510 ANIMAL NATURE OF DIATOMEjE. 



and multiplication, which takes place by division. Their 

 original formation is as yet unknown. The nuclei are 

 vesicles formed of albumen or fibrine, or perhaps of 

 caseine, containing, besides the nucleolus, an albuminous 

 liquid, oily drops, and elementary granules. The forma- 

 tion of nuclei is entirely obscure ; but it seems proved 

 that it cannot take place without the persistence of the 

 nucleoli. The nuclei are multiplied endogenously, and 

 in consequence of the multiplication of the nucleoli. 

 The contents of the nucleus collect round the nucleoli 

 originating from the division of the primitive nucleolus, 

 constitute their partial integument, and set them free by 

 the reabsorption of the maternal vesicles. The nuclei 

 are not only capable of uniform growth, and of multiplica- 

 tion, but also undergo numerous modifications, especially 

 conversion into fibre. The cells have walls soluble in 

 acetic acid, and are, therefore, composed of a combination 

 of proteine, either simple, or penetrated with horn, 

 gelatine, chondrine, calcareous salts, &c.; and they 

 contain, besides the nucleus, water, albumen, fat, ex- 

 tractive matter, salts, and other compounds peculiar to 

 the various organs. 



Whether the formation of the animal cell be endogenous 

 or exogenous, it requires the pre-existence of the nucleus, 

 upon which the membrane begins to appear, which by 

 degrees separates and dilates, surrounding it, and holding 

 it adherent to a point within its wall. 



More rarely, the formation of a cell takes place within 

 a collection of granules having a nucleus in its centre. 

 The gelatinous substance which superficially surrounds or 

 unites and amalgamates the granules, becomes coagulated, 

 so to speak, and is consolidated into a cellular membrane. 



These particulars are mostly taken from the work of 

 Kolliker, in which there may be found a critical analysis 

 of the physical and chemical theories proposed by authors 

 to explain the facts adduced. 



Page 16. — Nageli gives an ingenious explanation why 

 the multiplication of cells is always endogenous in 



