FERTILIZAWtX 



289 



(spermatium) comes in contact with a prolongation (trichogyne) of the 

 female sex organ (carpogonium). Solution of the intervening walla allows 

 the nucleus of the spermatium to pass into the 1 richogyne and down to I lie 

 female nucleus in the base of the carpogonium. In Polysiphonia we 

 have one of the few cases among lower plants in which the fusion of the 

 sexual nuclei has been minutely described. According to Yamanouchi 

 (1906) the male nucleus, by the time it has reached the female nucleus, 

 has resolved itself into a group of 20 chromosomes (Fig. 115, A). In i his 



: 



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 .. . 



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 i :■■'".•'■■ "-'■' *. 



V 



Fig. 115. 



A, fertilization in Polysiphonia. Group of male chromosomes about to enter female 

 nucleus. (After Yamanouchi, 1906.) B, fertilization in Albugo Candida. Female nucleus 

 lying in center of ooplasm near the " ccenocentrum " (larger dark body. I A ntheridial tube 

 about to discharge a male nucleus; another male nucleus in neck of tube. Additional nuclei 

 in periplasm surrounding the ooplasm. (After Davis, 1900.) 



condition it enters the female nucleus while the latter is yet in the reticu- 

 late state. Soon the female reticulum becomes transformed into 20 

 chromosomes, which arrange themselves with the 20 paternal chromo- 

 somes upon the spindle as the fusion nucleus divides. 



Fungi. — In the Phycomycetes sexual reproduction occurs in I wo princi- 

 pal forms, which serve to divide the group into two main divisions: 

 Oomycetes and Zygomycetes. 



In the Oomycetes the cytologica! phenomens are best known in the 

 Peronosporales and Saprolegniales. In the former there is differentiated 

 in the oogonium a single large egg Into which the contents of an antheri- 

 dium are discharged through a penetrating tube. In Albugo Writ and 

 A. portulaccce (Stevens 1899, 1901) the egg has a large number of nuclei, 



19 



