XX EXPLANATION OF PLATE 12. 



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, (i) The narrow passage connecting the peri- 

 cardial (pc) and body cavities (pp}. 



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 (6), and the 

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



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

 embryo as 9. Osmic acid specimens. 



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

 (gb, gc, gd). (2) The existence of the segmental tubes behind the anus (gb, 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. 1 1. 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 ( V r) which are seen to have the same segmentation as the muscle-plates. 



Fig. I2. 1 Longitudinal and horizontal section of an embryo belonging to the 

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

 illustrating the same points as the previous section. 



Fig. 13. Longitudinal and horizontal section of an embryo belonging to stage K. 

 Zeiss C, ocul. i, and illustrating same points as previous section. 



Figs. i+a, i^b, 14*:, i$d. Figures taken from preparations of an embryo of an 

 age intermediate between I and K, and illustrating the structure of the primitive ova. 

 Figs. 14 a and 14 b are portions of transverse sections. Zeiss C, ocul. 3 reduced -3-. 

 Figs. 14 c and 14 d are individual ova, shewing the lobate form of nucleus. Zeiss F, 

 ocul. i. 



Fig. 15. Osmic acid preparation of primitive ova belonging to stage K. Zeiss 

 immersion No. 2, ocul. i. The protoplasm of the ova is seen to be nearly filled with 

 bodies resembling yolk-spherules : and one ovum is apparently undergoing division. 



1 The apparent structure in the sheath of the notochord in this and the succeeding 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. 



