2-2 HISTOLOGY. 



The .-eeond e>sential part of the cell is the nucleus. This 

 i- often invisible in the living cell when the nucleus and the 

 protoplasm have the same refractive power. They react differ- 

 ently, however, to certain reagents. For example, acetic acid 

 causes protoplasm to swell up and the nucleus to shrink. 



The nucleus is usually spherical or oval ; sometimes 

 horseshoe-shaped, ring-shaped, or branched. 



The nucleus often holds a definite relation to the size of the 

 eell. For example, the nuclei of unripe egg cells are very 

 large. 



As a rule we find one nucleus in each cell. Often, however, 

 there are more than one, and exceptionally their number may 

 he as great as one hundred (e. g., in the giant cells of bone- 

 marrow). Such multinucleated cells are called syncyt'mm. 



The cell nucleus is not a simple structure. We are able to 

 recognize in it at least two and often as many as six proteids 

 which are chemically and microscopically different, namely: 



1. Nuclein chromatin ; 



2. Paranuclein pyrenin ; 

 'I. Linin ; 



4. Lantauin ; 



5. Nuclear fluid (Kernsaft) : 



6. A mphi pyrenin. 



The first two seem to be essential elements of the nucleus. 



1. Chromatin (nuclein) is the most characteristic constituent 

 of the nucleus. It is demonstrated by its great capacity for 

 taking up stains, and is distinguished from the other substances 

 by the tact that it contains phosphoric acid. Chromatin occurs 

 in the nucleus in the form of granules, fine threads, or as a 

 network which forms the so-called chromatin network. 



Paranuclein (pyrenin) occurs in the form of a small 

 highly refractive sphere which forms the true nucleolus. 

 These nucleoli are to be distinguished from chromatic enlarge- 

 ments formed in the angles of the nuclear network. Pyrenin 

 is distinguished from eliroinatin mainly by physical properties. 

 It does not swell in water, dilute alkaline solutions, lime-water, 

 or salt solution. Chromatin, on the contrary, swells in such 



