DEVELOPMENT OF THE OVUM. 319 



Development of the Ovum. 



The tubes which compose the ovaries and lead into the 

 oviducts begin as thin filaments, the ends of which are 

 usually connected on each side. Those thin filaments 

 consist of indifferent germinal cells, all of them potential 

 ova, and of mesodermic epithelial cells, which form the 

 ovarian tubes, &c., 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 sum- 

 mer 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 (mtcropyles). 

 Through a micropyle the spermatozoon finds entrance, 

 sometimes (as in the cockroach) after moving round and 

 round the shell in varying orbits. 



Development. 



The ripe egg usually consists of a central yolk-containing mass, sur- 

 rounded by a thin sheath of protoplasm. As is usual for Arthropods, 



