52 



THE CELL 



In Cyclas cornea and in the Naiadese a principal nucleolus, in 

 addition to a few smaller secondary nucleoli, is present in the 

 germinal vesicle. "The former consists of two differently consti- 

 tuted portions ; these may be seen in Fig. 30 as a smaller, strongly 

 stained more refractive part, and a larger, paler, less chromatic one, 

 which swells up more in acids. In Anodon these two portions 

 are closely coherent; in Unio they very frequently only just touch 

 each other, or, indeed, may lie apart. The smaller secondary 

 nucleoli, which lie in the meshes of the framework, show the same 

 power of refracting light, of swelling up, and of becoming stained, 

 as the larger portion of the principal nucleolus. If water is added, 



this larger portion disappears, 

 as well as the small nucleoli, 

 amongst the strands of the 

 framework ; the small, strongly 

 chromatic portion of the prin- 

 cipal nucleolus alone remains ; 

 this becomes more sharply de- 

 fined, shrinking up somewhat, 

 and developing a clearly marked 

 outline. The addition of strong 

 acetic acid (5 per cent, or more) 

 causes the larger paler portion 

 of the principal nucleolus to 

 swell up rapidly and to dis- 

 appear, whilst the smaller shin- 

 ing portion, though also swell- 

 ing up somewhat, remains visi- 

 ble." " When nuclear staining 

 reagents are used, both portions 

 of the nucleolus, and also the 

 secondary nucleoli, become 

 coloured to a considerable ex- 

 tent; the most strongly refrac- 

 tive part of the former, however, 

 is especially intensely stained." 

 "Such a differentiation of the 

 principal nucleolus into two 

 parts occurs in the egg-cells of 

 many animals. In Dreissena polymorpha the strongly refractive 

 chromatic portion covers the paler one like a hollow cap." 



. (Af ier Flemming, Fig. E', p. 104.; 

 a Nucleus of an egg from the ovary of Uirio; 

 it has just emerged from the cell into the 

 ovarian fluid. Nucleolus with two pro- 

 tuberances. A small portion of the nuclear 

 framework is visible; a a similar nucleus 

 after 5 per cent, acetic acid has been added. 

 The framework strands stand out more 

 clearly; the larger paler portion of the 

 principal nucleolus, as well as the minor 

 nucleoli, have similarly become swollen up 

 and faded ; the smaller portion of the 

 principal nucleolus is also swollen up, but 

 to a less degree, b Nucleolus of an egg 

 of Tichogonia polymorpha ; the principal 

 glistening portion rests like a cap upon the 

 larger one. /3 Diagrammatic representation 

 of an optical section of above. 



