28G Kolx^rt Willi elm Ilegner. 



The i)i'imitive genn-cells may be recognized in the parthenogenetic 

 eggs of Aphids shortly after the blastoderm is completely formed. 

 Some anthors were able to trace them back to a single cell which 

 separates from the inner surface of the blastoderm near the posterior 

 end of the egg (Balbiani, 1SG6 ; Witlaczil, 1884; Will, 1888). 



The only investigators who have recorded an early appearance of 

 the primitive germ-cells in the Leptidoptera are Balbiani (1869-72), 

 Woodworth (1889) and Schwangart (1905). These authors found 

 a thickening of the blastoderm near the posterior end of the egg. 

 The inner cells of this thickening differentiate into germ-cells; later 

 these migrate singly into the fourth to the eighth abdominal segments 

 (Schwangart, 1905). 



The foregoing accounts show that those embryologists who hold 

 that the germ-cells in the Insecta have a mesodermal origin are not 

 in harmony with the results obtained by recent investigators. Hey- 

 mons (1891), Korschelt and Ileider (1892), and Wheeler (1893) 

 all regarded the primitive germ-cells of Diptera and Ilemiptera 

 ''as derived by a process of precocious segregation from metameric 

 gonads like those of the Orthoptera-" (Wheeler, 1893). The germ- 

 cells in the Orthoptera (Blatta and Feri plane ta), however, do not 

 arise metamerically from the mesoderm and later migTate into the 

 primitive somites (Heymons, 1895). 



As noted above, the stage of embryonic development in which the 

 germ-cells can first be recognized varies considerably in different 

 species of insects. By the majority of authors the reproductive 

 cells have been considered of mesodermal origin, by others they are 

 supposed to arise from ectoderm-cells, blastoderm-cells, yolk-nuclei, 

 or early cleavage nuclei. I believe with Woodworth (1889) that 

 "the germinal cells do not belong to any layer, but are the products 

 of the first divisions of the egg cell; they take part generally in the 

 formation of the blastoderm and then migrate into the mesoderm. 

 In all cases where they are supposed to come from the 

 mesoderm, the later stages comparatively are the only ones kno\vii." 

 Ileymons (1893) four years later was led to similar conclusions. 

 This author states "dass die Geschlechtszellen der Insekten liberhaupt 

 nicht von diesem oder jenem 'Keirablatte' abzuleiten sind, sondern 



