SIGNIFICANCE OF THE SEXUAL PROCESS. 55 



most highly organized plants, at the close of their life-cycle, to the unicellular 

 condition: in a word it is the repetition of phylogeny in ontogeny (1. c., 

 P. 



This theory of reduction must still be regarded as a very helpful 

 working hypothesis, finding its greatest application in the higher 

 plants. In^the lower cryptogams the theory is confronted with facts, 

 many of which seem at present to be quite at variance with it. The 

 product of fecundation in the Thallophyta as a rule does not give rise 

 to a definite organism representing the asexual generation, and it is not 

 known at which point it} the life-cycle that reduction takes place. It 

 has been suggested that reduction may take place during the germina- 

 tion of the zygote or oospore. Conclusions respecting the time of 

 reduction in the lower cryptogams have been drawn chiefly from the 

 phenomena of certain cell-divisions that seem to be analogous with 

 divisions which follow the reduction in higher organisms, and not 

 from an actual determination of the number of chromosomes. On 

 account of the many difficulties in counting, the number of chromo- 

 somes is known in only a very few algae and fungi, and our knowledge 

 on this subject is so meager with respect to these plants that the few 

 definite facts that have been obtained, although apparently at variance 

 with the theory, may not as yet be considered as offering very serious 

 objections to it. 



If the reduction in the number of chromosomes signifies what is 

 attributed to it by the theory, it is possible, in the light of facts that 

 have been observed in such algae as Fucus and Dictyota, that what is 

 considered the sexual generation in the Thallophyta may not be 

 homologous with the gametophyte of higher plants, assuming that 

 homology is based upon the number of chromosomes. Farmer and 

 Williams ('96, '98), and Strasburger ('97) have found that the reduced 

 number of chromosomes in Fucus appears in the oogonium, while in 

 vegetative cells of the thallus twice that number is present. Stras- 

 burger finds that in the first nuclear division in the oogonium the 

 reduced number appears, fourteen to sixteen having been counted, 

 and this number persists throughout the two succeeding mitoses. In 

 vegetative cells of the thallus, which is regarded as the gametophyte, 

 the number is not far from thirty. In Dictyota I have found the 

 reduced number (sixteen) of chromosomes in the first nuclear division 

 of the tetraspore mother-cell, while in the vegetative cells of the thallus 

 bearing the tetrasporangia about twice that number was counted. 

 Whether in the nuclei of plants arising from tetraspores the reduced 



