II 4 INHERITANCE 



The problem is to explain how it is that in 

 the Metazoa, one particular cell, the ovum, should 

 have acquired and retained this special power of 

 transmitting the characters of the entire animal. 

 This problem Weismann proceeds to attack. He 

 calls special attention to the general agreement 

 among competent observers, that the part of the 

 cell directly concerned in the transmission of 

 hereditary features is the nucleus ; and he brings 

 forward observations of his own in support of this 

 view. He assumes the presence in the nucleus of 

 a special substance to which he gives the name 

 germ-plasma, and to which he supposes the power 

 of hereditary transmission to be confined. He 

 maintains that this germ-plasma is of exceedingly 

 complex structure, and that it has the power of 

 indefinite growth without loss of its essential 

 characters. He further supposes that the germ- 

 plasma of an egg is not wholly employed in build- 

 ing up the body of the embryo, or young animal, 

 but that a certain portion of it remains unchanged, 

 and produces the germ cells of the succeeding 

 generation. In this way the germ-plasma is 

 supposed to pass unchanged from one generation to 

 another, and this continuity of the germ-plasma is 

 regarded by Weismann as the fundamental cause of 

 heredity. It cannot be said that this explanation is 

 a satisfactory one. In the first place it is not 

 really an explanation of inheritance at all ; for 

 unlike Darwin's theory of Pangenesis, it does not 

 attempt to explain the actual modus operandi of 

 inheritance, but merely localises the power of 



