360 DEVELOPMENT OF ELASMOBRANCH FISHES. 



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. i4#, 14^, 14*:, \^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^ are portions of transverse sections. Zeiss C, ocul. 3 reduced \. 

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

 ocul. 2. 



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

 immersion No. i, 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. 



Fig. 1 5 a. Picric acid preparation shewing a primitive ovum partially filled with 

 bodies resembling yolk-spherules. 



Fig. 16. Horizontal and longitudinal section of Scyllium embryo belonging to 

 stage K. Zeiss A, ocul. i. Picric acid preparation. The connective-tissue cells are 

 omitted. 



The section shews that there is one segmental tube to each vertebral segment. 



Fig. 17. Portion of a Scyllium embryo belonging to stage K, viewed as a trans- 

 parent object. 



It shews the segmental duct and the segmental involutions two of which are seen 

 to belong to segments behind the end of the alimentary tract. 



Fig. 1 8. Vertical longitudinal section of a Scyllium embryo "belonging to stage K. 

 Zeiss A, ocul. i. Hardened in a mixture of osmic and chromic acid. It shews 



(1) the commissures connecting together the posterior roots of the spinal nerves ; 



(2) the junction of the anterior and posterior roots 



(3) the relations of the segmental ducts to the segmental involutions and the 



alternation of calibre in the segmental tube ; 



(4) the germinal epithelium lining the body-cavity. 



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. 



