,8,, RESEARCHES ON INSECTS AND ARACHNIDS. 55 



from the walls of the somites as these are developed, and, in tlie 

 younger stages of the embryo, are always to be found in the second, 

 third, fourth, and fifth abdominal segments. Thus, whether formed 

 from the primitive layer, or from the coelomic epithelium, the origin 

 of the germ-cells is clearly mesodermic. 



This result is contradictory to that lately obtained by Cholod- 

 kovsky, who believes that the germ-cells are probably formed from 

 small yolk-cells. Dr. Heymons, however, has no doubt that the 

 Russian observer fell into error owing to the imperfection of his 

 method of preparation, especially as he was unable to distinguish the 

 germ-cells before the stage at which the somites begin to form. 



Certainly the mesodermal origin of the germ-cells seems by far 

 the more probable of the two alternatives, and, after a review of the 

 results obtained by workers in other insect groups. Dr. Heymons 

 concludes that, among insects generally, though sometimes in a 

 modified way, the mesoderm gives rise to the generative organs. In 

 the origin and segmental arrangement of the germ-cells he sees a 

 similarity to what is observed in the Annelids. 



As development proceeds, the somites become largely filled up 

 with the fat-body, and the germ-cells begin to take up a dorsal 

 position. The differentiation of sex is now evident. Epithelial cells 

 of mesodermal origin arrange themselves around and amongst the 

 germ-cells. The epithelial cells ventral to the germ-cells form the 

 oviduct, those dorsal to them arrange themselves in linear series to 

 form the thread-like ends of the ovarian tubes, whilst others enclose 

 the germ-cells and so form the ovarian tubes. Special attention is 

 called by Dr. Heymons to the fact that, throughout the course of 

 the development, the germ-cells are always quite distinct from the 

 epithelium of the ovary. 



During the larval stages, changes take place in the nuclei of the 

 germ-cells, and they begin to assume the characters of ova. 



The thread-like prolongations of the ovarian tubes appear to be 

 of use only during the embryonic and larval periods, and their func- 

 tion is to enable the ovaries to change their position in the body of 

 the insect. They are all continuous with a single thread of cells 

 which stretches to the pericardial septum. 



Dr. Heymons states that the contents of the ends of the ovarian 

 tubes are ordinary epithelial cells and germ-cells. These portions 

 of the tubes in different insects have been called " Endkammer " by 

 various authors who have described a mass of undifferentiated cells 

 as found in them. In Fhyllodyomia, however, the germ-cells are, as 

 stated above, all along distinct from the epithelial cells. 



Some statements have lately been made with regard to the effect 

 produced by the parasitic Stylopidae on their hosts, which are Bees, 

 mostly of the genera Andvena and Halictus. Mr. R. C. L. Perkins 

 (" Entom. Monthly Mag." [2] , vol. iii., p. i) gave as the result of his 

 investigations on stylopised bees that the internal organs of the hosts 



