308 SEX IN MICROORGANISMS 



tip. Not infrequently the tips of several cilia of two mating animals 

 all adhered together to form a tight knot at the point of union. Even 

 single isolated cilia prepared by sonic treatment of reactive formalin- 

 killed animals adhered to living animals of opposite type. Again the 

 point of attachment involved one end of the isolated organelle. AVhen 

 tension was placed upon two joined cilia, they drew apart but re- 

 mained attached by a fine, apparently elastic, thread which finally 

 broke. This is believed to represent stretching and final breaking of a 

 cilium sheath. If such a break did not occur at the oriq-inal point of 

 union of the cilia, a piece of the cilium surface membrane would 

 necessarily be transferred from one animal to another. This would 

 readily account for Sonneborn's (1937, 1942b,c) observation that a 

 Paramecium of one mating type can clump with another of the same 

 mating type after it has first clumped with one of the opposite type.* 

 Unfortunately, these observations, though quite suggestive, do not 

 exclude other regions of the cilium or even the pellicle from participa- 

 tion in the mating reaction. The animals were necessarily under con- 

 siderable compression when observed. Furthermore, the behavior of 

 individual mating cilia could be studied only on very loosely united 

 animals or at regions where the pellicles of the mates were relatively 

 far apart. These observations (Metz and Pitelka, unpublished) leave 

 no doubt that the cilium surface possesses mating-type substances, 

 and that further study should be directed toward the structure of 

 these organelles, particularly their tips. 



Several workers have examined paramecium cilia with the elec- 

 tron microscope, but none of these studies has revealed any mating- 

 type-specific organization. Jakus and Hall (1946) found that cilia 

 contain a number of fine fibrils, but these workers were unable to 

 detect a membrane or limiting sheath about the cilium. Recent stud- 

 ies, however, leave no doubt that such a structure forms the limiting 

 boundary of the cilium. Such a membrane has been observed in sec- 

 tioned cilia by Lansing, Hillier, and Sonneborn (unpublished), and 

 its presence has been confirmed (Heilbrunn, 1952; Wichterman, 

 1953) in material prepared by the COo-critical-point method (Ander- 



*Sonneborn's observation has been confirmed repeatedlv (Metz, unpub- 

 lished) in mixtures of living and lyophilized (Metz and Fusco, 1949) P. cal- 

 kinsi of opposite type. The living and lyophilized animals clump on mixing, 

 and shortl)' thereafter numbers of living animals mav be seen adhering to one 

 another. 



