Germ-Colls in Chrysomelid Beetles. 243 



with it as it passed through the "Keirahautblastem." This was the 

 first cell. differentiation. At this time there were fifteen to twenty 

 pole-cells present, and not one to twelve, as Weismann (1863) and 

 others have claimed; these pole-cells divided forming a single layer. 

 This layer became several cells thick and formed the polar mass char- 

 acteristic of Dipterous eggs at this stage of development. This mass 

 of pole-cells was then passively carried into the dorsal groove of 

 the germ-band where it was connected with the tissue destined to 

 become the mid-intestine as Escherich (1900) had described it in 

 Musca vomHoria. The pole-cells always remained connected with 

 the yolk ; they later began to wander through the entoderm, not by 

 means of a definite canal (Escherich, 1900), but through an in- 

 definite gap. After this migration had taken place, the pole-cells 

 were found lying isolated among the entoderm cells from which 

 they were distinguished by their darker pigmentation and smaller 

 size. Soon, however, these distinguishing features became obliterated 

 and the further history of the pole-cells could not be followed. 



3. Hemiptera. 



Until recently the Hemiptera have held a position next to the 

 Diptera regarding the early appearance of germ-cells. As we shall 

 see later germ-cells have been found by modern methods of study 

 at a much earlier stage in the Ilymenoptcra and Coleoptera than 

 thus far reported in the Hemiptera. 



Huxley (1858) was the first to stud}^ the development of the 

 psendovarium in Aphis. He thought that the reproductive organs 

 arose from the inner layer of the two-layered blastoderm. 



Metschnikoff (1860) found in the viviparous Aphid, A pit is rosae, 

 sliorth' after blastoderm formation, a group of cells, "Keimhiigel," 

 projecting into the central yolk mass at the posterior end of the egg. 

 These cells could not be distinguished from those constituting the 

 blastoderm. The anterioi- part of this "Keimhugel" separated from 

 the remainder and became an oval mass, the rudiment of the repro- 

 ductive organs, ''Genitalhiigel." This rudiment, which lay within 

 the tail fold, was carried by the latter into the position of the defini- 

 tive germ-glands. Meanwhile, the nuclei increased in number, the 



