—After Korotneff.
The origin of numerous division-nuclei as the offspring of the first has been observed to take place in the eggs of those insects (Aphides, Cecidomyia, and Cynips) which have a slight amount of yolk. Yet in the large, ordinary eggs of insects with an abundance of yolk there is no doubt, say Korschelt and Heider, that these numerous division-nuclei, which soon after the process of oviposition are scattered within the egg between the yolk-spheres, and are enveloped by a star-shaped protoplasmic layer, and which constitute the formative elements of the blastoderm,—there is no doubt but that they have practically arisen through indirect nuclear division from the first division-nucleus.
The process of formation of the blastoderm in ordinary eggs with abundant yolk was first observed by Bobretsky in the eggs of a moth (Porthesia) and Pieris, also by Graber, and more recently by Blochmann in Musca, and by Heider in Hydrophilus.
In the earliest stage observed by Bobretsky there first appear after fertilization a few (the smallest number four) cell-like, minute amœboid masses of protoplasm, each with a distinct nucleus. A few (one at least) of these bodies gradually pass out of the centre of the yolk to the surface of the egg (Fig. 504, A, n), these becoming larger and rounder, and from one or two of these nuclei (B, bc) the blastoderm originates (C, bl). Those nuclei remaining in the yolk increase in number and afterwards become the nuclei of rounded masses of yolk-granules, forming the so-called yolk-spheres which Bobretsky regards as true cells.
To the few blastoderm cells situated on the upper end of the egg are added others which continue to pass from the yolk to the periphery, and then the blastoderm spreads out farther and farther from the upper end of the egg until finally it covers or envelops the whole yolk. This layer of cells is called the blastoderm.
As to the origin of the primitive amœboid cells, Bobretsky is in doubt, but is disposed to think that they are the result of the subdivision of the germinative vesicle or nucleus of the ovarian egg-cell. In this connection may be quoted the observations of Graber, who states that an examination of the ovarian cell at an early period has revealed the presence, in the centre of the yolk, of a number of amœboid cells, which appear to have been formed by the division of the germinal vesicle. These “primary embryonic cells” have a relatively large nucleus and a number of nucleoli. Several may be seen to unite with one another by means of their pseudopodia, and they may also be observed to undergo division. With this account may be compared the results obtained by Korotneff in his work on the embryology of the mole-cricket (Fig. 505).
Fig. 506.—Four successive stages in the formation of the blastoderm of Calliphora vomitoria (the figures represent segments of cross-sections through the fly’s egg): A, the nuclei of the division-cells have arranged themselves parallel with the outer surface of the egg. B, the division-cells fused with the “keimhautblastem.” C, the outer surface becomes furrowed by indentations; all the nuclei of the blastoderm-cells in process of division. D, the blastoderm-cells form a high cylinder-epithelium: b, “keimhautblastem”; bz, blastoderm-cells; d, nutritive yolk; dz, yolk-cell; fz, so-called division-cell; i, inner “keimhautblastem.”—After Blochmann, from Korschelt and Heider.
The result of these and of later observations, especially those of Blochmann on Musca, and those of Heider on Hydrophilus, show that the division-nuclei lie near the centre of the egg, along the longitudinal axis (Fig. 507, A). Each of these nuclei is enveloped by a star-shaped mass of protoplasm, and on the whole resembles a wandering amœboid cell. These isolated masses of protoplasm are all connected by a fine network of rays, which unite to form within the yolk a syncytium. Afterwards, in the later stages, these division-cells, as they may be, though somewhat incorrectly, regarded, move nearer the periphery and arrange themselves into a plane parallel with the surface (Figs. 506, A, 507, B). Continuing to divide, they reach the surface and fuse with the peripheral protoplasmic layer (Figs. 506, B, 507, C). Then follows the division into single cell-territories (Figs. 506, B, 507, C), corresponding to the division-nuclei, through the appearance of furrows which pass in from the outer surfaces of the egg into the interior and gradually penetrate the entire “keimhautblastem.” In this way the surface of the egg is covered with an epithelium (blastoderm). In many insects the so-called inner “keimhautblastem” (Fig. 506, D, i) is formed by the separation of a layer of protoplasm which contains larger granules and are accumulated between the blastoderm and the upper surface of the central nutritive yolk-mass. By the addition of this plasmic layer the cells of the blastoderm increase in height, and now form a cubical or cylinder epithelium, which continuously envelops the surface of the egg. (Korschelt and Heider.)