LECTURE XIII 



REPRODUCTION IN UNICELLULAR ORGANISMS 



Reproduction by division — In Amoebae — In Infusorians — Divisions following one 

 another in immediate succession — Formation of germ-cells in the Metazoa — Contrast 

 between germ-cells and body-cells — Potential immortality of unicellular organisms — 

 — Beginning of natural death— Budding and division in the Metazoa. 



We wish to consider the reproduction of organisms with special 

 reference to the problem of heredity, and it is most instructive 

 to begin with the lowest forms of life — the unicellulars — becau 

 their structure, as far as we can see with the instruments at our 

 command, is very simple, and, what is 

 even more important, is relatively homo- 

 geneous. 



Suppose that there are bacteria-like 

 organisms of quite homogeneous structure, 

 and that these multiply by simply dividing 

 into two, each rod-like creature dividing 

 transversely in the middle of its length, 

 the two halves would represent indepen- 

 dent daughter-organisms, whose structure 

 would correspond exactly with that of 

 the mother-organism, could not indeed in 

 any way deviate from it, and conse- 

 quently would take over all its characters, 

 that is, would inherit them. The size of 



body is the Only feature which is not Fig 59. An Amceba : the pro- 

 JKJ v i0 v±iK " «/ m ... cess of division. A, before the 



obviously inherited, but in reality it is beginning of the division. 



potentially heritable, since the structure £\™*£ £S£££J32; 

 of the divided portions involves the Magnified about 400 times, 

 capacity and the limits of their possible 



growth. Moreover, the size of body is not invariable in any species 

 a particular size is only reproduced under similar conditions of 

 development. Inheritance here consists simply in a continuation 

 of the mother-organism into its two daughter-cells. 



Even in an Amoeba (Fig. 59) we mi g ht P icture the P r0CeSS of 

 inheritance as equally simple, though in so doing we should probably 

 be making a fallacious inference, for the structure of these lowest 



