592 FERTILIZATION 



of the other member, leaving behind only a shrunken remnant of the 

 donor, which then detaches itself from the recipient and dies. This 

 process is functionally very similar to that which occurs in the Vorticelli- 

 dae, while the differences in structural details serve as a connecting link 

 to typical ciliate conjugation. 



Sexual phenomena seem to be fairly common in the Sarcodina, but 

 they are not so characteristic of the group as was thought by many of 

 the earlier workers. 



In spite of the many reports of sexual stages in Amoeba proteus, sev- 

 eral recent investigators have failed to observe any type of reproduction 

 other than fission. Johnson (1930), whose article includes a review 

 of the literature, believes that parasites and aquatic fungi have led to 

 many misinterpretations of the life cycle of Amoeba proteus. Liesche 

 (1938) carried A. proteus through 800 generations and observed no 

 sexual stages and no cysts. It is quite possible that there is a sexual 

 stage that occurs only at long intervals, or only under conditions which 

 ordinarily do not obtain in the laboratory. It is obvious that if sexual 

 stages occurred very often in this form, it would be reported more 

 frequently and more convincingly than it has been, in view of the fact 

 that this species is cultivated and studied so constantly in scores of 

 biological laboratories. For instance, at the University of Minnesota 

 A. proteus has been cultivated continuously for nine years, during which 

 period thousands of observations have been made and several hundred 

 permanent slides have been prepared from time to time. In spite of 

 this prolonged search, no sexual stages have ever been found. Binucleate 

 forms, presumably early dividing stages, have been observed frequently, 

 but nothing suggesting gamete formation has ever been noted. It is 

 true that sexual stages could have occurred and escaped observation, for 

 examinations have not been made daily, but it seems reasonable to sup- 

 pose that they would have been discovered at some time if they occurred 

 at any but the rarest intervals. 



However, Jones (1928) confirms the earlier work of Calkins (1907) 

 and others, with descriptions and photomicrographs of gamete forma- 

 tion by fragmentation of the primary nucleus. He further claims that 

 fertilization is accomplished by means of flagellated gametes. 



A skeptic might point out that his photomicrographs of gametes and 

 zygotes (his Figs. 13, 15, Plate 11) are strikingly similar to the figures 



