10b. This section is taken from the anterior part of the same embryo and shews the deep medullary groove and the commencing formation of the ventral wall of the alimentary canal from the nuclei of the yolk.
10c shews the disappearance of the medullary groove and the thinning out of the mesoblast plates in the region of the head.
Fig. 11. Small portion of the blastoderm and the subjacent yolk of an embryo at the time of the first appearance of the medullary groove × 300. It shews two large nuclei of the yolk (n) and the protoplasmic network in the yolk between them; the network is seen to be closer round the nuclei than in the intervening space. There are no areas representing cells around the nuclei.
Fig. 12. Nucleus of the yolk in connection with the protoplasmic network hardened in osmic acid.
Fig. 13. Portion of posterior end of a blastoderm of stage B, shewing the formation of cells around the nuclei of the yolk.
Fig. 14. Section through part of a young Scyllium egg, about 1/15th of an inch in diameter.
nl. Protoplasmic network in yolk. zp. Zona pellucida. ch. Structureless chorion. fep. Follicular epithelium. x. Structureless membrane external to this.
[116] Qy. Journal of Microsc. Science, Oct. 1874. [This Edition, No. V.]
[117] Centr. f. Med. Wiss. No. 38, 1875.
[118] Professor Bambeke (“Poissons Osseux,” Mém. Acad. Belgique 1875) describes a cavity in the blastoderm of Leuciscus rutilus, which he regards as the true segmentation cavity, but not as identical with the segmentation cavity of Osseous Fishes, usually so called. Its relations are the same as those of my segmentation cavity at this stage. This paper came into my hands at too late a period for me to be able to do more than refer to it in this place.