334 PHYL UM ARTHROPODA. 



saw-flies and gall-wasps. It occurs casually in the silk-moth and several 

 other Lepidoptera, seasonally in aphides, in larval life in some midges 

 {Miaslor, Chirouonnts}, and partially or "voluntarily" when the queen- 

 bee lays eggs which become drones. Parthenogenetic ova (in water- 

 fleas, Rotifers, etc.) usually form only one polar body; the egg which 

 becomes a drone forms two as usual. 



A few insects hatch their young within the body, or are " viviparous." 

 This is the case with parthenogenetic summer aphides, a few flies, the 

 little bee parasites Strepsiptera, and a few beetles. 



Development of the ovum. The tubes which compose 

 the ovaries and lead into the oviducts begin as thin fila- 

 ments, the ends of which are usually connected on each 

 side. These thin filaments consist of indifferent germinal 

 cells, all of them potential ova, and of mesodermic epithelial 

 cells, which form the ovarian tubes, etc., and are connected 

 anteriorly to the pericardial wall. 



But in most cases only a minority of these cells become 

 ova, the others become nutritive cells which are absorbed 

 by the ova, and follicle cells which line the walls of the 

 ovarian tubes and help to furnish the egg-shells. 



There may be, indeed, ovarian tubes without nutritive 

 cells (e.g. in Orthoptera), and then each tube is simply a 

 bead-like row of ova, which become larger and larger as 

 they recede from the thin terminal filaments and approach 

 the oviducts. In other cases the bead-like row consists 

 of ova alternating with clumps of nutritive cells (e.g. in 

 Hymenoptera and Lepidoptera). In other cases the nutri- 

 tive cells mostly remain in the terminal region, but their 

 products pass down to the receding ova. 



As there are numerous ovarian tubes in each ovary, 

 and as the same process of oogenesis is going on in each, 

 numerous eggs are ready for liberation at the same time, 

 and are simultaneously discharged into the oviduct of each 

 side. 



The eggs are large and contain much yolk. In relatively 

 few cases yolk is almost absent, as, for example, in the 

 suaimer eggs of the Aphides, which are hatched within the 

 body, and in some forms where the young are endoparasitic. 

 The ovum is surrounded by a vitelline membrane, and also 

 by a firm chitinous shell, secreted by the follicular cells, 

 which is often sculptured in a characteristic manner. This 

 shell is pierced by one or more minute holes (micropyles). 



