HEREDITARY RELATION IN UNICELLULARS 31 



life, which is at present insoluble. Analogies may be found in 

 methods of treating chemical molecules so that one gets at the 

 end of the operation twice as many molecules as one had to 

 start with ; or in the multiplication of crystals by breaking 

 them into fragments and placing them in solutions of the same 

 substance ; but, at the present time, these analogies are of no 

 particular service, since we do not understand the nature of 

 living matter. That a fragment of a unicellular's organisation 

 may, in an appropriate environment, reproduce an apparently 

 perfect replica of the original unit, is not in any way explained 

 by pointing out that there may be reproduction of like by like 

 in the case of crystals or chemical molecules. 



chr* 



Fig. 2. — Diagram of cell division (after Boveri). 

 chr. chromosomes, forming an equatorial plate; cs. centrosome. 



In slightly more complex cases there is a difference between 

 the two units into which the unicellular organism divides. 

 Thus, in the oblique division of the slipper animalcule {Parame- 

 cium), the one half goes off with the " mouth," the other has 

 none. In a short time, however, the mouthless half forms a 

 " mouth," and each half grows into a replica of the original. 

 But as the organisation of each half is essentially the same as, 

 and directly continuous with the organisation of the original 

 cell, the development of the halves into similar wholes presents 

 no special difficulty. Similar organisation and similar surround- 

 ings yield similar results. That an injured infusorian should 

 by re-growth repair its loss is an analogous phenomenon. Thus 



