The urinary tubes.—These excretory vessels arise as paired evaginations of the hind intestine or proctodæum. They are ectodermal structures arising as lateral diverticula of the intestinal cavity (Fig. 546). Figure 547 represents their mode of origin at the anterior end of the proctodæum of a locust. It will be seen that there are 10 primary tubes. There are 150 such tubes in locusts, or 10 groups of 15 each. The 15 secondary tubes probably arise from the primary ones in the manner described by Hatschek for Lepidoptera (see his Taf. III, Fig. 7).
While the Malpighian tubes usually first arise as diverticula of the proctodæum, in the Hymenoptera (Apis and Chalicodoma) they appear, even before the completion of the proctodæum, as invaginations of the ectoderm which at first open out on the outer surface of the primitive band. They seem, then, in some degree, to be similar to the tracheal rudiments, which perhaps is the reason why they have been homologized with them, a view which we do not share, and in which Carrière does not concur. They afterwards pass, with the growing proctodæum, into the interior of the embryo. (Korschelt and Heider.)
The heart.—The dorsal vessel is first indicated, according to Korotneff, by a long string or row of cells (cardioblasts), which on each side border the mesodermal layer of the primitive band (Figs. 544, h, 548, h). In the advancing growth of the primitive band around the yolk, this rudiment steadily passes up more towards the dorsal side. It is in connection with the wall of the primitive segment (Figs. 544 and 548), and represents the point at which the dorsal wall of the cœlom-sac passes into the lateral wall. According to Korotneff, the cardioblasts arise directly through a migration out from the wall of the primitive segment.
In Gryllotalpa the formation of the dorsal organ, which, as Korotneff states, is in this insect nothing else than a stopper which fills up the dorsal gap of the body-wall of the embryo, is effected by the rupture of the embryonal membranes. The serosa is drawn together to form a thick plate (Fig. 523, A, rp), and the much degenerated amnion-folds (am) which are laterally attached to it have moved from the edges of the primitive streak (*x-*y) far towards the dorsal side (see Fig. 539, C, which represents a similar stage). The distance between the rudiment of the amnion-fold and the lateral edge of the primitive band (*x, *y) is occupied by an epithelial lamella (l), in which we recognize the earlier amnion. This lamella does not lie directly on the yolk, but is separated from it by a spacious blood-lacuna (A, bs), in which can be seen numerous blood-corpuscles which have migrated in from the mesoderm of the primitive band. The cardioblasts which have arisen from the wall of the primitive segment (us) are on each side arranged into the form of a furrow (gr), which bounds the blood sinus below.
Fig. 548.—Cross-section through the abdominal part of an older primitive band of P. germanica when beginning to grow around the yolk: vm, ventral longitudinal muscle; other lettering as in Fig. 545.—After Heymons, from Korschelt and Heider.
By the continuous growth of the primitive band around the yolk, after the resulting invagination and degeneration of the dorsal plate, the two blood-lacunæ unite together on the dorsal side into a single one (B, bs). These constitute the first cavity of the heart. The vascular furrows (gr) come in contact with each other and grow together, and the wall of the heart is thus formed. Ayers states that in Œcanthus the heart is formed in the head region only after the yolk-sac has passed entirely within the body. The venous ostia arise by two paired invaginations of the lateral walls, forming a split at their bottom.
The rudiment of the heart stands, as we have seen, in intimate union with the primitive segments. Out of the lateral walls of these segments, after giving off the elements of the somatic mesoderm, arises an epithelial plate which becomes the rudiment of the pericardial septum or dorsal diaphragm (Figs. 523, A-C, dd, 544–545, ps). As soon as the two halves of the rudiments of the heart have united with each other in the dorsal middle line, the two halves of the pericardial septum unite with each other and form the wall to the pericardial cavity and shut it off from the rest of the body-cavity. For a long time the pericardial septum remains in union with the wall of the heart. Afterwards, however, it separates from it (Fig. 523, C, dd). (Korschelt and Heider.)
The statements of other authors (Ayers, Grassi, Patten, Tichomeroff, Carrière, Heider, Heymons, etc.) as to the mode of origin of the heart in insects of other orders are all similar to the type described in Gryllotalpa. The difference consists mostly in the fact that the two large blood-lacunæ are wanting or only exist to a slight extent. It results that the rudiment of the cavity of the heart in the earlier stages is of slight extent and often scarcely recognizable.
In Œcanthus (Ayers) and in Gryllotalpa, the hinder section of the heart is the first to develop, the development advancing from behind forward.