STUDIES ON GERM CELLS 431 



ing the substance after the fourth cleavage division when there 

 were four of them present, but he thinks this can be done and ex- 

 presses his ideas regarding their history and potency. Two 

 embryonic regions are formed in Copidosoma (1) an anterior 

 'massa germinigena' which produces normal larvae, and (2) a 

 posterior 'massa monembrionale' which produces larvae without 

 genital, respiratory, circulatory, and excretory systems; these 

 he calls sexless larvae. He believes that the cells provided with 

 nucleolar material are germ cells, whereas those lacking this 

 substance are somatic cells, and that the 'massa monembrionale' 

 contains only somatic cells, hence the larvae derived from this 

 region are sexless. 



Further studies were made by Silvestri ('08) on other species 

 of parasitic Hymenoptera, and several interesting variations in 

 the behavior of the nucleolus were observed. In Ageniaspis 

 (Encyrtus) fuscicollis, and A. fuscicollis praysincola the structure 

 of the egg is similar to that of Copidosoma and the nucleolus 

 becomes situated in one of the first two blastomeres. The cell 

 with the nucleolus divides more slowly than the other, and, before 

 its cleavage, the nucleolus breaks up into granules which are 

 distributed between the daughter cells. The cleavage stages 

 thus are as follows: (1) A two-cell stage, one cell with the nucleo- 

 lus; (2) a three-cell stage, one cell with and two without nucleolar 

 material; (3) a four-cell stage, two cells with and two without 

 nucleolar material; (4) a six-cell stage, two with and four without 

 nucleolar material; and (5) a twelve-cell stage, four with and 

 eight without nucleolar material. The further history of the 

 cells containing nucleolar material was not determined. 



In Encyrtus aphidivorus, which is not poly embryonic, the 

 nucleolus remains at the posterior end of the egg until a late 

 period of cleavage; then its substance becomes distributed among 

 the primordial germ cells, which, as in Copidosoma and Ageni- 

 aspis, divide more slowly than the somatic cells. In this case 

 there seems to be no doubt that the cells containing nucleolar 

 material become germ-cells, whereas all of the rest become 

 somatic cells. 



