28 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



association of the oosome with the germ cells has given rise to the assump- 

 tion on the part of some workers that this substance in some way acts 

 as a stimulus to convert undifferentiated cleavage nuclei into germ cells. 

 Recent writers, however, regard the oosome merely as an ivAicator of 

 the path of the germ cells from their original position at the posterior 

 pole of the egg to their definitive position in the gonad rather than a 

 determinant, or causal factor, in the production of germ cells. 



It is not the oosome but the cytoplasm lying adjacent to it that is 

 the factor governing the conversion of the cleavage cells into germ cells 

 according to Hegner (1914), all the cleavage cells being quahtatively 

 alike. Furthermore, Huettner (1923) states that the deciding factor 

 that determines whether a nucleus shall become somatic or germinal 

 appears to be the posterior pole plasm, which he regards as a differentiated 

 ooplasm. Any nucleus of the developing egg may be differentiated into 

 a polar nucleus {i.e., germ cell) if it comes accidentally into the region 

 of the posterior polar plasm. If one daughter nucleus enters the posterior 

 polar plasm and the other remains in the general ooplasm, the latter 

 becomes a somatic nucleus, whereas the former, surrounded by polar 

 granules (oosome), becomes a polar nucleus. The posterior polar plasm 

 referred to here is the periplasm (cortical ooplasm) which lies at the 

 posterior pole of the egg. 



As for the nature of the oosome substance, Huettner holds that in 

 the egg of Drosophila it is a by-product of the posterior pole plasm and is 

 not composed of minute yolk granules, or mitochondria. 



Among insects it has been demonstrated in Coleoptera (Chrysomelidae 

 and Brachyrhinus) , in Diptera (Miastor, Chironomus, Simulium, Sciara, 

 Drosophila, Musca, Phormia, Calliphora, Melophagus), and in Hymenop- 

 tera (Apis. Litomastix, Ageniapis, Encyrtus, Oophthora, Diastrophus, and 

 others) . 



FERTILIZATION AND MATURATION 



Fertilization. — With insects in general, after the eggshell has been 

 formed, fertilization takes place. The egg descends from the ovary and 

 in passing the opening of the spermathecal duct receives through the 

 micropyles the sperms that have previously been deposited in the 

 spermatheca (or in some cases in the bursa) by the male. Although 

 polyspermy is the rule with insects, in that a number of sperms enter the 

 micropyles, monospermy has been recorded for Sciara (Schmuck and 

 Metz, 1932), Diacrisia (Johannsen, 1929), parasitic Hymenoptera (Leiby, 

 Patterson, Silvestri), and some other insects. When polyspermy takes 

 place, the supernumerary sperms degenerate in the yolk. Huettner 

 (1924, 1927) reports that supernumerary sperm in Drosophila eggs 

 degenerate, only rarely forming mitotic figures. If, however, the degree 



