PLATES II AND 12. 359 



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 (/if) 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 (nip') 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. 



It shews (i) the formation of the anterior and posterior nerve-roots. (2) The solid 

 knob from which the segmental duct (sd) originates. 



Fig. 8. Section from the dorsal region of a Scyllium embryo belonging to a stage 

 intermediate between I and K. Zeiss C, ocul. i. Chromic acid specimen. 



It illustrates the structure of the primitive ova, segmental tubes, notochord, etc. 



Fig. 8 a. Section from the caudal region of an embryo of the same age as 8. 

 Zeiss A, ocul. i. 



It shews (i) the solid oesophagus. (2) The narrow passage connecting the peri- 

 cardial (pc) and body cavities (//) 



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

 Osmic acid specimen. 



It shews the formation of the liver (/), the structure of the anterior fins (/>), and the 

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



Figs. ga, gl>, gc, gd. Four sections through the anterior region of the same 

 embryo as 9. Osmic acid specimens. 



The sections shew (i) the atrophy of the post-anal section of the alimentary tract 

 (gb, gc, gd). (2) The existence of the segmental tubes behind the anus (gl>, gc, gd). 

 With 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 gc but reappears in gd. (3) The downward prolongation of the segmental duct to 

 join the posterior or cloacal extremity of the alimentary tract (9^). 



PLATE 12. 



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

 Zeiss C, ocul. i. Reduced by ^. Picric acid specimen. 



It shews (i) the structure of the notochord ; (2) the appearance of the early formed 

 band of muscles (nip 1 ) in the splanchnic layer of the protovertebra. 



Fig. u. Longitudinal and horizontal sections of an embryo belonging to stage I. 

 Zeiss C, ocul. i. 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. 



