Differences between the invaginated and overgrown primitive band.—In respect to the mode of origin of the primitive band and its relative position, there are two opposite types, though connected by transitional forms. In the one case the primitive band, i.e. its ventral portion, the “ventral plate” (Fig. 518, b, p) is pushed in or invaginated in the interior of the egg; in the other case it becomes overgrown by the folds of the amnion arising from its edges.
Fig. 517.—Primitive streak of a lepidopter in cross-section: ah, amniotic cavity; am, amnion; c, cœlomic cavity; do, nutritive yolk, divided into single nucleated masses; ec, ectoderm; m, mesoderm; pr, primitive thickenings of the ventral nervous cord; s, serosa.—Combined figure after those of Brobretsky and Hatschek, from Korschelt and Heider.
Fig. 518.—Five diagrammatic median sections representing the growth of a dragon-fly (Calopteryx). A-C, development of the primitive streak (k, k′) by invagination. D, the amnion-fold (af), growing over the head-end of the primitive streak. E, closing of the opening of the amnion-cavity (ah): v, ventral, d, dorsal side; a, fore, b, hind end of egg; bl, blastoderm; bp, ventral plate; do, yolk; k, head-end, k′, caudal end, of the primitive streak; kh, germinal thickening or initial point of invagination; s, serosa.—After Brandt, from Korschelt and Heider.
In insects with an overgrown primitive band, the band at the beginning is generally short and always situated on the ventral side of the egg, with the head-end looking forward, and remains in this position throughout embryonic life. There is no revolution of the embryo. The embryonal membranes arise through the formation of folds.
Fig. 519.—Three embryonic stages of Calopteryx: am, amnion; g, edge of the ventral plate; ps, germ of primitive band; se, serosa.—After Brandt, from Balfour.
Fig. 520.—Three farther stages of growth of Calopteryx. B and C show the inversion of the embryo: a, opening of the amniotic-cavity, out of which the embryo emerges; ab, abdomen; am, amnion; at, antenna; md, mandible; mx1, mx2, 1st and 2d maxillæ; œ, œsophagus; p1, p2, p3, legs; se, serosa; v, anterior end of the primitive streak.—After Brandt, from Balfour.