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. 
i82,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 ob'gonial 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. 
236. Life-cycle of Dictyota. The life-cycle of Dictyota 
may be diagrammed thus: 
