ANIMAL NATURE OF DIATOMUJG. 509 



after the primordial utricle and the nucleus have dis- 

 appeared. Whatever may be the mode in which the new 

 cells appear, we may assert, on good foundation, that tie 

 previous formation of a nucleus is constantly followe 1 

 by the primordial utricle, and in consequence by the 

 cellular wall. The uncertainty and variety are referable 

 to the first formation of the nucleus, to the contents which 

 remain confined, or are successively formed between the 

 nucleus and the primordial utricle, and to the mechanism 

 forming the cell wall. It is not yet decided whether this 

 origin of the cells is exclusively endogenous, or may not 

 even be exogenous. Recent inquiries as to the nature of 

 yeast, (Hygrocrocis, Leptomitus, &c.,) seem to prove that 

 nuclei alone, or, perhaps nucleoli, may separately give 

 origin to cells. In such a case the mucous granules would 

 be foreign to the new plant, and would contribute imme- 

 diately to its formation ; the nucleoli would be its germs. 

 Every one sees the difficulties that are to be encountered 

 in this research, the cautions that are to be used in 

 drawing conclusions from every fact observed, and the 

 necessity of advancing always from the known to the 

 unknown. Hence, in denominating the organic elements 

 which are before our eyes, we are not to commence with 

 what appears the most simple, but rather with that which 

 is decidedly organised and clearly characterised. 



In animals, besides the solid elementary granules 

 already referred to, there are small vesicles incapable of 

 ulterior growth and reproduction. They are simple 

 drops of oil enveloped in a stratum of coagulated 

 albumen, and may be imitated artificially. Other vesicles 

 are capable of growth, but not of multiplication. They 

 contain liquid albumen, and one or more oily drops, as, 

 for example, the vitelline vesicles. Perhaps they are 

 analogous to globules of chlorophyll in plants, or those 

 of starch. 



The nucleoli are vesicles similar to the preceding; 

 they consist of a contained fatty substance, and an 

 envelope of proteine. They are susceptible of growth 



