660 THE GENERATIVE ORGANS. 



1. DESCRIPTIVE ANATOMY OF THE INTERNAL GENERATIVE 

 ORGANS OF THE MALE BLOW-FLY. 



The internal generative organs of the male Blow-fly are 

 (i) a single pair of testes and their vasa efferentia ; (2) the para- 

 gonia, usually termed vesiculae seminales ; (3) the vas deferens ; 

 (.4) the ejaculatory sac; and (5) the ejaculatory duct. 



These parts are found very generally in male insects, but those 

 in which the penis is absent have usually no ejaculatory sac or 

 duct, and the vas deferens opens on the surface between a pair 

 of more or less complex genital appendages. In some larvae, 

 Ephemeridse (Palmen), the external openings are paired, so that 

 the vasa efferentia (segmental ducts) open directly on the 

 surface. 



The Testes are a pair of simple follicles, situated one on either 

 side of the median line in the dorsal part of the fourth segment 

 of the abdomen. They are pyriform, and often exhibit an 

 hour-glass constriction between the fundus and duct of the 

 gland. They are of a pale orange colour externally, due to the 

 pigmented epithelium which forms the wall of the follicle. In 

 the adult insect the testes consist of an epithelial capsule filled 

 with semen. It measures 75 mm. to '8 mm. in its long dia- 

 meter, and '5 mm. in its transverse diameter. The cells of the 

 capsule are so thin that they are readily mistaken in sections 

 for a cuticular layer ; but in surface views their outlines are 

 very distinct. In fresh preparations the capsule is readily torn 

 into fragments by moving the cover-glass, when if any cuticular 

 layer existed it could not fail to be apparent. 



These cells are hexagonal, they have an average diameter of 

 02 mm., and are pigmented with orange-brown granular pig- 

 iiH-nt they have large oval nuclei. 



Externally to the epithelium just described, the testis is 

 covered by a thick layer of small, usually multi-nucleated fat- 

 cdls, precisely similar to those which separate the ovary from 

 the abdominal wall in the female insect. Each nucleus in 

 these cells is surrounded by a stellate protoplasmic mass, the 



