Fig. 551.—First developmental stages of the parthenogenetic eggs of the larva of Cecidomyia: b, peripheral protoplasmic layer (Keimhautblastem); bl, blastoderm; d, central yolk; f, division-nuclei; n, nutritive cell (“corpus luteum”) about to break up; pz, polar cells.—After Metschnikoff, from Korschelt and Heider.
In the Hymenoptera Ganin has observed in the embryo of Platygaster the rudiments of the sexual glands in the form of two rounded masses situated near the posterior intestine and apparently derived from the same blastems or buds as the latter.
Uljanin studied these organs in the larva of the honey-bee. They are two reniform bodies in the middle of which will soon appear the ovarian tubes. They also give birth to the internal parts of the excretory ducts, while the external part of the genital tube, as also the accessory glands which are connected with it, are derived by an invagination of the hypodermis at the surface of the penultimate segment.
Dohrn observed in the larva of ants the rudiments of the ovaries in the form of two pyriform masses, each with eight prolongations which he regarded as young ovarian tubes.
Fig. 552.—Three longitudinal sections through the embryo of Chironomus. In A, the blastoderm (bl) is beginning to form, the polar cells (p) outside of it; in B, the polar cells have pressed in between the blastoderm cells; in C, they lie in the interior of the embryo: b, protoplasmic layer (Keimhautblast); d, yolk; k, nucleus of the forming blastoderm.—After Ritter, from Korschelt and Heider.
In Encyrtus Bugnion observed the rudiments of the sexual glands in the middle of the larval period; they were rounded and with no apparent connection with the neighboring organs. Afterwards these rudiments elongated, approached nearer to the ventral surface, and placed themselves in relation with some small cell-groups which appeared under the rectum, and seemed destined to form the efferent canal (vas deferens) and accessory glands of the genital organs. He thought the sex could be recognized in the second half of larval life, the male gland being distinguished by its rounded shape and smaller size; the ovary by its oval form and larger size. In larvæ ready to be transformed the testis formed a cellular mass enveloped by a cuticle, and at its hinder end prolonged into an epithelial cord, which is undoubtedly the vas deferens. The ovary had a similar envelope, and from its cellular mass arose epithelial cords which were destined to become the ovarian tubes.
m. Length of embryonic life
The duration of embryonic life varies greatly in different insects. The embryo of the blow-fly is fully developed in less than 24 hours, that of the house-fly in 24 hours. In the locusts and tree-cricket the embryos begin to develop at the end of the summer, continuing to grow until the cool weather of autumn, when growth is arrested, the later stages being finished in the latter part of the spring. It is so, likewise, with the embryos of many moths and other insects.