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THE AMERICAN NATURALIST [Vol. XLIV 



We have from the investigations of Williams 9 an ac- 

 count of the cytology of Dictyota throughout the critical 

 phases of its life history. There are three forms of this 

 alga, the male and female sexual plants and an asexual 

 plant which develops spores in groups of four (tetrads) 

 within a tetrasporangium. Earlier research of Mottier 

 indicated clearly that the tetraspore mother-cell is the 

 seat of chromosome reduction, and Williams by studies 

 of the sexual plants as well as the asexual was able to 

 present convincing cytological evidence of an alternation 

 of generations. 



Williams found that the sexual plants were character- 

 ized by nuclei with sixteen chromosomes and that this 

 number was passed on to the eggs and sperms respec- 

 tively. Fertilization takes place following the discharge 

 of the gametes, which have the peculiarity of being de- 

 veloped in fortnightly crops, ''each crop being initiated a 

 little before the lowest neap tide, and arriving at matur- 

 ity about the period of the highest succeeding spring 

 tide". The motile sperms gather about the eggs which 

 for a short time exert a strong chemotactic influence. 

 Following the union of the gamete nuclei, a second nu- 

 cleole appears in the fusion nucleus which Williams be- 

 lieves to be the chromatin brought by the sperm. The 

 first mitosis in the egg presents thirty-two chromosomes 

 at the equatorial plate, and this is believed to be the first 

 mitosis of a sporophyte generation represented by the 

 asexual plant and terminating with the development of 

 the tetraspores. 



The spindle poles of the first mitosis in the eggs ap- 

 pear to arise by the division of an aster the two poles of 

 which separate until they come to lie on opposite sides of 

 the nucleus. It is an interesting fact that unfertilized 

 eggs begin a parthenogenetic development, but the spin- 



