TR AC HE AT A. 345 



by indifferent embryonic cells'. Their protoplasm then fuses, and their 

 nuclei divide, and they give rise to the larval ovaries, for which the 

 enclosing cells form the tunics. 



In Aphis Metschiiikoff (No. 423) detected at a very early stage a mass 

 of cells which give rise to the generative organs. These cells are situated 

 at the hind end of the ventral plate ; and, except in the case of one of the 

 cells which gives rise by division to a green mass adjoining the fat body, 

 the protoplasm of the separate cells fuses into a syucytium. Towards the 

 close of embryonic life the syncytium assumes a horse-shoe form. The mass 

 is next divided into two, and the peripheral layer of each part gives rise 

 to the tunic, while from the hinder extremity of each part an at first solid 

 duct the egg-tube grows out. Tlie masses themselves form the ger- 

 mogens. The oviduct is formed by a coalescence of the ducts from each 

 germogen. 



Ganin derives the generative organs in Platygaster (vide p. 347) from 

 the hind end of the ventral plate close to the proctodseum j while Suckow 

 states that the generative organs are outgrowths of the proctodseum. 

 According to these two sets of observations the generative organs would 

 appear to have an epiblastic origin an origin which is not incompatible 

 with that from the pole cells. 



In Lepidoptera the genital organs are present in the later periods of 

 embryonic life as distinct paired organs, one on each side of the heart, in 

 the eighth postcephalic segment. They are elliptical bodies with a duct 

 passing off from the posterior end in the female or from the middle in the 

 male. The egg-tubes or seminal tubes are outgrowths of the elliptical 

 bodies. 



In other Insects the later stages in the development of the generative 

 organs closely resemble those in the Lepidoptera, and the organs are usually 

 distinctly visible in the later stages of embryonic life. 



It may probably be laid down, in spite of some of Metschnikoff's 

 observations above quoted, that the original generative mass gives rise to 

 both the true genital glands and their ducts. It appears also to be fairly 

 clear that the genital glands of both sexes have an identical origin. 



Special types of larvce. 



Certain of the Hymenopterous forms, which deposit their eggs in the 

 eggs or larvse of other Insects, present very peculiar modifications in their 

 development. Platygaster, which lays its egg in the larvae of Cecidomyia, 

 undergoes perhaps the most remarkable development amongst these forms. 

 It has been studied especially by Ganin (No. 410), from whom the following 

 account is taken. 



The very first stages are unfortunately but imperfectly known, and the 

 interpretations offered by Ganin do not in all cases appear quite satis- 

 factory. In the earliest stage after being laid the egg is enclosed in a 

 capsule produced into a stalk (fig. 190 A). In the interior of the egg 

 there soon appears a single spherical body, regarded by Ganin as a cell 

 (fig. 190 B). In the next stage three similar bodies appear in the vitellus, 

 no doubt derived from the first one (fig. 190 C). The central one presents 

 somewhat different characters to the two others, and, according to Ganin, 



1 This point requires further observation. 



