128 THE GERM-CELLS 



to the formation of chromosomes. It cannot therefore be directly com- 

 pared to the net-knots or karyosomes of tissue-cells. This nucleolus is 

 often vacuolated and sometimes assumes the form of a hollow vesicle. 

 It is rarely double or multiple. The accessory nucleoli, on the other 

 hand, are in general coloured by plasma-stains, thus resembling the 

 plasmosomes of tissue-cells ; they are often multiple, and as a rule 

 they arise secondarily during the growth of the ^^g (Fig. 6i). The 

 accessory nucleoli often have no connection with the principal ; but 

 in some mollusks and annelids an accessory and a principal nucleolus 

 are closely united to form a single compound body (Figs. 60, 61). 

 The numerous nucleoli of the amphibian or reptilian Q.^g appear to be 

 of the "accessory" type. The singular inconstancy of the nucleolus 

 is evidenced by the fact that even closely related species may differ 

 in this regard. Thus, in Cyclops brevicornis, according to Hacker, the 

 very young ovum contains a single intensely chromatic nucleolus ; at 

 a later period a number of paler accessory nuclcoH appear ; and still 

 later the principal nucleolus disappears, leaving only the accessory 

 ones. In C. strenmis, on the other hand, there is throughout but a 

 single nucleolus. 



The physiological meaning of the nucleoli is still involved in doubt. 

 Many cases are, however, certainly known in which the nucleolus 

 plays no part in the later development of the nucleus, being cast out 

 of degenerating z« situ at the time the polar bodies are formed. It 

 is, for example, cast out bodily in the medusa yEq?iorea {W acker) and 

 in various annelids and echinoderms, afterward lying for some time 

 as a " metanucleus " in the egg-cytoplasm before degenerating. In 

 .these cases the chromosomes are formed in the germinal vesicle inde- 

 pendently of the nucleoh(Fig. 125), which degenerate /;/ sit?i when 

 the membrane of the germinal vesicle disappears. In such cases it 

 seems quite certain that the nucleoli do not contribute to the forma- 

 tion of the chromosomes, and that their substance represents passive 

 material which is of no further direct use. Hence we can hardly 

 doubt the conclusion of Hacker, that the nucleoli of the germ-cells 

 are, in some cases at least, accumulations of by-products of the nuclear 

 action, derived from the chromatin either by direct transformation of 

 its substance, or as chemical cleavage-products or secretions. It will 

 be shown in Chapter V. that in some cases a large part of the 

 chromatic reticulum is cast out, and degenerates at the time the polar 

 bodies are formed. The immense growth of the chromatin during 

 the ovarian development is probably correlated in some way with the 

 intense constructive activity of the cytoplasm (p. 339); and when this 

 latter process has ceased a large part of the chromatin-substance, 

 having fulfilled its functions, is cast aside. It seems not improbable 

 that the nucleoli are tributary to the same general process, perhaps 



