SEX 



297 



of other plants are the 

 result of the successive 

 divisions of the proto- 

 plasm of some preexisting 

 cell. But the sporelike 

 cell of Spirogyra origi- 

 nated in the union of two 

 preexisting cells. This 

 conjugation product is 

 called a zygospore, which 

 means a spore produced 

 by a joining together ; it 

 may be called a zygote 

 for short. 



347. Gametes. The 

 cells that conjugate to 

 produce zygotes are called 

 gametes, which comes 

 from a Greek word mean- 

 ing " to marry" that 

 is, to join, or unite with. 

 In the case of the Spiro- 

 gyra, it is impossible to 

 tell, from the appearance 

 of the cells, which are to 

 become the resting, or 

 receiving, gametes and 

 which the moving, or 

 supplying, gametes. No 

 doubt there is some 

 difference in the chemical 

 conditions between the 

 two strings of cells, but 

 what the difference is 

 has not been found out. 



H 



FIG. 129. Conjugation in Paramecium 



There are two nuclei : a, the small (micronucleus); 

 and l>, the large (macronucleus) . A : Two individuals 

 become attached and their micronuclei begin to 

 divide. B : The half nuclei divide a second time. 

 Of the four units resulting, three are called polar 

 bodies, c, and the fourth is a germ nucleus, g, 

 which again divides. C : The germ nuclei are 

 interchanged, one of each pair passing over to 

 the opposite animal. Z>, completion of the inter- 

 change. , same, further enlarged. F, the active 

 germ nucleus fuses with the stationary one. G, same, 

 enlarged. In the meantime the macronucleus has 

 broken up and disappeared. After the restoration 

 of the micronucleus through fusion, /% the two in- 

 dividuals float apart. H ': The new micronucleus, d, 

 breaks up into two. /, each portion splits again. 

 /, after a third division. K\ Four of the 

 nuclei become new macronuclei and four remain 

 as the micronuclei. The rest of the protoplasm 

 divides and four individuals result, each with a 

 micronucleus and macronucleus. (From Calkins, 

 alter Hertwig, and Maupas) 



