REPRODUCTION IN UNICELLULAR ORGANISMS 255 



something new added, something which requires explanation ; we are 

 confronted with the first riddle of heredity. Simple growth does not 

 explain the phenomenon, for what has to be added to complete the 

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

 accessory apparatus from any that the halves themselves possess. It 

 in no way affects this state of matters that in the normal process 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 mere 

 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 expressed 

 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 a.re 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 Aeinetee. In relation to the phenomena of inheritance 

 the problem raised by the equal division of the Infusorians repeats 

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

 take place several times, or many times in succession, so that from one 

 animal a large number of small pieces of the same size may be 

 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 that 

 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 found in 

 many Infusorians. Thus the holotrichous species, Holophrya mvltifiliis 

 (Fig. 6i), reproduces by first becoming enclosed in a cyst or capsule, 



