O SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 89 



11. MORPHOLOGY OF THE GONADS AND THE GENITAL DUCTS 



The embryonic genital ducts of insects are simple mesodermal tubes 

 continuous with the mesodermal walls of the gonads. The primitive 

 ducts had separate openings to the exterior of the body, the apertures 

 in the male being located probably on the tenth abdominal segment, 

 those of the female on the seventh. In the majority of modern insects, 

 however, a median ectodermal exit apparatus, together usually with 

 accessory structures, has been added to the primitive ducts, and in 

 many cases the latter even are largely replaced by ingrowths from 

 the ectodermal parts. The definitive median exit duct of the male 

 (except possibly in Collembola) opens in the ventral membrane 

 between the ninth and tenth abdominal segments, which membrane 

 probably belongs to the ninth primary somite. The definitive median 

 egg passage of the female varies in its extent and point of opening 

 from the posterior part of the seventh abdominal segment to the 

 posterior part of the ninth, or it may open in common with the rectum 

 into a small cloacal pouch on a terminal segment of the abdomen rep- 

 resenting the combined ninth and tenth somites. 



The gonads are mesodermal structures developed in the dorsal parts 

 of the splanchnopleure as ridges of the latter extending continuously 

 through several abdominal segments to enclose the germ cells here 

 finally located. The gonadial ducts arise from ridges of cells ex- 

 tending posteriorly from the gonads. The subsequent compound struc- 

 ture of the gonads results from a secondary subdivision of each organ 

 into a series or group of egg tubes or sperm tubes. There is no 

 evidence from ontogeny that these gonadial tubes ever had separate 

 openings or ducts to the exterior, and yet much theoretical speculation 

 has been based on the idea that the component tubes of the ovaries 

 and testes represent primitively segmental organs. The following 

 discussion of the subject, therefore, may not be superfluous as an in- 

 troduction to a study of the appendicular structures associated with 

 the definitive genital apertures. 



Early history of the germ cells. — In many insects, representing 

 widely separated groups, the germ cells and the somatic cells are dif- 

 ferentiated at the time of cleavage ; and in such cases the germ cells 

 are distinguishable when the blastoderm is formed as a group of special 

 cells at the posterior end of the germ band. From this position the 

 germ cells later migrate into the interior of the embryo, and are 

 eventually associated with the dorsal part of the splanchnic wall of the 

 mesoderm in the abdominal region of the body. Since it is not possible 

 in all cases to recognize the germ cells as such prior to their enclosure 

 in the abdominal mesoderm, some of the earlier, otherwise reliable 



