TRACHEATA. 



417 



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 larva. 



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

 eggs or larvte 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, 

 gives rise to the whole embryo. The two peripheral bodies increase by 

 division, and soon ap- 

 pear as nuclei imbed- 

 ded in a layer of pro- 

 toplasm (fig. 190 D, 

 E, F). The layer so 

 formed serves as a 

 covering for the em- 

 bryo, regarded by 

 Ganin as equivalent 

 to the amnion (? se- 

 rous membrane) of 

 other Insect em- 

 bryos. In the em- 

 bryo cell new cells 

 are stated to be 

 formed by a process 

 of endogenous cell formation (fig. 190 D, E). It appears probable that 

 Ganin has mistaken nuclei for cells in the earlier stages, and that a blasto- 

 derm is formed as in other Insects, and that this becomes divided in a way 

 not explained into a superficial layer which gives rise to the serous 

 envelope, and a deeper layer which forms the embryo. However this 



B. II. 27 



FlG. 190. A SERIES OF STAGES IN THE DEVELOPMENT 



OF PLATYGASTER. (From Lubbock ; after Ganin.) 



