Mating of Similar Parts 185 



fusorian Trichomastix, found in lizards. The flagellate 

 forms a small round cyst (Figure 41), in which the single 

 nucleus divides into two (A), the two nuclei coming to lie 

 one on each side of a large mass of reserve nutritive material. 

 Then each nucleus divides twice unequally, giving off two 

 small nuclei (Figure 41, C and D). [These two small nuclei, 

 commonly known as reduction nuclei, are absorbed and dis- 

 appear. The remaining two nuclei, which we can now, for 

 reasons to be clearly seen later, call half nuclei, move 

 toward each other (E), and finally unite (F). The process 

 of mating or of fertilization is now finished. 



Here again we have a case in which the two half nuclei 

 that unite have been developing continuously together, in 

 a single nucleus; they separate, then (after two more divi- 

 sions) reunite. And in this case, it is the same two nuclei 

 that have separated that come together again, save for the 

 fact that each has divided off two small nuclei. 



The same sort of thing occurs in many lower organisms. 



Figure 42. Two methods of conjugation in Spirogyra, after Walton, 

 1915. A, conjugation between adjoining cells of the same filament. 

 The contents of the cell to the right have passed into the other cell and 

 united with its contents. B, conjugation between the cells of different 

 filaments. From the filament above, the cell contents have passed into 

 the cells of the filament below. 



