328 CYTOLOGICA.L METHODS. 



employed in solutions of 40 to 50 per cent, strength. If it be 

 desired to remove all the chromatin from a nucleus the reac- 

 tion must be prolonged, sometimes to as much as two or three 

 days, especially if the operation be conducted on a slide and 

 under a cover-glass, which is the safer plan. 



It must be remembered that these operations must be per- 

 formed on fresh cells, for hardening agents bring about very 

 considerable modifications in the nature of chromatin, render- 

 ing it almost insoluble in ammonia, potash, or sodic phosphate, 

 &c. Hydrochloric acid, however, still swells and dissolves it, 

 though with difficulty. 



Partial digestion may render service in the study of the 

 chromatic elements of nuclei. Chromatin resists the action 

 of digestive fluids much longer than the albumens do ; so that 

 a moderate digestion serves to free the chromosomes from 

 any caryoplasmic granulations that may obscure them, whilst 

 at the same time it clears up the cytoplasm. 



In the last edition the term " nuclein " was used throughout this section 

 in all the places where the term " chromatin " has been used in the above 

 paragraphs. It is now Jmown that there exists a whole series of nucleins, 

 differing chiefly in respect of their richness in phosphorus and proteids. At 

 one end of the chain is nucleic acid, with 9 to 11 per cent, of phosphorus, and 

 without any proteid (this compound occurs in nature in the heads of sper- 

 matozoa) ; in the middle are what are generally termed the nucleins, con- 

 sisting of proteid with varying amounts of nucleic acid ; and at the other 

 extreme are nucleins which are nearly all proteid, containing only 0'5 to 1 

 per cent, of phosphorus, and are in fact the same substances which have 

 received the name of " nucleo-albumin ;" they may also be termed the 

 artificial plastins. 



These substances have both been isolated from the most diverse tissues of 

 the animal body, and have been prepared artificially. A corresponding series 

 of nucleins exists within the nucleus itself. There are those that contain 

 most nucleic acid ; these are readily soluble in alkalies, and precipitable with 

 difficulty by acid : chromatin is one of these. There are others more in- 

 soluble in alkalies and poorer in nucleic acid: these are the plastins, the 

 pyrenin of nucleoli being one of them. And there are others even poorer in 

 nucleic acid : these are the nucleo-albumins (which exist also in the cyto- 

 plasm) ; the paralinin, or nuclear sap, appears to be in part composed of 

 these, in part of phosphorus-free compounds. 



There appears to be some doubt whether chromatin is or is not nucleic 

 acid itself. The principal reactions in which it resembles nucleic acid are 

 given by HALLIBUBTON (Goulstonian Lectures on the Chemical Physiology 

 of the Animal Cell, 1893, p. 574 of the Eeport in the British Medical 

 Journal, No. 1681, March 18th, 1893, from which place also I have con- 

 densed the above remarks on the chemistry of the nucleins) as follows : 



