272 SCIENCE PROGRESS 



inclusions I have called " keimbahn-determinants," since they 

 make it possible for us to determine the keimbahn with certainty 

 during the early cleavage stages of the egg. 



Although a large number of reports on the segregation of 

 the germ cells were published during the latter part of the 

 nineteenth century, including the well-known cases of Ascaris 

 and Cyclops, we need not go back beyond the year 1906 for our 

 illustrative material. In that year the first account of Silvestri's (4) 

 researches on the development of hymenopterous parasites 

 appeared. This was followed two years later by additional 

 studies by the same investigator. The development of both 

 monembryonic and polyembryonic species was worked out. 

 In all cases, both in parthenogenetic and fertilised eggs, the 

 ovum at the time of deposition contains near the anterior end 

 a nucleus, and near the posterior end a body called by Silvestri 

 the "nucleolo," and considered by him as a nucleolus (meta- 

 nucleolus) which had escaped from the germinal vesicle. In 

 the monembryonic species studied, early development proceeds 

 as in the typical insect egg ; the cleavage nuclei multiply rapidly 

 and migrate toward the periphery, where they become cut off 

 from one another by cell walls and form a single cellular layer. 

 During most of this process the " nucleolo " remains intact at 

 the posterior end. When the cleavage nuclei reach its vicinity, 

 however, it disintegrates and becomes distributed in the cyto- 

 plasm of a few of the cells formed at the posterior pole. These, 

 the primordial germ cells, were traced during subsequent 

 development, and found to produce the eggs or spermatozoa 

 of the succeeding generation. The cleavage processes are 

 different in the polembryonic species studied. The first two 

 cleavage nuclei are at once separated by cell walls, and the 

 " nucleolo " is segregated in one of them. During several 

 succeeding divisions the " nucleolo " is likewise limited to one 

 cell. Finally, however, it disintegrates and becomes divided 

 between two daughter cells, and later is distributed among four 

 cells. These were not traced further, but it seems very probable 

 that, as in the monembryonic species, the cells containing 

 granules from the "nucleolo " may be considered as primordial 

 germ cells which multiply and form part of the morula-like 

 groups of cells from which the adult insects arise. Certain 

 larvae were found to lack several systems of organs, including 

 reproductive organs, and were hence called sexless larvae. The 



