112 Regulation of Size in Unicellular Organisms 



Its causes. The cause of this assortation of sizes 

 in mating is not fully known. Pearl ('07) believed it 

 was due to the requirement that the oral grooves of 

 the two pairing individuals must fit in order to unite 

 fully. Jennings ('11) made more complete observa- 

 tions and confirmed this belief, observing that animals 

 which did not fit upon coming together, often sepa- 

 rated and sought other mates. 



Both Pearl and Jennings sought for evidence of a 

 change in length during the process of union in con- 

 jugation, but demonstrated that such did not exist for 

 Paramecium. But in Chilodon, Enriques ('08) found 

 that one individual of each conjugating pair did 

 shorten during the process of pairing. On the average 

 the shortening in length amounted to 18 per cent. The 

 shorter length was kept by the ex-conjugant, but not 

 by its progeny, if one may judge from the variabilities 

 of size measured among the non-conjugants. It is not 

 clear whether the breadth and the thickness of a con- 

 jugating individual decreased with the length; it can- 

 not be definitely concluded that the volume of the body 

 diminished. 



It is probable that similarity of size is a mechanical 

 necessity for mating. Whether this is the only factor, 

 in such species as show assortation, is uncertain. It 

 must be remembered that there are some species, even 

 among ciliates, in which gametes are differentiated 

 that are always of very diverse sizes. Such gametes 

 have no difficulty in uniting. 



2. The Effects of Conjugation 



Before conjugation. Some observations indicate 

 that in the period just before conjugation there occur 

 modifications of size which are distinct from the in- 

 hibition of growth at mating. But, again, it is not 

 proven that these are essential accompaniments of con- 



