12 SUMMARY OF CUREENT RESEARCHES RELATING TO 



determinants to form one or more primordial germ-cells ; the appar- 

 ently equal distribution of the Keimbahn determinants between the 

 daughter germ-cells at each mitotic division {Sagitta possibly excepted) ; 

 the disappearance of the Keimbahn determinants in the oogonia and 

 spermatogonia; the reappearance of the Keimbahn determinants in 

 the oocyte or mature egg. The most interesting period in the germ- 

 cell cycle is that extending from the formation of the ultimate oogonia 

 and spermatogonia to the complete segregation of the germ-cells in the 

 developing egg. A little known and important part of this period is 

 that during which, in some animals, visible substances (Keimbahn 

 determinants) peculiar to the germ-cells appear, become localized in a 

 definite part of the egg or in certain blastomeres, and are equally dis- 

 tributed among the primordial germ-cells. The Keimbahn in animals 

 was first traced in dipterous insects. Keimbahn determinants appear 

 in the eggs of all Diptera that have been carefully studied (Miastor, 

 Chironomus, CaUipkora). In Miastor there are six cell-divisions during 

 the multiplications of the oogonia. The somatic cells lose part of their 

 chromatin by diminution processes, whereas the germ-cells possess a 

 complete amount of chromatin. The nurse-cells are of mesodermal 

 origin. A peculiar mass of cytoplasm becomes situated at the posterior 

 end of the oocyte ; within this one of the first eight cleavage nuclei 

 (with a complete amount of chromatin) becomes embedded ; it is then 

 cut Off from the rest of the egg as the primordial germ-cell. The 

 origin of this peculiar mass of cytoplasm could not be determined. 

 The eggs of the ovoviviparous Dipteron Compsilura, and of the willow- 

 cone gall-fly, Cecidomyia, contain Keimbahn determinants which have a 

 history like that of similar bodies in other insects. An early segrega- 

 tion of germ-cells has been reported for certain Chrysomelid beetles, 

 and Keimbahn determinants have been found in the eggs of those 

 carefully examined. An examination of all stages in the early cleavage 

 of Chrysomelid eggs failed to reveal a chromatin-diminution process 

 such as occurs in Ascaris and Miastor. The conclusion is reached that 

 the cleavage nuclei are all potentially alike, and that the cytoplasm 

 controls their differentiation into the nuclei of blastoderm cells, pri- 

 mordial germ-cells, and vitellophags. What appears to be amitotic 

 nuclear division among the vitellophags is described. The pole-disc 

 granules in Chrysomelid eggs form a recognizable mass just before the 

 oocyte reaches its full size ; their genesis could not be definitely de- 

 termined In the testis of Leptinotarsa the germ-cells in each cyst arise 

 from a single spermatogonium. Spindle- remains connect the daughter- 

 spermatogonia up to the time when sixty-four cells are present in each 

 cyst. This process is homologous with the differential divisions in 

 Dytiscus and other beetles, and certain Hymenoptera, during which an 

 ultimate oogonium and a definite number of nurse-cells arise from a 

 single oogonium. What appears to be amitotic nuclear division was 

 found among the nurse-cells of Leptinotarsa, but no nuclear phenomena 

 which could be interpreted as amitosis were observed among the oogonia 

 or spermatozoa. Most of the author's own experiments and observations 

 refer to Chrysomelid beetles, but he describes work done by other 

 investigators on the Keimbahn in Crustacea, in Ascaris, Sagitta, and 



