NO. 14 INSECT ABDOMEN SNODGRASS 9 



each other in a continuous tube (E, G). The two tubes thus formed 

 become the definitive gonads. The germaria are now parts of the 

 epithehal walls of the gonadial tubes ; consequently the germ cells 

 given off from the germaria are imprisoned in the gonads, and must 

 be given some special means of escape to the exterior. 



The genital exits of the Onychophora are formed in the last of the 

 series of genital segments, which is the penultimate body somite. In 

 this somite the coelomic sacs are retained entire (fig. 3 B) ; though 

 they become much narrowed, they are not divided into dorsal and 

 lateral compartments as in the preceding somites (fig. 2C). Their 

 dorsal ends unite with the posterior ends of the gonadial tubes (fig. 

 2 E), and there is thus established a pair of exit passages from the 

 gonads opening externally through the primitive coelomoducts (d) 

 of the segment. In other words, a relatively generalized condition is 

 retained in the last genital segment, which allows the gametes to be 

 discharged in the primitive manner through the coelomic sacs and 

 their outlet ducts. The open passages take on a tubular form and 

 become the definitive lateral gonoducts. Eventually the lateral ducts 

 (fig. 3 C, Gdl) come together medially on the ventral surface of the 

 last genital segment, and those of the male open into an ectodermal 

 gonoductus communis (Gdc), or ejaculatory duct (fig. 6 C, D, Dej). 



Among the Arthropoda a direct development of the gonads and 

 gonoducts from coelomic sacs in a manner similar to the development 

 of these organs in the Onychophora has been described by Heymons 

 (1901) in Scolopendra. The dorsal parts of the coelomic sacs of the 

 centipede embryo persist as a double series of closed chambers, which 

 unite on each side, thus forming two long tubes with segmental com- 

 partments, in the epithelial walls of which are contained the germ 

 cells. Later the dissepiments disappear, giving each gonadial tube a 

 continuous lumen, and finally the two tubes combine to form the single 

 definitive gonad. The gonoducts of Scolopendra are derived from the 

 coelomic sacs of the greatly reduced last two somites. Each of these 

 sacs gives off a small ventral diverticulum, or ampulla (fig. 3 D, 

 Amp), into the corresponding rudimentary appendage of its segment 

 {2Gp). The two sacs on each side then combine in a single vessel, 

 and the sacs of the first pair join with each other above the alimentary 

 canal and unite here with the posterior end of the gonadial tube. The 

 ampullae of the second pair of sacs now withdraw from the appen- 

 dages and migrate toward the median line on the ventral body surface, 

 where finally they open into a median ectodermal invagination formed 

 behind the sternum of the small last somite. The passages thus estab- 

 lished from the gonadial tube to the exterior constitute the gono- 



