580 TEXT-BOOK OF ENTOMOLOGY 



On reviewing the facts as to the origin of the sexual organs, as in Phyllo- 

 dromia, 1 as just described, it will be seen that they afford proof that in the 

 derivation of the genital cells from the epithelial cells of the ccelom-sacs, there 

 is a direct agreement with the annelids. In the later development of the 

 paired genital glands, and of an efferent passage standing in direct union with 

 the glands themselves, there is a certain agreement with the conditions in 

 reripatus. In the first place, the dorsal position of the genital glands is the 

 same in the two groups. On the other hand, the genital glands of Peripatus, 

 according to Sedgwick, are formed by direct fusion of the successive coelom- 

 sacs (and a similar point of view has been taken by Heathcote for the my no- 

 pods), hence it results that in Peripatus the genital cavities arise out of the 

 coalom-cavities. In the insects, on the other hand, the genital rudiment lies, 

 to be sure, in the wall of the ccelom-sac, but the genital cavity (lumen of the 

 oviducts) in them arises separately from the coelom-sacs, while the coelom-cavi- 

 ties finally become a small part of the definite body-cavity. We must consider 

 the conditions in Peripatus and the myriopods as the more primitive, directly 

 pointing to the annelids ; on the other hand, those of the insects as derived 

 and secondary. 



If we attempt to homologize the sexual efferent passages of insects with those 

 of Peripatus, we are compelled to refer them to a modified pair of nephridia, 

 and the origin of the latter (Peripatus) from the mesoderm agrees with that 

 of insects. In general, however, in the development of the sexual outlets of 

 insects, there are no characters which can be regarded as favorable to such a 

 view. We must here accept the fact that the mode of development is secondary. 



Mention should be specially made of the fact we owe to Heymons, that in the 

 genital rudiment of Phyllodromia the genital cells and epithelial cells can be 

 distinguished from each other from the very beginning. This fact does not 

 favor the generally accepted view that the follicle-cells and egg-cells arise 

 through a later differentiation from one and the same kind of cell. From their 

 first origin, indeed, in Phyllodromia, both kinds of cells may be referred to the 

 same source. 



The mode of origin of the genital rudiments in Diptera and Aphides deserve 

 special mention. In these groups the sexual germs are present in very early 

 stages of life. This certainly in part is the result of the parthenogenetic and 

 psedogenetic mode of reproduction in the two groups, which leads to an early 

 differentiation of the sexual germs. 



In the Diptera the first germs of the genital glands are represented by the 

 polar cells (Fig. 551, pz). In the asexual developing eggs of the oviparous 

 Cecidomyia larva, before the formation of the blastoderm, there separates from 

 the hinder pole (Z>) a rather large cell rich in granules, which soon divides into 

 two and afterwards four polar cells. After the completion of the blastoderm 

 these polar cells then pass in among the blastoderm cells (G) and into the 

 interior of the embryo, where they are in later stages symmetrically arranged 

 in two groups, and, enveloped by the cells of surrounding tissues, transformed 

 into the genital rudiments. (Metschnikoff.) 



In Chironomus (Fig. 552, p), according to Balbiani, two polar cells almost 

 simultaneously separate from the hinder pole of the egg, which, by division, 

 form a group of four and eight cells. Exactly as in the case in Cecidomyia, 

 these cells are taken within the embryo, where they lie divided into two groups 

 on each side of the proctodaeum. In all the young, freshly hatched larvae these 



1 The description perhaps applies not only to the cockroaches, but, as seen from 

 the similar but fragmentary notices of Heider and of Wheeler on the Coleoptera, 

 may be common to insects in general. 



