f. Formation of the external form of the body
Origin of the body-segments.—As we have seen, the first traces of segments appear very early, the primitive band being divided by superficial transverse furrows into segments. This segmentation into arthromeres (somites or metameres) can be observed in Hydrophilus and Chalicodoma at a time when gastrulation begins (Figs. 515, 536). The segmentation extends not only across the median portion of the primitive band, through whose invagination the inner layer (endomesoderm) results, but also to the lateral portions which become a part of the ectoderm of the primitive band. These transverse furrows correspond to thinner places in the epithelium, which in this stage forms the embryonal rudiment. It thus happens that, in the forms named, after the end of gastrulation not only the ectoderm, but also the endomesoderm, is already segmented.
So early an appearance of segmentation as that observed in Hydrophilus and Chalicodoma we must regard as a falsification of the process of development due to heterochrony. We must consider the conditions observed in other forms as the primitive ones, in which (as, for example, in Lina and in Stenobothrus, according to Graber) the gastrulation and separation of the ectoderm occurs in the still unsegmented primitive band, the division into segments occurring in later stages (Fig. 524). In these forms, then, the segmentation affects the invaginated endomesoderm, as well as the ectoderm. (Korschelt and Heider, p. 789.)
Fig. 523.—Diagrammatic cross-section through three successive stages of Gryllotalpa, showing the formation of the heart. (Compare Fig. 505.) The germs of the glandular intestinal layer (darmdrüsenblatt) are omitted. A, earliest stage; the primitive streak extends from *x to y*. The embryonal membranes are torn and pressed against the back: am, edge of the rent; rp, dorsal plate (serosa); l, lamella (amnion turned up) standing in connection with the ectoderm of the primitive streak. B, second stage; the primitive streak has completely grown around the yolk; the dorsal organ is absorbed. C, third stage, dorsal portion; the formation of the heart is finished: am, vestige of the amnion-fold; bs, blood-sinus; dd, rudiment of the dorsal diaphragm; dv, ventral diaphragm (compare Fig. 505); do, yolk; dz, yolk-cells; ec, ectoderm; gr, vascular groove (rudiment of the heart); l, lamella of the upturned amnion; lh, definite body-cavity; m, transverse muscle; n, nervous cord; r, heart; rp, dorsal plate; sp, splanchnic; so, somatic layer of the mesoderm; us, primitive segmental cavity; *x, y*, lateral terminations of the primitive streak.—After Korotneff, from Korschelt and Heider.
In the completely segmented primitive band may be distinguished two regions of a peculiar appearance (Figs. 515, 527), one at the anterior, and the other at the hinder end. The anterior, the primary head-section, contains the mouth-opening, and is characterized by its lateral expansions, or procephalic lobes. The other section, or posterior section, the so-called anal segment or telson, contains the anus. Between the two sections lies the segmented primary trunk-segment, which in insects consists of 17 segments. Of these the three most anterior are those destined to bear the mandibles and two pairs of maxillæ; the three following are the thoracic, which are succeeded by 10 abdominal segments, besides the 11th or telson (pygidium, or suranal plate).
It is now generally believed that there are primarily eleven abdominal segments, while Heymons has detected twelve in the embryos of Blattids and Forficula (see p. 162). In the later stages of embryonic development the number of abdominal segments is diminished, the 10th and 11th abdominal segments becoming fused. In Hydrophilus and Lina this is the case, but according to Graber, in the Lepidoptera there is a fusion of the 9th and 10th abdominal segments, the llth remaining free.
According to Wheeler, in Doryphora, and also in Chalicodoma (Carrière), between the primary head-region and the mandibular segment is interpolated a rudimentary and transitory body-segment, the premandibular segment. According to Carrière this segment corresponds to a rudimentary pair of limbs, and also to a ganglion, which participates in the formation of the œsophageal commissure (see p. 51).
Fig. 524.—Three embryonic stages of a leaf-beetle (Lina): A, unsegmented primitive streak; in B and C the segmentation becomes distinct on the lower layer (u). B, with the germs of the gnathal segments (k′-k‴), and in C the three thoracic segments (t’-t‴), with the first two abdominal segments (a′, a″): bl, blastopore; kl, head-lobes; th, extension of the primitive streak into the thoracic region.—After Graber, from Korschelt and Heider.