174 



Embryogenesis: Preparatory Phases 



in the gall midges (Cecidomyidae), which 

 have been studied most recently by White 

 ('46, '47a, b, '48). In Miastor, for example, 

 there are 48 chromosomes in the zygote but 

 36 of these are eliminated from the so- 

 matic nuclei of the female and of the pedo- 

 genetic larvae at the third and fourth cleav- 

 age divisions, while 42 are eliminated in 

 the males. The full complement is retained 

 by the germ cells, which are set aside at 

 the posterior pole of the egg at this time. 

 Chromosome- or chromatin-diminution 

 also occurs in various animals without rela- 



of the germ cells, have been described in 

 several species of animals among the scyph- 

 ozoans (Equorea), chaetognaths (Sagitta), 

 rotifers (Asplanchna), insects (Chironomus, 

 Calliphora, Calligrapha, Lepintotarsa, Copi- 

 dosoma, Trichograma, Apanteles), crustace- 

 ans (Cyclops, Diaptomus, Polyphemus, 

 Moina) and amphibians (Rana) (see Heg- 

 ner, '14; Wilson, '25; Bounovure, '39). Ac- 

 cording to the accounts the cells that receive 

 these granules ultimately become germ cells. 

 However, there is, as yet, no direct experi- 

 mental evidence, such as might be derived 



Fig. 50. Geigy's method for exposing the posterior pole of the egg of Drosophila to ultraviolet radiation. C, 

 wax trough; U, ultraviolet beam; S, brass screen; E, egg; P, pole cells. (After Geigy, '31.) 



tion to germ-cell determination (see Berry, 

 '41, for references). For example, it occurs 

 in the polar body divisions of several species 

 of Lepidoptera. Here, however, the elimi- 

 nated chromatin is Feulgen-negative whereas 

 in Ascaris, Sciara, etc., it is Feulgen-positive 

 (Bauer, '32, '33). One of the most striking 

 examples has been described in the grass 

 mite, Pediculopsis graminum (Cooper, '39, 

 '41). Feulgen-negative "chromatin" bodies 

 that correspond in number and position to 

 the metaphase chromosomes are left at the 

 equator of the spindle at both polar body 

 divisions and at each of the first nine cleav- 

 age divisions, but not in the succeeding 

 mitoses of the embryo or of the oogonia. 



Such cases do not, however, rule out the 

 possibility that, where elimination occurs 

 in connection with germ-cell determination, 

 the chromatin that is eliminated from the 

 somatic cells may contain genes that are of 

 importance for the development of the 

 gametes. 



Localized Cytoplasmic Factors in Germ-Cell 

 Determination. Special cytoplasmic granules 

 (termed germ-cell determinants), that are 

 considered to be important for the formation 



from centrifugation experiments, to show 

 that the particular granules are causative 

 agents in germ-cell determinations. 



Apart from the questions of particular 

 granules and of chromatin diminution, the 

 importance of localized cytoplasmic factors 

 in germ-cell determination is very well 

 illustrated in the experiments of Geigy 

 ('31), Aboim ('45), and Geigy and Aboim 

 ('44) on Drosophila. Geigy ('31) succeeded 

 in inhibiting the formation of germ cells by 

 subjecting the posterior pole of the egg to 

 ultraviolet irradiation. The method of local- 

 ized irradiation is shown in Figure 50. After 

 irradiation the cleavage nuclei that wander 

 into the damaged pole plasm degenerate 

 and the formation of pole cells is partially 

 or completely suppressed. However, even 

 in cases of complete suppression, adult flies 

 are obtained which appear completely nor- 

 mal and have gonads of normal structure 

 although of reduced size. This work not 

 only strikingly confirms the earlier indica- 

 tions that factors essential for germ-cell 

 formation are localized in the posterior cyto- 

 plasm of the egg, but also furnishes further 

 convincing evidence that the mesodermal 



