THE OVUM 93 



second form comprises the so-called "accessory nucleoli," which 

 stain less intensely, are often numerous, and perhaps correspond 

 with the true nucleoli of tissue-cells. As growth proceeds, they 

 usually increase in size and number, and may finally become very 

 numerous, in which case they often occupy a peripheral position 

 in the germinal vesicle. This is typically shown in amphibia and 

 selachians, where there are a large number of nucleoli, which are 

 at first scattered irregularly through the germinal vesicle but at a 

 certain period migrate towards the periphery. In some of the 

 mollusks and Crustacea both forms coexist ; but even closely related 

 species may differ in this regard. Thus, in Cyclops brevicornis, 

 according to Hacker, the very young ovum contains a single in- 

 tensely chromatic nucleolus; at a later period a number of paler 

 accessory nucleoli appear; and still later the principal nucleolus 

 disappears, leaving only the accessory ones. In C. strciuins, on 

 the other hand, there is throughout but a single nucleolus. In 

 some of the mollusks and annelids the "germinal spot" is double, 

 consisting of a deeply staining principal nucleolus and a paler 

 accessory nucleolus lying beside it, as in Cyclas and in Nereis 



(Fig. 43)- 



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 or degenerating /;/ situ at the time the polar bodies are 

 formed. It is, for example, cast out bodily in the medusa ^-Eqnorea 

 (Hacker) and in various annelids and echinoderms, afterwards lying 

 for some time as a " metanucleus " in the egg-cytoplasm before 

 degenerating. In many cases — for example in amphibia, in sela- 

 chians, in many Crustacea, annelids, and echinoderms — the chromo- 

 somes are formed in the germinal vesicle independently of the 

 nucleoli (Fig. 96), which degenerate in situ when the membrane of 

 the germinal vesicle disappears. The evidence is, therefore, very 

 strong that the nucleoli do not contribute to the formation of the 

 chromosomes, and that their substance represents passive material 

 which is of no further direct use. There is, furthermore, strong 

 evidence that the nucleoli of both kinds are directly or indirectly 

 derived from the chromatin. Hence we can hardly doubt the 

 conclusion of Hacker, that the nucleoli of the germ-cells are ac- 

 cumulations 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 



1 Hiicker regards the principal nucleolus as a more highly differentiated modification of 

 tlie accessory nucleolus, and regards it as a pulsating excretory organ comparable with the 

 contractile vacuoles of Protozoa. 



