458 THE AMERICAN NATURALIST. [VOL. XXXIX. 



involving the development of a vegetative structure before the 

 period of sporogenesis. From the studies of Wolfe (: 04) we 

 know that the sporophyte portion of Nemalion (the cystocarp) 

 contains nuclei with double the number of chromosomes (about 

 1 6) present in the gametophyte (about 8) and that the period of 

 chromosome reduction is apparently just previous to the devel- 

 opment of the carpospores (sporogenesis). Williams (:O4a and 

 b) has recently determined that the asexual plant of Dictyota is 

 a sporophyte generation with double the number of chromosomes 

 (32) found in the sexual plant (16). The reduction occurs here 

 during a rather long period of preparation on the part of the 

 nucleus in the tetraspore mother-cell and the reduced number 

 appears in the two mitoses that form the tetraspores. These 

 events closely parallel those in the spore mother-cell of higher 

 plants and will be discussed further under " Sporogenesis." 



William's (: O4b) account of gametogenesis in Dictyota is the 

 most complete that we have for any thallophyte. The oogonia 

 and antheridia are cut off from a stalk cell by a mitosis which 

 presents 16 chromosomes, the number characteristic of the 

 gametophyte. The contents of the oogonium forms a single 

 egg and consequently presents no mitotrc phenomena. The 

 antheridium develops over 1500 sperms thus exhibiting a large 

 number of successive divisions. These all show 16 chromosomes 

 and the mitoses are typical, not differing in any essential from 

 the division in the stalk cell. The entire absence of mitoses in 

 the oogonium and the great number in the antheridium are 

 striking facts which show that no especial significance can be 

 attached to nuclear divisions within sexual organs of this type. 

 There is no place for reduction phenomena within these sexual 

 organs and none precede their development. 



These studies of Williams and Wolfe justify us in expecting 

 that other thallophytes will support their discoveries that the 

 product of the sexual act will have a fusion nucleus with double 

 the number of chromosomes present in the sexual plant (game- 

 tophyte) and that reduction phenomena may be expected to fol- 

 low the sexual act and not precede it as in animals. In such 

 thallophytes as have no sporophyte generation we may suppose, 

 as Strasburger ('94a) suggested, that the number of chromo- 



