192 



EVOLUTION, GENETICS, AND EUGENICS 



organism and its special function. Thus a nerve cell and a muscle 

 cell of a given organism may be utterly different in general form and 

 function, but the nuclei of both are the same. Since the specific heredi- 

 tary materials of the individual are possessed equally by all of the cells 

 of that individual, it follows that the differences in cells and tissues 

 must be solely cytoplasmic. How these differences arise is a problem 



Central bodies 



Golgi bodies 



Plasraosome or the 

 true nucleolus 



Basichromatin 



Oxychromatin or 

 linin 



Karyosome or chro- 

 matin-nucleolus 







■hue wall or membrane 

 Plasma-membrane 



Cortical layer 

 Plastids 



Chondriosomes 



Vacuole 



Passive metaplasmic or 

 paraplastic bodies 



Fig. 40. — Diagram of a typical cell. Its cytoplasmic basis is shown as a granu- 

 lar mesh work or framework in which are suspended various differentiated granules, 

 fabrillae, and other formed components. (From E. B. Wilson.) 



of individual development rather than one of racial evolution. Many 

 of the slow and steady trends in evolution, however, are believed by 

 some authorities to be the result of general hereditary changes in the 

 cytoplasm, but there is a great deal of evidence favoring the conclusion 

 that the nucleus is the chief organ of heredity and of variation in the 

 cell. 



Details of the nucleus. — The nucleus, except during the process of 

 mitotic cell division, is surrounded by a definite membrane, the nuclear 

 membrane, which separates the clear nuclear sap from the cytoplasm 

 outside the nucleus. During the resting stage, between two cell divi- 

 sions, there is a network of linin fibers running throughout the nucleus. 



