MESODERMAL DERIVATIVES 123 



about the germ cells dorsally and laterally, but ventrally they form 

 three thick cellular masses, which later give rise to the ovarian pedicles 

 and the lateral ducts of the ovaries. According to Lautenschlager the 

 follicle cells of the definitive egg tubes of Solenohia also take their origin 

 from the cells of the mesodermal sheaths of the gonads. 



In Pyrrhocoris apterus Seidel (1924) found that the segmental groups 

 of germ cells, formed at an early embryonic stage, become surrounded 

 individually by mesoderm epithelium and remain thus, up to a relatively 

 late period of development, as a series of distinctly separated gonadial 

 rudiments segmentally distributed on each side of the body in somites 

 two to eight of the abdomen. Later they all assemble in segments two 

 and three, where those of each lateral group develop directly into the 

 seven genital tubes of the adult organ. The lower tubular parts of the 

 primary elements unite to form the lateral duct. 



In the honeybee the genital ridges of the female at first extend along 

 the splanchnic walls of the mesodermal tubes from the second to the 

 seventh abdominal segments, inclusive. Later they become shorter 

 and thicker and finally reach only from near the anterior end of the fourth 

 abdominal segment into the anterior end of the seventh. 



Beyond the region of the germ cells the genital ridges are continued 

 posteriorly, but here they are much reduced in size and consist of simple 

 cellular strands. These parts of the ridges will form the mesodermal 

 parts of the lateral genital ducts. In many insects the primitive ducts 

 of the female in embryonic and larval stages turn downward posteriorly 

 and are attached to the ectoderm at the posterior end of the ventral wall 

 of the seventh abdominal segment or at the intersternal membrane. The 

 male ducts of the Orthoptera, as described by Wheeler and Heymons, 

 extend into the tenth segment and are here similarly attached to the 

 ectoderm. 



The primary paired gonads of insects are segmentally arranged, and 

 the continuous genital ridges later formed are secondary structures result- 

 ing from the fusion of the series of primary gonads on each side of the 

 body. It seems scarcely probable, therefore, that the subsequent division 

 of the genital ridges into a series of ovarial or testicular tubes can repre- 

 sent a primitive segmental structure of the reproductive organs, though in 

 a few adult apterygote insects and in the embryo of Pyrrhocoris apterus 

 the tubes do coincide with the abdominal segments. 



The early segregation of the germ cells in Blattella into groups within 

 the dorsal parts of the intercoelomic septa, says Snodgrass, is highly 

 suggestive of a similar arrangement of the germ cells in many adult 

 Annelida in which the gonads are simple swellings of the dissepiments, 

 retaining the germ cells beneath a thin mesodermal epithelium. In the 

 annelids, however, the gonads may occur on almost any part of the 



