Kowalevsky has already proved that it is the median parts only of the inner layer which at the two ends of the primitive band become separated as endodermal rudiments through the advance of the stomodeal and proctodeal invaginations: the lateral portions become mesoderm.
Kowalevsky has compared the germ-layers of insects with those of Sagitta. This comparison is supported by the later researches of Heider and of Wheeler on Coleoptera. (See Korschelt and Heider, p. 809.)
Relations somewhat different from the common type of formation of germ-layers occur in Hymenoptera. Kowalevsky and also Grassi agree that here also the endoderm originally forms a part of the lower (inner) layer. But the separation of the endoderm from the mesoderm goes on in Apis in such a way that the two ends of the inner layer pass up to the dorsal side of the egg, where the fore and hind rudiments of the endoderm extending along the back of the embryo grow together. When the two horseshoe-shaped rudiments have met each other and become fused, the enclosing of the yolk begins, which accordingly here proceeds from the dorsal towards the ventral side, instead of vice versa. As a result the endodermal cell-layer in Apis (and also Chalicodoma) at first does not lie under the primitive band, but on the dorsal side of the egg under that flat epithelium, which, arising from the amnion-fold, completes the provisional closure of the back.
The yolk-cells and secondary yolk-segmentation are discussed by Korschelt and Heider at this point. The yolk-cells are elements scattered throughout the yolk and which partly remain in the yolk during the formation of the blastoderm (Fig. 507, C and D), but which in part through a later immigration pass out of the blastoderm into the yolk. Graber has proved the fact of the migration of cells from the lower layer into the yolk, and his observations have been confirmed by other authors. Indeed, in certain cases (Melolontha), these later immigrant cells are clearly distinguishable by their histological characters from those originally found in the yolk.
The yolk-cells are regularly scattered throughout the yolk. Their use to the embryo lies in the fact that they absorb the particles of yolk, which they digest and thus reduce to a fluid condition. It usually happens that after the complete formation of the primitive band there results a delimitation of the areas enclosing each yolk-cell, and this occurrence is called secondary yolk-division. In special cases (Apis, Musca) this occurrence seems not to take place. The yolk-cells are still, after the complete formation of the mid-intestine, to be recognized in the yolk-remnants filling the interior of the same, and gradually become absorbed.
k. Farther development of the mesoderm. Formation of the body-cavity
We have seen that by means of an invagination extending throughout the entire length of the primitive band a layer of cells is produced which soon spreads out on the inner side of the band and thus forms a second lower (inner) layer (Fig. 539, C). From this inner layer is separated at the anterior and posterior ends of the primitive band, the endoderm, which lies in direct contact with the invaginations of the proctodæum and stomodæum. The remainder, by far the most extensive part of the inner layer, is the mesoderm.
The mesoderm now becomes divided into two lateral streaks (mesodermal streaks), by the withdrawal of its cells from the median line (Fig. 539, D). This withdrawal is not, however, always a complete one. In the free median space thus formed, the yolk often forms the so-called median yolk-ridge. Segmentally arranged cavities soon appear in the lateral region of the mesoderm (the primitive segmental cavities), and the bordering mesoderm-cells arrange themselves in the form of an epithelium, and constitute the wall of the primitive segments or cœlom-sac. (Korschelt and Heider).
The primitive segmental cavities in general arise through a split in the mesoderm. In Phyllodromia, according to Heymons, the primitive segments are very extensive. The mesoderm, at the time of the formation of the rudiments of the appendages, is raised with the ectoderm from the surface of the yolk, and in this way there arise in each segment cavities, which, since they are surrounded by mesodermal elements, become the closed cœlom-sacs (Fig. 540, c, c′, c″).
The cœlom-sacs differ in different groups. They are largest in Orthoptera (Phyllodromia), where they take up almost all the cell material of the mesoderm in their formation, and exhibit certain conditions recalling those of Peripatus. The very large primitive segmental cavities, which in Orthoptera also extend into the rudiments of the appendages (Fig. 540, B, ex), in their later stages are, through the formation of a constriction, divided into a dorsal and a ventral half (Fig. 540, B, c′, c″). The ventral portions of these cavities extending into the extremities soon disappear, while the cells of their walls lose their epithelial nature, and group themselves irregularly into a sort of mesenchym. In this tissue, then, arises, partly through a separation among its cells, partly through the elevation of the same from the upper surface of the yolk, the definite body-cavity. The dorsal portions of the primitive segmental cavities remain unchanged a longer time in order to play a rôle in the formation of the intestinal muscular layer, of the heart, pericardial septum, and sexual organs.