180 GERM-CELL DETERMINANTS [CH. xn 



A germ-cell determinant of a different kind has been 

 described by SILVESTRI and others in the parasitic Hymen- 

 opteran Litomastix (Copidosoma). It consists of a deeply- 

 staining compact body lying in the egg before maturation, 

 called by SILVESTRI the oosoma (Text-fig. 20). The egg 

 is very small, and undergoes total segmentation (very 

 rare in Insects) and the stained body remains undivided 

 and is enclosed in one cell at the four-cell stage. It then 

 enlarges and breaks up, so that when this cell divides 

 both daughter-cells will contain part of it. As these cells 

 divide, always containing portions of the stained body, 

 they form a mass at the anterior end of the embryo. 

 This mass then breaks up into a number of cell-groups 

 or morulae, some of which again divide in the same way. 

 Ultimately each of the morulae develops into a larva, 

 so that one egg gives rise to scores or hundreds of larvae. 

 The larvae are said to be of two kinds; some are normal 

 Hymenopterous larvae with all the usual organs, including 

 gonads; others are "sexless" larvae with no gonads and 

 reduced alimentary canal. The sexless larvae never develop 

 to adults, and SILVESTRI believes that they are formed from 

 groups of cells which have received no portion of the stained 

 body of the egg. In related species in which polyembryony 

 does not occur it seems almost certain that a similar stained 

 body determines the cells which will become primitive germ- 

 cells. Of the origin of the stained body little is known; 

 SILVESTRI'S supposition that it is the nucleolus extruded 

 from the egg-nucleus, and HEGNER'S, that it is derived from 

 the chromatin of a degenerate oocyte, have both been dis- 

 proved, and if seems more probable that it is a portion of 

 modified and concentrated protoplasm, as in the Diptera 

 and Coleoptera. 



The early differentiation of primitive germ-cells has also 



