286 BIOLOGY OF THE PROTOZOA 



importance has been attached to the relationship of gametes, and 

 fertilizations have been described as exogamous, endogamous, 

 autogamous, or pedogamous. Of these the third refers to self- 

 fertilization and the second and fourth to union of closely related 

 individuals. Such terms serve a useful purpose for descriptions 

 but are without significance in the matter of effective fertilization. 

 It is quite possible, however, that a brood of gametes from the 

 same gametocyte will have a common physical and chemical make-up 

 and will not be attracted to one another but will meet and fuse with 

 apparently identical gametes from another gametocyte. This 

 appears to be the case with Polystomellina crista according to Schau- 

 dinn (1903) and also of gregarines. The significance of ancestry 

 however, appears to be in the matter of mating rather than in that 

 of effective fertilization and belongs to the same group of phenomena 

 as the fact that sperm cells do not unite with sperm cells or eggs with 

 eggs. With Infusoria Maupas' conclusion has not been supported 

 by later observers. Calkins (1904) found that fully as many 

 conjugations between closely related forms of Paramecium caudatum 

 were fruitful as between forms of diverse ancestry, and one such 

 ex-conjugant from a closely-related pair, was followed through 379 

 generations by division. Similar evidence has been furnished 

 by isolation cultures of Didinium nasutum, Paramecium aurelia, 

 Paramecium burs-aria, Stylonychia sp., Blepharisma undulans, 

 Spathidium spathula, Oxytricha fallax, and Chilodon cucullus. 

 With Uroleptus mob His the protoplasm of one individual gave rise 

 to progeny which would conjugate whenever the proper conditions 

 were provided, and the 140 series derived from ex-conjugants from 

 such unions furnish ample proof that the conjugations were fruitful. 

 Such results indicate that Maupas' conclusion regarding the neces- 

 sity of diverse ancestry was incorrect. 



(b) Environment.^ One unmistakable conclusion can be drawn 

 from the many diverse observations and interpretations of the 

 conditions under which fertilization occurs in ciliates, viz., the pro- 

 toplasmic state with which conjugation is possible is induced in 

 large part, but not wholly, by environmental conditions. 



In practice the simplest way to obtain conjugations in ciliates 

 is the method adopted by Maupas. A pure culture of the organism 

 to be tested is allowed to multiply freely in a rich culture medium ; 

 a large number of these are then transferred to a smaller container 

 with enough of the original medium in which they had developed 

 to make it unnecessary to add fresh medium. In this second con- 

 tainer, conjugations will appear in from twelve to thirty-six hours 

 provided a mixed population is present or if the organisms are 

 mature. In a similar way conjugation tests are made at regular 

 intervals in all complete isolation culture work. Such tests have 

 been made with Uroleptus mobilis approximately every ten days. 



