41 6 INSECTA. 



mass of yolk external to the mesenteron takes the form of a median and two 

 lateral streaks. Some of the yolk cells either prior to the establishment of 

 the mesenteron, or derived from the unenclosed portions of the yolk, pass 

 into the developing organs (Dohrn, 408) and serve as a kind of nutritive cell. 

 They also form blood corpuscles and connective-tissue elements. Such yolk 

 cells may be compared to the peculiar bodies described by Reichenbach in 

 Astacus, which form the secondary mesoblast. Similar cells play a very 

 important part in the development of Spiders. 



Generative organs. The observations on the development of the 

 generative organs are somewhat scanty. In Diptera certain cells known 

 as the pole cells are stated by both Metschnikoff (No. 423) and Leuckart to 

 give rise to the generative organs. The cells in question (in Chironomus 

 and Musca vomitoria, Weismann, No. 430) appear at the hinder end of the 

 ovum before any other cells of the blastoderm. They soon separate from 

 the blastoderm and increase by division. In the embryo, produced by the 

 viviparous larva of Cecidomyia, there is at first a single pole cell, which 

 eventually divides into four, and the resulting cells become enclosed within 

 the blastoderm. They next divide into two masses, which are stated by 

 Metschnikoff (No. 423) to become surrounded by indifferent embryonic cells 1 . 

 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 Metschnikoff (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 syncytium. 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. The 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 proctodaeum ; while Suckow 

 states that the generative organs are outgrowths of the proctodicum. 

 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. 



1 This point requires further observation. 



