LECTUKE XIII 



REPEODUCTION IN UNICELLULAR ORGANISMS 



.r.^f?^^™'^"^"*'"''.^'!'''''''''"'-^"'^™'*^'*-^'^ Infusorians-Divisions following one 

 another in immediate succession-Formation of germ-cells in the Metazoa-CoStrast 

 B^t-l^'™"r f 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 unieellulars— because 

 their structure, as far as we can see Avith 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 

 obviously inherited, but in reality it is 

 potentially heritable, since the structure 

 of the divided portions involves the 

 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 Amceba (Fig. 59) we might picture the process of 

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

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



Fia. 59. An Amceba : the pro- 

 cess of division. A, before the 

 beginning of the division. B, 

 the nucleus divided into two. 

 C, the two daughter-Amoebfe. 

 Magnified about 400 times. 



