248 STRUCTURE AND LIFE HISTORIES 



vegetatively alike. Obviously we would expect all the 

 cells of the gametophyte to bear the reduced or haploid 

 number of chromosomes, and all the cells of the zygote or 

 sporophyte, the diploid number. Such is the case. Fig. 

 182, C, shows a section of an oogonium with the nucleus 

 in its first division, and the number of chromosomes (16) 

 may be easily counted. In Fig. 182, A, is shown the divid- 

 ing nucleus of the antheridium also with 16 chromosomes. 

 In the fertilized egg (Fig. 182, B) the diploid number (32) 

 may readily be counted. Since Dictyota is dioecious, we 

 must infer that the tetraspores, though looking alike ex- 

 ternally, are fundamentally different internally, since part 

 of them give rise to antheridial or male plants^ and part 

 to oogonial or female plants. 



234. Reduction. — Since all the cells of the sporophyte 

 (tetrasporic plant) contain the diploid number of chromo- 

 somes, and all the cells of the gametophytes the reduced 

 number, the question naturally arises, where does re- 

 duction take place? It occurs in the two divisions that 

 result in the formation of the tetraspores; each of the 

 latter possess the reduced number, whereas the spore- 

 mother-cell is diploid. From this we learn that reduction 

 may occur at different points in the life-cycle in different 

 forms. In Ascophyllum it occurs in the nuclear divisions 

 immediately preceding the formation of the gametes; in 

 Dictyota, immediately preceding the formation of the 

 spores, while between reduction and the formation of 

 gametes occur the innumerable cell-divisions that give 

 rise to the body of the gametophyte. 



235. Life-cycle of Dictyota. — The life-cycle of Dictyota 

 may be diagrammed thus : 



