CHAPTER V.
Stages B to G.
The present chapter deals with the history of the development of the Elasmobranch embryo from the period when the medullary groove first arises till that in which it becomes completely closed, and converted into the medullary canal. The majority of the observations recorded were made on Pristiurus embryos, a few on embryos of Torpedo. Where nothing is said to the contrary the statements made apply to the embryos of Pristiurus only.
The general external features for this period have already been given in sufficient detail in the last chapter; and I proceed at once to describe consecutively the history of the three layers.
General Features of the Epiblast.
At the commencement of this period, during the stage intermediate between B and C, the epiblast is composed of a single layer of cells. (Pl. 10, fig. 1.)
These are very much elongated in the region of the embryo, but flattened in other parts of the blastoderm. Throughout they contain numerous yolk-spherules.
In a Torpedo embryo of this age (as determined by the condition of the notochord) the epiblast presents a very different structure. It is composed of small spindle-shaped cells several rows deep. The nuclei of these are very large in proportion to the cells containing them, and the yolk-spherules are far less numerous than in the cells of corresponding Pristiurus embryos.
During stage C the condition of the epiblast does not undergo any important change, with the exception of the layer becoming much thickened, and its cells two or three deep in the anterior parts of the embryo. (Pl. 10, fig. 2.)
In the succeeding stages that part of the epiblast, which will form the spinal cord, gradually becomes two or three cells deep. This change is effected by a decrease in the length of the cells as compared with the thickness of the layer. In the earlier stages the cells are wedge-shaped with an alternate arrangement, so that a decrement in the length of the cells at once causes the epiblast to be composed of two rows of interlocking cells.