Fig. 13. Longitudinal and horizontal section of an embryo belonging to stage K. Zeiss C, ocul. 1, and illustrating same points as previous section. Figs. 14_a_, 14_b_, 14_c_, 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_b_ are portions of transverse sections. Zeiss C, ocul. 3 reduced 1/3. 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. 2, ocul. 1. The protoplasm of the ova is seen to be nearly filled with bodies resembling yolk-spherules: and one ovum is apparently undergoing division. Fig. 15_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. 1. 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 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 tract. 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. CHAPTER VII. GENERAL DEVELOPMENT OF THE TRUNK FROM STAGE H TO THE CLOSE OF EMBRYONIC LIFE. _External Epiblast._ The change already alluded to in the previous chapter (p. 317) by which the external epiblast or epidermis becomes divided into two layers, is completed before the close of stage L. In the tail region at this stage three distinct strata may be recognized in the epidermis. (1) An outer stratum of flattened horny cells, which fuse together to form an almost continuous membrane. (2) A middle stratum of irregular partly rounded and partly flattened cells. (3) An internal stratum of columnar cells, bounded towards the mesoblast by a distinct basement membrane (Pl. 13, fig. 8), unquestionably pertaining to the epiblast. This layer is especially thickened in the terminal parts of the paired fins (Pl. 13, fig. 1). The two former of these strata together constitute the epidermic layer of the skin, and the latter the mucous layer.