Germ-Cells in Chiysoiuelicl Beetles. 28Y 



niir scheiiibar je iiacli clem Zeitpuiikt ilires Ilervortretens Laid dieser, 

 bald jener Zelleiiscliieht angelioren. 



''Wenn aiieh die Treiinung zwiscben somatiscben Zellen und 

 Gcscblecbtszelleu bei den meisten Insekten erst spat bemerkbar wird, 

 so werden wir somit docli annehmen miissen, dass ein solcber Unter- 

 sebied bereits vom Beginne der Entwicklung an vorhanden ist. 



"Es mag nocli bervorgeboben werden, dass die Gescblecbtszcllen 

 der Insekten iiicbt, wie man bisber geglaubt bat, in metamerer An- 

 ordnnng in den einzelnen anf einander folgenden Abdominalseg- 

 menten znr Anlage kommen, sondern dass ibr Ursprnng am hin- 

 tersten Ende des Keimstreifs zn sucben ist, von wo aus erst im 

 Laufe der Entwicklnng eine Wandernng oder Verscbiebnng nacb 

 vorn bin erfolgt. Dies trifft zunacbst fiir die bier bescbriebenen 

 Formen zn, bat moglicherweise aber fiir sammtlicbe Insekten Giiltig- 

 keit." Heymons is an investigator wbo bas not been content to 

 work on a few widely separated types of insects, but bas made 

 comparative studies of nearly allied species. Tbe value of tbis kind 

 of researcb is strikingly sbown by the results be obtained from a 

 study of Blatta and Periplaneta. 



In an early paper. Ileymons (1891) stated tbat tbe germ-cells of 

 Blatta are derived from tbe mesoderm just previous to tbe segmenta- 

 tion of tbe germ-band. Later (1895) Periplaneta was also examined. 

 In tbis Ortbopteron tbe germ-cells were found to arise from tbe 

 ectoderm at tbe posterior end of tbe egg ; tbey separate from one an- 

 other, migrate anteriorly, and arrange themselves intcrsegmentally. A 

 re-examination of Blatta convinced Heymons tbat tbe germ-cells 

 originate in tbis cockroach as they do in Periplaneta, but can be dis- 

 tinguished from the mesoderm-cells only when tbey reach the primi- 

 tive somites. This is made more certain by the discoveiy of a similar 

 origin and migration in Forficida (Heymons, 1895) and Lepisma 

 (Heymons, 1897). 



In C allirjraplia all the nuclei of the egg are apparently alike, 

 potentially, until in their migration toward the surface tbey reach 

 the '^Keimbautblastem;" then those which chance to encounter the 

 granules of the pole-disc are differentiated by their environment, 

 i. e., tbe granules, into germ-cells. In other words, whether or 



