284 Robert Wilhelm Heffncr. 



to^ 



Pctriinkewitscli (1901 and 1903) has described the origin of the 

 primitive germ-cells in the ''drone eggs" of the bee at a period earlier 

 than has been recorded for any other insect. The inner half of the 

 first polar body of these nnfertilized eggs nnites with the second 

 polar body to form the "Richtungscopulationskern'' which is the 

 primordial germ-cell. Weismann (1904) vouches for the exactness 

 of Petrunkewitsch's results. In other Hymenoptera no germ-cells 

 have been found previous to the appearance of the mesoderm (Car- 

 riere and Burger, 1897). 



The primitive germ-cells (pole-cells) of several species of Diptera 

 have a very early origin. Weismann (1904), discussing the develop- 

 ment of the reproductive cells in connection with his ''germ-plasm 

 theory," says: "If we could assume that the ovum, just l)eginning 

 to develop, divides at its first cleavage into two cells, one of which 

 gives rise to the whole body (soma) and the other only to the germ- 

 cells lying in this body, the matter would be theoretically simple. 

 As yet, however, only one group of animals is known to 

 behave demonstral)ly in this manner, the Diptera among insects. . ." 

 I have been unable to find in embryological literature any account 

 of such a phenomenon in this order of insects. The pole-cells of 

 Diptera are always found at the posterior end of the egg. The time 

 of their first appearance varies in the different species described. 

 In Miastor the primordial pole-cell nucleus can l)e distinguished 

 when there are only eight to fifteen nuclei in the pseudovum 

 (Leuckart, 1865; Metschnikoff, 1865). In Chiroiiomns the single 

 primordial pole-cell appears before the blastoderm is formed and is 

 closely followed by a second, these two then divide, re-enter the egg, 

 and develop into the germ-glands (Grimm, 1870; Weismann, 1882; 

 Jaworowski, 1882; Balbiani, 1885; Ritter, 1890). The first pole- 

 cell nucleus in this species may divide before it separates from the 

 <'gg. Weismann (1863) noted four pole-cell nuclei lying in the 

 "Keimhautl)lastem," wliile several authors have described the ap- 

 pearance of two, before separation takes place (Grimm, 1870; 

 Weismann, 1882). 



In other species of Diptera the primordial pole-cell nucleus ap- 

 f)arently divides several times before it reaches the surface of the 



