „ „ anterior root of spinal nerve. 

 b. anterior fin. 77ip. muscle-plate. 



mp. early formed band of muscles. Vr. vertebral rudiment. 



pj?. body-cavity. um. umbilical cord. sd. segmental duct. 



St. segmental tube. sr. inter-renal body. ^^o. primitive ovum. 



ge. germinal epithelium. vc. visceral cleft. ao. dorsal aorta. 



ca V. cardinal vein. v. splanchnic vein. 



ua. umbilical artery. uv. umbUical vein. 



V. blood-vessel. ch. notochord. 



sh. cuticulur sheath of notochord. x. subnotochordal rod (except 



in fig. 14«). 



al. alimentary tract. I. liver. 



an. point where anus will be formed. 



Fig. 9. Section of a Pristiurus embryo belonging to stage K. Zeiss A. 

 Ocul. 1. Osmic acid specimen. 



It shews the formation of the liver (l), the structure of the anterior fins 

 (b), and the anterior opening of the segmental duct into the body-cavity (sd). 



Fig. 9a, 9b, 9c, 9d. Four sections through the anterior region of the 

 same embryo as 9. Osmic acid specimens. 



The sections shew (1) the atrophy of the post-anal section of the ali- 

 mentary tract (9^, 9c, 9d). (2) The existence of the segmental tubes behind 

 the anus {9b, 9c, 9d). AVith reference to these it deserves to be noted that 

 the segmental tubes behind the anus are quite disconnected, as is proved 

 by the fact that a tube is absent on one side in 9c but reappears in 9d. 

 (3) The downward prolongation of the segmental duct to join the posterior 

 or cloacal extremity of the alimentary tract {9b). 



Fig. 10. Longitudinal and horizontal section of a Scyllium embryo of 

 stage H. Zeiss C. Ocul. 1. Reduced by ^. Picric acid specimen. 



It shews (1) the structure of the notochord ; (2) the appearance of the 

 early formed band of muscles {inp') in the splanchnic layer of the proto- 

 vertebra. 



Fig. 11. Longitudinal and horizontal sections of an embryo belonging 

 to stage I. Zeiss C. Ocul. 1. Chromic acid specimen. It illustrates the same 

 points as the previous section, but in addition shews the formation of the 

 rudiments of the vertebral bodies ( Vr) which are seen to have the same 

 segmentation as the muscle-plates. 



Fig. 12.^ Longitudinal and horizontal section of an embryo belonging to 

 the stage intermediate between I and K. Zeiss C. Ocul. 1. Osmic acid 

 specimen illustrating the same points as the previous section. 



1 The apparent structure in the sheath of the notochord in this and the suc- 

 ceeding figure is merely the result of an attempt on the part of the engraver 

 to represent the dark colour of the sheath in the original figure. 



