Anura. The formation of the germinal layers has so far only been studied in some Anura and in the Newt. The following description applies to the Anura, and I have called attention, at the end of the section, to the points in which the Newt is peculiar.
The segmentation of the Frog’s ovum has already been described (Vol. II. pp. 95-7), but I may remind the reader that the segmentation ([fig. 69]) results in the formation of a vesicle, the cavity of which is situated excentrically; the roof of the cavity being much thinner than the floor. The cavity is the segmentation cavity. The roof is formed of two or three layers of smallish pigmented cells, and the floor of large cells, which form the greater part of the ovum. These large cells, which are part of the primitive hypoblast, will be spoken of in the sequel as yolk-cells: they are equivalent to the food-yolk of the majority of vertebrate ova.
Fig. 69. Segmentation of Common Frog. Rana Temporaria. (After Ecker.)
The numbers above the figures refer to the number of segments at the stage figured.
The cells forming the roof of the cavity pass without any sharp boundary into the yolk-cells, there being at the junction of the two a number of cells of an intermediate character. The cells both of the roof and the floor continue to increase in number, and those of the roof become divided into two distinct strata ([fig. 70], ep).
The upper of these is formed of a single row of somewhat cubical cells, and the lower of several rows of more rounded cells. Both of these strata eventually become the epiblast, of which they form the epidermic and nervous layers. The roof of the segmentation cavity appears therefore to be entirely constituted of epiblast.
The next changes which take place lead (1) to the formation of the mesenteron[48], and (2) to the enclosure of the yolk-cells by the epiblast.
