THE PHYSICAL BASIS OF HEREDITY. 151 



magnification, it is found to consist essentially of a 

 delicate network or system of threads, the " linin net- 

 work," bearing granules of a substance rich in phos- 

 phorus, which, from its affinity for certain staining 

 fluids, has been termed " chromatin." One or more 

 rounded structures, behaving in certain definite ways 

 toward reagents, may also be found in the nucleus. 

 These, the " nucleoli," are probably not always of the 

 same nature in different cells, and their significance is 

 at present much less clearly understood than is the case 

 with the other nuclear structures. Filling the meshes 

 of the nuclear network is found a clear semi-fluid ma- 

 terial, the " karyolymph," and a more or less clearly de- 

 fined wall, the nuclear membrane, incloses the nuclear 

 substances and separates them from the cytoplasm. 



In 1876 Van Beneden announced the discovery of a 

 minute rounded body at the poles of the spindle in the 

 dividing eggs of Dicyemids, which has since been found 

 in nearly all kinds of animal cells, both in division and 

 in the " resting condition," and may probably be re- 

 garded as of universal occurrence, so far at least as 

 animal cells are concerned. In plants, however, it has 

 thus far been identified with certainty in but few forms. 



To this structure Boveri, in 1888, gave 



The centrosome. , ^ ,, ,, , 1 . 



the name of " centrosome, and showed 



it to be a cell organ of probably constant occurrence 

 and of the greatest significance in cell multiplication. 

 It often lies in a more or less specialized area of the 

 cytoplasm, the " attraction sphere," or " archoplasm," 

 near the nucleus, but in some forms it is doubtless with- 

 out any such surrounding structure. 



The foregoing paragraphs must be taken merely as 

 the briefest outline of cell structure. It would far ex- 

 ceed the limits of this article to attempt to discuss the 

 finer detail of the subject, or to enter upon the many 



