568 POPULAR SCIENCE MONTHLY. 



Its structure and chemical composition; its mode of origin; the part 

 which it plays in the formation of new cells, and its function in nutri- 

 tion and secretion have been investigated. 



When examined under favorable conditions in its passive or resting 

 state, the nucleus is seen to be bounded by a membrane which separates 

 it from the cell plasm and gives it the characteristic sharp contour. It 

 contains an apparently structureless nuclear substance, nucleoplasm or 

 enchylema, in which are embedded one or more extremely minute par- 

 ticles called nucleoli, along with a network of exceedingly fine threads 

 or fibers, which in the active living cell play an essential part in the 

 production of new nuclei within the cell. In its chemical composition 

 the nuclear substance consists of albuminous plastin and globulin; and 

 of a special material named nuclein, rich in phosphorus and with an 

 acid reaction. The delicate network within the nucleus consists ap- 

 parently of the nuclein, a substance which stains with carmine and 

 other dyes, a property which enables the changes, which take place in 

 the network in the production of young cells, to be more readily seen 

 and followed out by the observer. 



The mode of origin of the nucleus and the part which it plays in 

 the production of new cells have been the subject of much discussion. 

 Schleiden, whose observations, published in 1838, were made on the 

 cells of plants, believed that within the cell a nucleolus first appeared, 

 and that around it molecules aggregated to form the nucleus. Schwann 

 again, whose observations were mostly made on the cells of animals, 

 considered that an amorphous material existed in organized bodies, 

 which he called cytoblastema. It formed the contents of cells, or it 

 might be situated free or external to them. He figuratively compared 

 it to a mother liquor in which crystals are formed. Either in the 

 cytoblastema within the cells or in that situated external to them, the 

 aggregation of molecules around a nucleolus to form a nucleus might 

 occur, and, when once the nucleus had been formed, in its turn it 

 would serve as a center of aggregation of additional molecules from 

 which a new cell would be produced. He regarded, therefore, the 

 formation of nuclei and cells as possible in two ways — one within pre- 

 existing cells (endogenous cell-formation), the other in a free blastema 

 lying external to cells (free cell-formation). In animals, he says, the 

 endogenous method is rare, and the customary origin is in an external 

 blastema. Both Schleiden and Schwann considered that after the cell 

 was formed the nucleus had no permanent influence on the life of the 

 cell, and usually disappeared. 



Under the teaching principally of Henle, the famous Professor of 

 Anatomy in Gottingen, the conception of the free formation of nuclei 

 and cells in a more or less fluid blastema, by an aggregation of elemen- 

 tary granules and molecules, obtained so much credence, especially 



