Fig. 542.—Two stages of development of the 5th of the six ocelli of larva of Acilius: c, cuticular striated band; cl, germ destined to form the corneal lens; h, hypodermis; l, crystalline-lens layer; n, optic nerve; r, retinal germ; sp, vertical fissure of the retina; x, the retina-cells bordering this fissure.
Fig. 543.—Two later stages of development of the same eye as in Fig. 542: i, iris; m, middle inverted layer of the eye; r, retina; sp, vertical fissure of the retina; st, rods; other letters as in Fig. 542.—This and Fig. 542 after Patten, from Korschelt and Heider.
The inner, deeper layer of the eye, which forms the contracted cup-shaped portion, appears to be the rudimentary retina (r). From its cuticular rod-like or fibrous edge arise the visual rods. There soon arise certain peculiarities characteristic of the eye of Acilius, i.e. the fissure (sp) bordered by the horizontally situated rods of the large retina-cells (x).
In the farther developed eye (Fig. 543) there is a flattening of the cup-shaped inner edge, by which the bottom of the eye is levelled and the little rods belonging to it stand up vertically (Fig. 543, B, st). Then the cells belonging to the edge of the retinal cup (m) are turned in, forming an inverted layer constituting the germs of a third layer interpolated between the two chief layers of the eye. (Korschelt and Heider, from Patten.) Patten concludes that the structure of the retina in the larval ocelli of insects is much like that of myriopods, and that the whole eye is constructed on the same plan as that of Peripatus and most molluscs.
Intestinal canal and glands.—The intestinal or digestive canal is primitively divided, as already stated on p. 299, into three sections, of which the anterior and posterior are called respectively the stomodæum and proctodæum, and are invaginations of the ectoderm, forming sacs whose blind ends face the future site of the mid-intestine. The fore-intestine (stomodæum) in most cases arises earlier than the proctodæum. Its muscles are derived from the mesoderm. From the stomodæum arises at an early date an unpaired dorsal invagination out of which develops the ganglion frontale and the pharyngeal nerve.
The absorption of the ends of the blind sacs of the fore and hind intestine, and opening up of the passage into the mid-intestine, occur rather early in embryonic life. In the wasps and bees, as well as the larva of the ant-lion, the mid-intestine remains closed at the end, not communicating with the proctodæum, which has an exclusively excretory function (Fig. 497).
The mid-intestine arises from two originally separate rudiments, i.e. the fore and hind endodermal rudiments, which at the outset stand in the most intimate relation with the invagination of the fore and hind intestine. Originating as a simple collection of cells, so closely adjoining these invaginations that Voeltzkow, Patten, and Graber derived them directly through outgrowths of them, they become extended by advancing cell-multiplication until they assume a U-shaped form. The legs of the U-shaped rudiment are in the anterior endodermal mass, directed backwards; those in the posterior mass, on the other hand, are directed anteriorly. These legs grow towards each other until they become fused together, forming two paired endodermal streaks, which pass under the primitive band along its whole length, and are fused with it at the fore and hind ends. In these places they stand in intimate union with the proctodeal and stomodeal invaginations.
The paired endodermal streaks belong to the lateral portions of the primitive band. As a rule, they lie directly under the row of cœlom-sacs (Fig. 539, F). The dorsal wall of the primitive segments stands consequently in intimate contact with the endodermal streaks. On this wall of the primitive segments an active cell-growth takes place, and the cell-material produced in this way, which separates from the dorsal wall of the primitive segments, forms the outer or splanchnic layer of the rudiment of the mid-intestine (spm, Figs. 539, F, 544, sp). What remains of the dorsal wall of the cœlom-sacs after this separation joins the genital rudiments and gives rise to the so-called terminal thread-plate (Fig. 544, ef). The endodermal streaks, with the splanchnic layer lying next to them, may now be considered as the rudiments of the mid-intestine (Fig. 530, m, etc.). These are noticeable in the following stages by their considerable lateral growth; they spread out over the upper surface of the yolk, around which they finally entirely grow (Figs. 539, C-F, 544, 545). This growth around the yolk goes on in most cases in such a way as to unite the two mid-intestinal streaks in the region of the ventral median line with each other. Then afterwards their union on the dorsal side takes place (Figs. 539, F, 545). The yolk thus passes completely into the interior of the mid-intestine, and with it the remains of the dorsal tube or dorsal organ, when such an one is present.