NO. O INSECT ABDOMEN SNODGRASS I3 



close over the germ cells and afford them a protected coelomic space 

 in which to complete their development. A similar mesodermal en- 

 velope surrounds the gonads in some of the Annelida, and may be 

 continuous with an exit duct. On the other hand, if the condition 

 described by Wheeler in Conocephalus is primitive, it would appear 

 that the segmental clusters of germ cells in the splanchnic mesoderm 

 are the primary gonads ; and even in Blattella Heymons observes that 

 some of the germ cells remain within the intercoelomic septa, where 

 they become later a part of the continuous series of germ cells con- 

 tained in the genital ridges. In most other insects in which the origin 

 of the reproductive organs has been studied, it is found that the gonads 

 are formed as mesodermal ridges which from the beginning contain 

 the germ cells. 



By whatever method the germ cells may become aggregated in the 

 coelomic walls, it is clear, at least, that the primary paired gonads of 

 insects are segmentally arranged, and that the continuous genital 

 ridges later formed are secondary structures resulting from the fusion 

 of the series of primitive gonads on each side of the body. It would 

 seem scarcely probable, therefore, that the subsequent division of the 

 genital ridges into a series of ovarial or testicular tubes can represent 

 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 abdominal segments. In some of the other 

 Apterygota, and in Protura, the mature gonad is a single, elongate 

 sac continuous posteriorly with the duct, as it is also in most other 

 members of the Arthropoda. In such cases, however, the gonadial sac 

 has the structure of a single secondary tube and not that of a primi- 

 tive genital ridge. 



Origin of the gonadial tubes. — The definitive ovaries and testes of 

 the majority of insects consist each of a series or group of diverticula 

 arising from the common lateral duct (fig. 3G). The ovarian tubes 

 are free and end in terminal filaments, which are generally united in 

 a cluster at their distal ends. In the male the testicular tubes usually 

 lack terminal filaments, and those of each organ are generally bound 

 together in an investing sheath ; otherwise they do not differ from 

 the egg tubes of the female. 



The differentiation of the secondary continuous genital ridges into 

 a tertiary series of tubes has been minutely followed by Heymons 

 (1892) in the female of Blattella, and most other accounts of the 

 development of the reproductive organs show that the process is es- 

 sentially the same in both the female and the male of insects having 

 compound gonads. By a multiplication of the cells along the bases 



