312 BIOLOGY AND ITS MAKERS 



investigators reached their conclusions independently 

 though there is great similarity between them. Although tl 

 credit for the first formulation of the law of germinal coi 

 tinuity does not belong to Weismann, that of the greatest 

 elaboration of it does. This doctrine of germinal continuity 

 is now so firmly embedded in biological ideas of inheritance 

 and the evolution of animal life that we may say it has become 

 the corner-stone of modern biology. 



The conclusion reached — that the hereditary substance is 

 the germ-plasm — is merely preliminary; the question remains, 

 Is the germ-plasm homogeneous and endowed equally in all 

 parts with a mixture of hereditary qualities? This leads 

 to the second step. 



The More Precise Investigation of the Material Basis of 

 Inheritance. — The application' of the microscope to critical 

 studies of the structure of the germ-plasm has brought 

 important results which merge with the development of the 

 idea of germinal continuity. Can we by actual observation 

 determine the particular part of the protoplasmic substance 

 that carries the hereditary qualities? The earliest answer 

 to this question was that the protoplasm, being the living 

 substance, was the bearer of heredity. But close analysis 

 of the behavior of the nucleus during development led, 

 about 1875, to the idea that the hereditary qualities are located 

 within the nucleus of the cell. 



This idea, promulgated by Fol, Koelliker, and Oskar 

 Hertwig, narrowed the attention of students of heredit) 

 from the general protoplasmic contents of the cell to tl 

 nucleus. Later investigations show that this restriction was, 

 in a measure, right. The nucleus takes an active part 

 during cell-division, and it was very natural to reach the 

 conclusion that it is the particular bearer of hereditary 

 substance. But, in 1883, Van Beneden and Boveri made 

 the discovery that within the nucleus are certain dis- 



