Fig. 13. Longitudinal anrl horizontal section of an embryo belonging to 

 stage K, Zeiss C. Ociil. 1, and illustrating same points as previous section. 



Fig. 14«, 141), I4c, 14d. Figures taken from preparations of an embryo 

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

 the primitive ova. Fig. 14a and 14?> are portions of transverse sections. 

 Zeiss C. Ocul. 3 reduced ^. Fig. 14c and 14d 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. 2, Ocul. 1. The protoplasm of the ova is seen to be 

 nearly filled with bodies resembling yolli-spherules : and one ovum is ap- 

 parently undergoing division. 



Fig. 15a. Picric acid preparation shewing a primitive ovum partially 

 filled with bodies resembling yolk-spherules. 



Fig. 16. Horizontal and longitudinal section of Scyllium embryo be- 

 longing to stage K. Zeiss A. Ocul. 1. Picric acid preparation. The con- 

 nective-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 transparent 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 

 tracts. 



Fig. 18. Vertical longitudinal section of a Scyllium embryo belonging 

 to stage K. Zeiss A. Ocul. 1. 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. 



