542 



INSECTA. 



Dr 



some of the (Estridce and of the Pupipara, are viviparous. As a 

 rule, the eggs are laid shortly after fertilization, and before the 

 commencement of the development of the embryo. In rare cases 

 the embryo is already formed when the egg is laid. In the last case 

 the segmentation and formation of the embryo take place in the vagina 

 (fig. 451). The fertilization of the egg usually takes place during 

 its passage through the oviduct, at the place where the receptaculum 

 seminis opens. Since the eggs become invested with their resistant 

 chorion in the ovarian tubes, from the epithelial cells of which they 

 originate for the most part during the larval life, it is necessary 

 that there should be special arrangements which render possible the 

 entry of the spermatozoa and the fertilization of the ovum. For this 

 object there exist on the upper pole of the egg (the pole turned 

 towards the egg-tubes during the passage of the egg) one or more 



pores known as micropyles* 

 which pierce the chorion 

 and present a characteristic 

 form and arrangement (fig. 

 452). 



The ova originate in the 

 narrow terminal portion of 

 the egg-tubes, which is 

 often prolonged into a thin 

 thread. Here the growth 

 of the egg-tube takes place, 

 as well as the differentiation 

 of its contents into egg cells and ovarian epithelium. The ovarian 

 tubes increase continuously in diameter towards the oviduct, in 

 correspondence with the gradual increase of size undergone by the 

 eggs, which are arranged one behind another in its lumen. Each 

 egg occupies a chamber, and obtains an external resistant mem- 

 brane (chorion), which is secreted by the epithelium which lines the 

 chamber. The chorion shows in its external markings the pecu- 

 liarities of the epithelium from which it was formed. 



Besides this type, which is found in Pulex and in many of the 

 Neuroptera and Ortlwptera, there is a second type of ovarian tube, 

 distinguished from the first by a more complicated structure of the 

 ovarian chambers. The lumen of such egg-tubes encloses above the 



.FiG. 451. Female generative organs of the viviparous 

 Melophagus ovinus (Pupipara) (after R. Leuckart). 

 Oc, Egg in the ovarian tube of one side; lit, 

 uterus ; Dr, the glands opening into the uterus ; 

 Va, vagina. 



* Compare R. Leuckart, " Ueber die Mikropyle und den feineren Bau der 

 Schalenhaut bei den Insecten." Miiller's Archie., 1855. 



