THE MECHANISM OF LIFE 505 



vital phenomena were regarded as taking place in the protoplasm, 

 and no one knew what to do with the nucleus ; hence the latter was 

 considered unessential and received little attention. 



It is psychologically interesting and a characteristic phenomenon 

 in the history of human thought that the knowledge of the truth 

 swings to either side of the middle point before it comes to a stand- 

 still at the latter. An extreme view, which in the course of time 

 proves to be untenable, causes a swing to the opposite extreme, and 

 the true medium is found only gradually by means of a healthy 

 reaction. Thus it was with the cell-doctrine. 



After it was found that the nucleus undergoes profound changes, 

 especially in the reproduction of the cell by division and in the 

 fertilisation of the ovum, while the protoplasm remains apparently 

 quiet, the original idea of the dominant role of the protoplasm 

 changed to the opposite one of control by the nucleus. The latter 

 regarded the nucleus as the essential bearer of the cell-life, and the 

 protoplasm as performing merely an accessory function. What in the 

 earlier cell -doctrine was ascribed exclusively to the protoplasm, in the 

 later one was ascribed exclusively to the nucleus. During the last 

 few years a healthy reaction against this swing to the other extreme 

 is beginning to make itself felt. 



It is not possible to examine all the individual facts, relative to 

 the function of the nucleus and the protoplasm, that have recently 

 been brought together. It will suffice to indicate some of the more 

 important observations and experiments that have led to important 

 deductions. 



The idea that the nucleus plays a dominant role in the cell has 

 obtained at the present time wide acceptance and has been 

 expressed in various forms. One prominent view is defended by 

 eminent investigators, such as Weismann, Hertwig, Boveri and 

 others, and takes special account of the remarkably complex and 

 regular changes that the newer morphology has demonstrated in 

 the nucleus in connection with the phenomena of fertilisation and 

 division of the ovum. The essence of this view is that the nucleus 

 is the bearer of certain substances called hereditary substances, that 

 hereditary transmission takes place by means of a transference of 

 these substances to the descendants, and that the protoplasm 

 contains no substances necessary to heredity. 



The fact that in the fertilisation of the ovum by the spermato- 

 zoon only a very small quantity of protoplasm is transferred by 

 the latter to the descendants, since the spermatozoon consists in by 

 far the greater part of nuclear substance, induced biologists to 

 neglect completely this small quantity of protoplasm, and to ascribe 

 the transference of the paternal characteristics to the descendants 

 exclusively to the nucleus of the spermatozoon. This assumption 

 appeared the more probable because the small mass of protoplasm 

 in the spermatozoon, which is contained chiefly in the flagellum, 



