CHEMICAL BASIS OF THE ANIMAL BODY. 1205 



source from which it is obtained. It is characterized by its 

 somewhat greater resistance to those decomposing agents whose 

 action on keratin has been already described. 



Chitin. 



Although it is not found as a constituent of any mam- 

 malian tissue, this substance composes the chief part of the 

 exoskeleton of many invertebrates. It is by many regarded 

 as the animal analogue of cellulose of plants, and from this 

 point of view it possesses considerable morphological interest. 

 The most convenient source from which it may be prepared is 

 the cleansed exoskeleton of crabs or lobsters. This is first 

 thoroughly extracted with dilute hydrochloric acid and caustic 

 potash, after which it is treated with boiling alcohol and ether, 

 and may be finally completely decolorized by the action of per- 

 manganate of potash. It is a white amorphous substance w^hich 

 often retains the shape of the integument from which it has 

 been prepared. It is insoluble in any reagents other than con- 

 centrated mineral acids, such as sulphuric or hydrochloric. The 

 immediate addition of water to these solutions probably repre- 

 cipitates the chitin in an unaltered form. When heated with 

 concentrated hydrochloric acid it is decomposed into glucosa- 

 mine and acetic acid, of which the former is the characteristic 

 product. A similar decomposition is observed when sulphuric 

 acid is employed. 



Glucosamine (C 6 H n 5 .]SrH 2 ). Crystallizes from alcohol in fine 

 needles, is dextrorotatory, and reduces Fehling's fluid to the same 

 extent as does dextrose, but is not fermentable. By treatment with 

 nitrous acid a carbohydrate (C 6 H 12 6 ) is obtained which also reduces 

 cupric oxide, but is similarly unfermentable. This is doubtless the 

 substance which led to certain erroneous statements as to the pro- 

 duction of a true dextrose from chitin. 



Nuclein. 



The nuclei of cells, both animal and vegetable, differ dis- 

 tinctly in chemical composition from the remaining substance 

 of the cells. As a result of this difference it is possible to sepa- 

 rate the nuclei approximately by various means from the adja- 

 cent cell-substance, and the name nuclein was given originally 

 to the material of which the nuclei or parts of nuclei thus iso- 

 lated seemed chiefly to consist. As research proceeded the fact 

 soon became apparent that the various investigators were deal- 

 ing with distinctly different substances under the one name 

 nuclein, and it is only recently that our knowledge of the 

 nucleins has begun to take a more definite and systematic 

 shape. It is however still in a too transitional condition to 



