CHAPTER XII 



Heredity General observations Darwin's theory of pangenesis and 

 Weismann's theory of the continuity of the germ plasm The nucleus 

 as the bearer of inheritable characters. 



WHEN we study the life histories of the unicellular Protista 

 we find ourselves face to face with the problem of heredity in 

 its simplest form. The Amoeba divides into two parts by simple 

 fission, the division of the cell body being preceded by that of 

 the nucleus. The two daughter cells exactly resemble one 

 another, and, except in point of size, also resemble the parent, 

 while the latter ceases to exist as an individual in the very act of 

 reproduction. Here we may suppose that we are dealing with a 

 division which is qualitative as well as quantitative, that every 

 organ possessed by the parent cell is divided into two similar 

 parts and the total inheritance thus fairly apportioned between 

 the offspring, which will therefore be exactly similar to one 

 another and will need only to feed and grow in order to become 

 exactly similar to the parent. 



The young Amoebae may be supposed to resemble the parent 

 because they arise by duplication of the parent and of its organs j 1 

 moreover, there is a perfect continuity of the living substance, or 

 protoplasm, of which the body is composed from one generation 

 to the next, and the whole of the body of the parent is used up 

 in providing the bodies of the offspring. Thus, although the 

 individuality oflthe parent comes to an end, the Amoeba never 

 dies, for there is never anything left over to die, and, barring 

 accident, it goes on multiplying for ever. We have here a 

 typical illustration of the so-called immortality of the Protista. 



The case is very different amongst the higher plants and 

 animals. Here, as we have already seen, each individual starts 



1 We cannot, however, say this of all Protista, for in many cases the division 

 takes place asymmetrically and entirely new organs have to be formed by one or both 

 of the daughter cells, as, for example, in the transverse fission of Bodo (Fig. 38, D F). 

 In such cases it looks as if the nucleus might be the real seat of the inherited 

 tendencies, and as if it were able, by its influence, to mould the daughter cell into 

 the form of the parent. 



