A survey of the cleavage-process in the later blastodiscs comprised in Group I shows that the division of the blastomeres follows no very regular plan and that there is often a considerable time-lag between nuclear and cytoplasmic division.
The central blastomeres, somewhat more flattened and more ovoidal in form than in the 31- and 32-celled stages may divide before or after they have become delimited from the yolk-bed. In the former case division usually appears to be effected in a plane oblique to the surface and is slightly unequal. It results in the formation of a larger superficial cell and a smaller deep, which lies not directly below its sister-cell but obliquely thereto. Both remain for a time in continuity with each other and with the yolk-bed. In the latter case, when the blastomere divides after delimitation, it may do so in the vertical plane to furnish two superficial cells or in the horizontal plane to form a superficial and a deep cell.
The deep cells separate sooner or later from their superficial sister-cells and then become delimited from the yolk-bed, first in the central region of the disc. It seems probable that some few of them may divide prior to their delimitation, the deeper of the two daughter-cells forming a sub-disc cell, which is later incorporated in the disc. The distinction between marginal and central cells (so obvious in the 31- and 32-celled blastodiscs) is soon lost (in LC. 4), the marginal cells being used up in the production, on the one hand, of peripheral disc-cells, and, on the other, of peculiar cells possessed of migratory properties and the capacity of enclosing yolk-spheres which lie at first just outside the periphery of the disc, but some of them soon wander outwards below the egg-membrane, away from the latter, whilst others migrate inwards below the yolk-membrane, to underlie the periphery of the disc. These cells, or vitellocytes as we have termed them, are already present in the 41-celled disc (LC. 1). In succeeding stages they increase in number by division and are destined to fuse together to form a continuous zone of nucleated cytoplasm which encircles the disc and extends inwards for a very short distance below its periphery. This zone constitutes the germ-wall, the presence of which conditions the growth in surface-extent of the blastodisc and its concomitant transformation into a thin blastoderm, capable under its continued influence of growing round to enclose the yolk.
In the earlier stages under review, the superficial cells increase in number much more rapidly than the deep; in LC. 4, for example, the former number 44 and the latter 12, but in the interval between LC. 4 and LC. 5 the deep cells, as well as the superficial, have undergone division so actively that the disc in its central region has attained a thickness of about four cells, thinning out peripherally to a thickness of two cells and to one at the margin. Except over a very restricted area below the central, region, the underside of the disc is still very irregular and lacks a definite contour, many of the deep cells being still open to the yolk.
The delimitation of the deep cells from the yolk-bed results from a process of rounding off or constriction affecting the region of continuity of the cytoplasm of the cell-body with the. cytoplasmic reticulum of the yolk-bed. As soon as a number of contiguous deep cells have become delimited, a thin limiting membrane (the yolk-membrane) is differentiated on the surface of the yolk-bed. It first appears in LC. 5 below a restricted area of the central region of the disc, and in LC. 6 it has also been established in places below the periphery of the disc. In LC. 7 it has been completed below the greater part of the disc. In this way the disc, as a whole, becomes delimited from the yolk-bed, though more or fewer of its peripheral cells remain open to the yolk. It is circular and biconvex in form, measures just over 0-5 mm. in diameter x 0-08 mm. in maximum thickness and centrally is about four cells deep.