REPRODUCTION IN UNICELLULAR ORGANISMS 



:z.jo 



something new added, something which requires explanation : we ar. 

 confronted with the first riddle of heredity. Simple growth , .- 

 explam the phenomenon, for what has to be added to complete I 

 halved portions has a different structure, a different form, different 

 accessory apparatus from any that the halves themselves poss li 



in no way affects this state of matters that in the normal pr of 



division in Infusorians the formation of the new mouth and peristome- 

 region begins before the halves have actually separated, for even if a 

 Stentor be cut in two artificially the cut halves form complete animals. 

 And, indeed, a Stentor may be cut into three or four pieces, and in 

 certain conditions each piece will develop into a complete animal. 

 These pieces must therefore possess something more than the mi 

 power of growth. We shall try later on to discover whether this 

 marvellous invisible transmission of characters, this implication of the 

 whole in each of the parts, can be in any way theoretically expiv^r.l 

 and included in our scheme of conceptual formulation. 



Now that we have become familiar with these facts it will no 

 longer surprise us to learn that the reproduction of unicellular animals 

 does not always depend on equal division, but that unequal spontaneous 

 divisions are also possible, so that one or several smaller portions of 

 the cell-body, containing a portion of the cell-nucleus, can separate off 

 from the mother-animal. This occurs especially among the suctorial 

 Infusorians or Acinetaa. In relation to the phenomena of inheritance 

 the problem raised by the equal division of the Infusorians repeats 

 itself, and it is in no way affected by the fact that equal division can 

 take place several times, or many times in succession, so that from on*- 

 animal a large number of small pieces of the same size may '»«■ 

 produced in rapid succession. The characteristic marks of the mother- 

 animal are not infrequently lost sight of, wholly or partially, when 

 this occurs, and the divided portions seem to consist of a homogeneous 

 cell-body and nucleus; but they possess the power of regenerating 

 themselves, or of developing, if the expression be preferred, into 

 animals similar to the maternal-organism. Such divided portions 

 might very well be called germs, only it must not be forgotten thai 

 the relation of the mother-animal to these germs is a different one 

 from that of a higher animal or plant to its germ-cells ; the unicellular 

 animal breaks up by continued division into these 'germs,' while the 

 Metazoon continues its individual existence unimpaired by the pro- 

 duction of its germ-cells. 



The beginning of a so-called 'spore-formation' is to be Pound in 

 many Infusorians. Thus the holotrichous species, Holophrya m idtiJUiis 

 (Fig. 61), reproduces by first becoming enclosed in a cyst or capsule, 



