38 



INSECTA. 



The terminal threads are fine prolongations of the ends of the egg-tubes 

 usually continued close up to the heart. At their extremities they fre- 

 quently anastomose, or even unite into a common thread. In some cases 

 they are absent. They form either direct continuations of the germogen 

 and have the same histological structure, or in other cases are simply pro- 

 longations of the tunica propria, and serve as ligaments. 



The germogen usually consists of 

 two parts : an upper, filled with nuclei 

 imbedded in protoplasm, and a lower, 

 in which distinct cells have become 

 differentiated. 



The lower part of the egg-tubes 

 is filled with ova which advance in 

 development towards the oviduct, and 

 lie in chambers more or less distinctly 

 constricted from each other. In these 

 chambers there are in most forms in 

 addition to the true ova a certain num- 

 ber of nutritive cells. The true egg- 

 tubes are moreover lined by an epithe- 

 lial layer which passes in and forms 

 more or less complete septa between 

 the successive chambers. The points 

 which have been especially controvert- 

 ed are (1) the relation of the ovum 

 to the germogen, and (2) the relation 

 of the nutritive or yolk cells to the 

 ovum. To the controversies on these 

 points it will only be possible to give 

 a passing allusion. 



As has been already hinted there 

 are two distinct types of ovaries, viz. 

 those without the so-called nutritive 

 or yolk cells and those with them 1 . 



The formation of the ovum is most 

 simple in the type without yolk-cells, 

 which will for that reason be first con- 

 sidered (fig. 17 A). 



The germogen is constituted of a 

 number of nuclei imbedded in a scanty 

 cementing protoplasm. In the lower 

 part of the germogen the nuclei are larger, and become separated 

 off from the nucleated protoplasm above, as distinct cells with a 

 thin layer of protoplasm round the germinal vesicle. These cells 

 are the ova. As they pass down the egg-tube their protoplasm in- 

 creases in bulk, and they become isolated by ingrowths of the epithelial 



FIG. 17. A. OVARIAN TUBE OF THE 

 FLEA, PULEX IRRITANS. (From 

 Gegenbaur). 



o. ovum. g. germinal vesicle. 

 B. OVARIAN TUBE OF A BEETLE, 

 CARABUS VIOLACEUS. (After Lub- 

 bock.) 



o. ovarian segment, formed of an 

 ovum a, and a mass of yolk-cells, b. 



1 For a list of the genera \vith and without nutritive cells, vide Brandt, pp. 47 and 48. 



