7 6 



FECUNDATION ; NON-MOTILE ISOGAMETES. 



the others being disorganized ; and in the desmids only two out of the 

 four in the germination of the zygote develop into perfect cells. 



It is not known whether the reduction in the number of chromo- 

 somes, if a reduction actually occurs in either desmids or diatoms, is 

 in any way associated with the nuclear divisions in question, as has 

 been assumed by some authors (see Wilson, " The Cell," p. 198) ; 

 consequently, in the light of our present knowledge, it cannot be said 

 with any certainty that these nuclear divisions represent a preparation 

 for the sexual act, that in the diatoms taking place just before fecun- 

 dation while in the desmids it occurs at the beginning of an ontoge- 

 netic development. 



BASIDIOBOLUS. 



A sexual process similar to that in the Conjugates is found in 

 Basidiobolus, one of the Phy corny cetes. I have selected Basidio- 



FIG. 24. Fusion of the sexual nuclei in Rhtp*- 



loctiagibta.(Micr Klebahn.) 



I, the two young zygotes or auxospores have elon- 

 gated and begun to assume form of adult; sexual 

 nuclei now in contact. 



J, middle portion of two auxospores, each with a 

 fusion nucleus. 



bolus ranarum because of its close re- 

 semblance to certain Mesocarpacece, 

 especially Mougeotia, both in structure 

 (the cells possess only one nucleus) and 



in the sexual process, and because the development of the sexual 

 organs and the fusion of the gametes are well known in detail. Sex- 

 uality in this genus has recently been subjected to a critical study by 

 Fairchild ('97), whose results form the basis of the following account. 

 Two neighboring cells of a filament send out near the transverse 

 wall a beak-like protuberance, into which the nuclei of the respective 

 cells pass (Fig. 25, A). 



The nucleus in each of the protuberances now undergoes a karyo- 

 kinetic division, which is followed by the formation of a transverse 



