MYRIAPODA 403 



to become the definitive genital duct. The left duct, on the other hand, 

 is stretched over the dorsal side of the alimentary canal, forming a 

 slender transverse arch. 



The connection between the mesodermal and ectodermal parts of the 

 reproductive organs is established during postembryonic life. After the 

 rupture of the chorion the pregenital and the genital segments telescope 

 more or less into the preceding rump segments of the young fetus. The 

 sternite of the twenty-third segment (genital segment) becomes modified 

 into two small lateral processes which may be called the "genital lobes." 

 The two-lobed genital sternite in the female is fully telescoped into the 

 body so that between it and the sternite of the twenty-second segment 

 (pregenital segment) a deep chamber, or pocket, is formed which Hey- 

 mons designated as the " genital atrium." It is lined with the ectodermal 

 conjunctiva of the two sternites. In the male a similar invagination 

 occurs, but here the lobes of the genital sternite are longer and more 

 slender and not wholly retracted into the atrium. Consequently the 

 atrium is smaller. Furthermore, in the male a small median invagination 

 is formed between the lobes of the genital sternite which will develop 

 into the ejaculatory duct. At this time, also, two pairs of ectodermal 

 glands are formed in both sexes. In the female the anterior pair of gland 

 rudiments retracted into the atrium will form the receptacula seminis 

 (spermatheca) ; the posterior pair which opens between the lobes of the 

 genital sternite will form the accessory glands. Homologous glands are 

 also developed in the male. 



A free communication between the mesodermal genital ducts and the 

 atrium is not established until the free-living stage is reached. In the 

 female the two ampullae of the twenty-third segment (genital segment), 

 which mark the extremities of the two future genital ducts (left one non- 

 functional) and which lie in contact with the blind anterior end of the 

 atrium, fuse below the alimentary canal into a single chamber, and soon 

 thereafter the union of atrium and ampullae takes place. The ampullae 

 of the pregenital segment at this time gradually contract, shorten, and 

 soon are no longer apparent. Finallj^ a cuticula lines the entire atrium 

 and covers the lobes of the genital sternite. 



In the male the development follows a similar course, except that the 

 posterior pair of ampullae, instead of uniting with the atrium, fuse with 

 each other and then unite with the invagination which is to form the 

 ejaculatory duct. As with the female, the left primary genital duct 

 becomes nonfunctional; but whereas in the female it becomes a slender 

 duct, in the male it is more or less distended, sac-like. This sac has 

 formerly been considered the seminal vesicle, but Heymons states that it 

 does not contain spermatozoa. The paired lobes derived from the 

 twenty-third sternite doubtless represent the intromittent organ. 



