396 Introduction to Animal lilorphology. 



{ovipositors), in the forms of piercers or saws, &c. The 

 ovipositor and penis are both altered metameres. 



In the ovary arise two series of products. The 

 terminal cells of the ovarian tubes produce the ova 

 proper, but in the epithelial wall of the tube lower 

 down is secreted a yelk mass for the nutrition of the 

 germ. 



Insects' eggs have an outer, often sculptured, shell, 

 which has usually a micropyle at one pole. In some 

 Diptera the eggs are retained and hatched in a caecal 

 process from the vagina. In Melophagus the coUeterial 

 glands secrete a fluid whereby the young are nourished 

 in the larval stage. 



True or pseudo-parthenogenesis (p. 36) occurs in 

 many insects. Metagenesis has been described in 

 Cecidomyia, where a larva produces embryos : but as 

 there is a peculiar pseudovarum present, though with 

 no oviduct, this is probably a form of pseudoparthe- 

 nogenesis. 



In the developing ^^%, after the partial (rarely 

 complete, Poduridae) fission of the yelk, a germinal 

 membrane forms usually around the yelk, with a pri- 

 mitive ventral streak, in which the abdominal and 

 head segments begin to appear. Eggs are thus 

 usually ectoblastic, but some seem to have a periphery 

 of yelk, and be endoblastic. The germ is inclosed in 

 a cellular amnion (chitinoid, structureless in Poduridae, 

 Packard). Finally, the germ breaks the shell, some- 

 times by the help of provisional organs (Pentatoma, 

 Phryganea, Mantispa). The young thus produced is 

 a larva, and this may only differ from the perfect 

 adult in size, and in the number of corneal facets, or 

 else the larva may, by developing wings and a more 



