Figs. 8_a_, 8_b_, 8_c_. Modified nuclei from the yolk from the same germinal disc.
Fig. 8_d_. Segment in the act of division from the same germinal disc.
Fig. 9. Section through a germinal disc in which the segmentation is completed. It shews the larger collection of cells at the embryonic end of the germinal disc than at the non-embryonic. _ep._ epiblast.
In the last chapter the blastoderm was left as a solid lens-shaped mass of cells, thicker at one end than at the other, its uppermost row of cells forming a distinct layer. There very soon appears in it a cavity, the well-known segmentation cavity, or cavity of von Baer, which arises as a small space in the midst of the blastoderm, near its non-embryonic end (Pl. 7, fig. 1.).
This condition of the segmentation cavity, though already[116] described, has nevertheless been met with in one case only. The circumstance of my having so rarely met with this condition is the more striking because I have cut sections of a considerable number of blastoderms in the hope of encountering specimens similar to the one figured, and it can only be explained on one of the two following hypotheses. Either the stage is very transitory, and has therefore escaped my notice except in the one instance; or else the cavity present in this instance is not the true segmentation cavity, but merely some abnormal structure. That this latter explanation is a possible one, appears from the fact that such cavities do at times occur in other parts of the blastoderm. Dr Schultz[117] does not mention having found any stage of this kind.
Footnote 116: _Qy. Journal of Microsc. Science_, Oct. 1874. [This Edition, No. V.]
Footnote 117: _Centr. f. Med. Wiss._ No. 38, 1875.
The position of the cavity in question, and its general appearance, incline me to the view that it is the segmentation cavity[118]. If this is the true view of its nature the fact should be noted that at first its floor is formed by the lower layer cells and not by the yolk, and that its roof is constituted by both the lower layer cells and the epiblast cells. The relations of the floor undergo considerable modifications in the course of development.
Footnote 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.