MESODERMAL DERIVATIVES 127 



lomic walls, they probably originate as channels between epithelial folds 

 continuous with the epithelial covering of the germ cells, which eventually 

 close to form tubes. 



The idea that the reproductive ducts of the arthropods are modified nephridial 

 tubes is purely theoretical, and there is nothing to suggest that they are not 

 canals of the mesoderm formed specifically for the conduction of the germ cells 

 to the outside of the body such as exist among some of the Annelida (Snod- 

 grass, 1933). 



The primary mesodermal oviducts of insects undoubtedly opened to 

 the exterior on the seventh abdominal segment, since in the embryos and 

 larvae of many modern insects they are attached to the ectoderm at the 

 posterior end of the venter of the seventh segment. The termination 

 of the mesodermal oviducts in the seventh segment is now known to occur 

 in all the larger orders and in some of the smaller ones including the 

 Thysanura. The mesodermal oviducts retain in the adult stage the 

 primitive position of their openings on the seventh venter or between 

 the seventh and eighth sterna onl}^ in the Ephemerida. The paired 

 female gonopores of the Ephemerida are in some species widely separated ; 

 in others they are approximated and open into a slight median depres- 

 sion. In the Dermaptera, however, the lateral oviducts open into a short 

 median ectodermal pouch between the seventh and eighth sterna. Many 

 insects in their embryonic and postembryonic development recapitulate 

 the primitive condition found in Dermaptera, in that the first rudiment 

 of the common oviduct appears as a median ingrowth on the posterior 

 part of the seventh venter with which the approximated lateral ducts 

 become united. 



The first rudiment of the median duct is formed in the late embryonic 

 or an early nymphal stage as an ectodermal invagination on the posterior 

 part of the venter of the seventh abdominal segment. In Acrididae and 

 Philaenus the primitive gonopore here located runs out in a median groove 

 on the eighth venter. Later the edges of this groove, beginning ante- 

 riorly, grow together and convert the channel into a cuticle-lined tube. 

 In this way the primary opening on the seventh segment is lost, as the 

 median duct is extended through the eighth segment and acquires its 

 definitive opening behind the sternal plate of this segment. 



A second invagination is formed in most insects at the posterior end 

 of the eighth abdominal segment which gives rise to the sperm receptacle. 

 Hence, with the posterior extension of the oviduct, the gonopore and 

 the spermathecal opening come to lie close together. In the majority 

 of insects these two openings are carried inward by the formation of a 

 copulatory pouch, or bursa copulatrix, invaginated above the eighth 

 sternum. The pouch finally may take the form of an elongate sac or 



