STUDIES ON GERM CELLS 387 



tende Protoplasmamasse fast farblos erscheint (fig. 2, B, p.g.c). Bei 

 der Fortentwicklung der Polzellen schwindet allmahlich die scharfe 

 Grenze zwischen Zellprotoplasma und Polplatte. Letztere lost sich 

 auf und es entsteht eine gleichmassige Pigmentirung, welche den Pol- 

 zellen noch auf lange Zeit ein ganz charakteristisches Aussehen ver- 

 leiht. 



Concerning the nature of this 'Dotterplatte' Noack says: 



Dass die Platte am hinteren Pole des Musciden-Eies sich aus Dot- 

 terelementen zusammensetzt. Sie scheint den Zweck zu haben, das 

 Wachsthum am hinteren Pol zu beschleunigen, ferner durch Eintritt 

 in die Polzellen es diesen zu ermoglichen, sich auch weiterhin lebhaft 

 zu vermehren, obgleich sie vom Dotter her keine Nahrung mehr erhalten. 

 Schhesshch verursacht sie die charakteristische Pigmentirung dieser 

 Zellen. 



B. The Keimbahn in Miastor americana Felt 



The paedogenetic flies of the genus Miastor furnish especially 

 favorable material for the study of the germ-cell cycle. The 

 process of paedogenesis was discovered by Wagner, and a short 

 statement was published by him in 1862; three years later a more 

 detailed account appeared by Wagner ('65) whose extraordinary 

 discovery was confiirmed by Meinert ('64), Pagenstecher ('64) 

 and Ganin ('65); but of the early authors Leuckart ('65) and 

 Metschinkoff ('65, '66) have given the best descriptions of the 

 developmental stages. In 1870 Grimm announced the occur- 

 rence of paedogenesis in a species of Chironomus. From that 

 date until 1907 nothing new concerning this peculiar method of 

 reproduction in insects was learned. Zavrel ('07) then reported 

 paedogenesis in the genus Tanytarsus, and this has been con- 

 firmed for T. dissimilis by Johannsen ('10). In the meantime 

 Kahle ('08) published perhaps the best account that has ever 

 appeared on the 'Keimbahn' of any animal, using Miastor me- 

 traloas for this purpose. He was able to trace the germ cells 

 from one generation to the next with remarkable clearness. 

 Many of Kahle's results have been confirmed (Hegner, '12) 

 for Miastor americana Felt and reference will be made to them 

 more in detail in the following pages. Finally a short study 

 of the life history of Miastor metraloas has been made by G. W. 

 Muller('12). 



