GENERATIVE ORGANS OF ARTHROPOD A. 



301 



formed in these terminal filaments, the cell-masses of which represent 

 ovarian germs; these, while undergoing continual differentiations, 

 gradually make their way out 

 of the ovarian tube. The ovum 

 is a true cell at the place where 

 it is formed, but on its way 

 through the ovarian tube it in- 

 creases in size, so that we find 



the largest 



po'O'g 



farthest from 



the germinal region, and nearest 

 the oviduct, while behind them 

 there is a continual series of 

 smaller and younger formations 

 up to the above-mentioned blind 

 end of the ovarian tube. The 

 separate eggs cause the ovarian 

 tube to appear to be divided 

 into segments or chambers. The 

 gradual descent of the egg is not 

 only correlated with its growth, 

 but with various changes also in 

 the substance of the yolk ; each 

 egg is provided, especially in the 

 last segment of the tube, with 

 an external cuticular investment, 

 the so-called chorion ; this is 

 formed by the epithelial layer of 

 the ovarian tube. 



As each egg passes into the 

 so-called oviduct, a portion of the 

 ovarian tube is degenerated, and 

 so the egg next in front is 

 brought close to the oviduct. 

 The differentiation of the egg is 

 accompanied by the growth of 

 the thin end of the ovarian tube, 

 which is made up for by its 

 shortening at the other end. In 

 many Insects a group of cells is 



differentiated with each egg, in addition to the epithelial layer sur- 

 rounding it; this vitellogenous layer occupies the portion (b) of 

 the chamber (a) behind the egg-cell (Fig. 158, B a), but is gradually 

 used up by the latter, as it grows. An ovarian tube, or a collec- 

 tion of such tubes, does not therefore correspond merely to a germ- 

 producing reproductive gland, but is an organ entrusted with a 

 much larger series of functions, and its blind end only is analogous 

 to an ovary. 



The length of the ovarian tube depends on the number of eggn 

 in it. The smallest number of chambers is found in most of the 



Fig. 158. A Ovarian tube of the Flea. 

 o Ovum, fj Germinal vesicle. B Ovarian 

 tube of a Beetle (Carabus violaceus). 

 o Ovarian segment, divided into two por- 

 tions, of which the ovarian cell is marked 

 a, and the vitellogenous layer b. The 

 ovum of the last segment has been ex- 

 polled ; the walls of the ovarian tube are 

 collapsed (after Lubbock). 



