36 NAGELl's THEOEY. [BOOK i. 



activity of the cell,, and the support of it. They lie free in 

 the cavity of the cell, single or in large numbers, and appear 

 with the primary nucleus, or not till after its absorption. 

 The presence of secondary nuclei has only been determined, 

 as yet, in cells with a parietal primary nucleus, not in such as 

 have a free primary nucleus. Their presence is as yet 

 restricted to the pollen cells and the pollen tube, to the 

 spores and the embryo -sac, to the parent cells of pollen 

 grains, and those spores which originate in fours. 



" Where the nuclei were large enough to allow of an accu- 

 rate microscopical examination, the following structure was 

 perceived : -the nucleal vesicle consists of a peculiar mem- 

 brane, and incloses fluid and solid contents distinct from it, 

 and, in addition to one or more dense mucilage corpuscules, 

 a nucleolus. That the nuclei possess a membrane, and there- 

 fore are utricles, follows from the correspondence of the 

 results of researches in Algae, Fungi, Floridese, Hepaticae, 

 Musci, Filices, Characeae, Equisetacese, Lycopodiacese, and 

 Phanerogamia. This structure is demonstrated both in the 

 free and the parietal nuclei. Where the membrane cannot 

 be seen, it is on account of the minute size of the nucleus, 

 of its being densely filled with homogeneous substance, or of 

 the opacity of the cell-contents. In larger nuclei, where a 

 more certain examination is practicable, the absence of the 

 membrane can never be demonstrated. Even where the 

 membrane is overlooked in the natural condition, on account 

 of the similarity of the contained matter, it may be made 

 visible by the action of re- agents. (Filices.) There is no 

 reason, therefore, against stating, as a general proposition, 

 that the nuclei are utricles. 



" The nucleus is not merely universally an utricle ; it also 

 possesses a proper membrane, and a peculiar metamorphosis 

 of its contents. It cannot be demonstrated by direct exami- 

 nation, that the membrane is not merely a deposit (distinct 

 from the nucleus) from the surrounding fluids of the cell. 

 That it constitutes in its totality a distinct organ, is shown 

 by the power of the nucleus to propagate as an utricle. I 

 have pointed out the division of the nucleal vesicle in the 

 Hepaticse (Anthoceros,) and in the Phanerogamia (Trades- 



