Fig. 539.—Cross-section through the primitive streak of Hydrophilus in six successive stages: A, gastrula-stage (compare Fig. 515, A, corresponding to the point a). B, cross-section through stage, Fig. 515, D, in the most anterior section of the primitive band, where the same is not completely overgrown by the amnion-folds. C, cross-section through the trunk-segment of stage, Fig. 515, E. D, E, F, cross-sections through later stages: am, amnion; b, lower (inner) layer; d, yolk; dz, yolk-cells; ec, ectoderm; en, entoderm; l, definite body-cavity; pr, primitive groove (= neural groove); pw, primitive roll, or strip, of the ventral nerve-cord; r, blastopore; sp, fissure in the mesoderm (remains of the cavity of the primitive intestine); se, serosa; s, lateral cord of the rudiment of the nervous cord; spm, splanchnic layer of the mesoderm; tr, rudiment of a trachea (in E appearing as an invagination of the ectoderm) in F in cross-section; us, primitive segment (= cœlomic sac).—After Heider, from Lang.
Fig. 540.—Cross-sections through the abdominal part of three successive stages of evolution of Phyllodromia germanica: am, amnion; bg, rudiment of the ventral nervous chord; c, cœlomic cavity; c′, dorsal, and c″, ventral, section of the cœlomic sac; cz, cells of the walls of the primitive segment, which are joined to the genital rudiments; gz, genital cells; dw, dorsal wall of the cœlomic sac; d, yolk; ec, ectoderm; ep, epithelium-cells; ex, rudiment of the abdominal appendages; f, germ of the fat-body; lw, lateral wall of the cœlomic sac; m, mesoderm cells, which take no part in the formation of the cœlomic sac; mw, median wall of the cœlomic sac; so, somatic mesoderm layer; vm, ventral longitudinal muscle.—After Heymons, from Korschelt and Heider.
In the highest groups of insects (Coleoptera, Lepidoptera, and Hymenoptera) the primitive segments are not so extensively developed (Fig. 539, D-F, us). They here form only relatively small sacs situated in the lateral parts of the primitive band which correspond to the dorsal section of the cœlom-sacs of Orthoptera. The ventral part is here from the very outset replaced by a mesenchym. As a result in these forms also no cœlomic diverticula occur in the rudiments of the extremities.
The definite body-cavity of insects arises entirely independent of the cœlom cavities, and in fact, as Bütschli showed, through the separation of the primitive band from the yolk (Fig. 539, F, l). It appears bounded on the one hand by the surface of the yolk, on the other side by the irregularly arranged mesenchym cells. Originally we can in cross-sections distinguish three separate cavities of the definite body-cavity (in Hydrophilus according to Heider), a median and two larger paired lateral ones which later fuse with each other and with wide lacunæ (e.g. in the appendages) arising by the separation of the mesenchym cells. We refer the compartments of the definite body-cavity, as in Peripatus, to the primary body-cavity or segmentation-cavity. They are only lacunæ in the area of the mesenchym, and throughout bear the character of a pseudocœl.
In later stages of embryonic development the cœlom-sacs and the definite body-cavity enter into communication with one another (Fig. 523, A, us, lh). (Korschelt and Heider.)
Then the hinder cœlom-sacs unite through the degeneration of the transverse dissepiments which separate them. After this a fissure opens in the median wall of the cœlomic sac, through which its cavity unites with the definite body-cavity. In the subsequent changes which the wall of the cœlom-sacs undergoes, these can be recognised no longer as separate divisions of the whole body-cavity.
l. Formation of organs
The nervous system.—As we have already seen (p. 554), the rudiments of the ventral nervous cord arise, after the gastrula invagination is completed, as two ectodermal thickenings situated on each side of the median line, the so-called primitive rolls or strips (Fig. 528, s), which extend from the centre of the procephalic lobes of the head to the last segment, enclosing between them the single median “primitive groove” (Fig. 539, C, pr, and pw).