512 Wisconsin Academy of Sciences, Arts, and Letters. 



protoplasm bet^veen themselves and the cell wall (Fig. 41). 

 The nucleus and pvrenoid are generally situated in the cen- 

 tral region of the cell where the protoplasm still extends be- 

 tAveen the vacuoles clear through the diameter of the cell, as 

 is clearly shown in a cross section (Fig. 42). In these early 

 stages in living cells, the chloropliyl can only be seen in the 

 thicker portions of protoplasm as the middle part in which the 

 nucleus and pyrenoid lie and the ends where the vacuoles do 

 not press out to the cell wall. But this appearance is prob- 

 ably due to the fact that the layer of cytoplasm between the 

 cell and vacuoles is so verv thin as to render the color indis- 

 cernible when viewed from the surface. There is certainly 

 no indication in sections of anv differentiation of the cvto- 

 plasm to form a chromatophore in this or any other stage of 

 the cell life. The arrangement described above is undoubt- 

 edly what led to Artari's (2) statement that in the earlier 

 stages the chromatophore is a branched structure similar to 

 the chromatophore of DraparnalcJia. But if Artari had had 

 better prepared material he would have seen that there is no 

 trace of any differentiation of the protoplasm into an or- 

 ganized cliromatophore. 



As the cell becomes larger the vacuoles fuse into one large 

 central vacuole and the protoplasm, now containing numerous 

 nuclei and pyrenoids, becomes evenlv distributed in a layer 

 next to the cell wall (Fig. 1). 



If we attempt now to apply the facts described in the fore- 

 going pages to the problems suggested in the introduction, 

 several points of general interest are at once apparent. Xot- 

 withstandino; the very small size of the nuclei the structure 

 and essential features of mitotic divisions are identical ^vith 

 those of the higher plants. Hydrodictyon agrees in this re- 

 spect ^Yit\\ all of the carefully investigated cases of nuclear 

 structure and division in the other thallophytes with the pos- 

 sible exception of Spirogyra, in which it seems quite probable 

 according to the recent researches of Mitzkewitsch and Wis- 

 senlingh that the nuclear structure is different from that of 

 most other forms. But in that case the peculiar structure of 

 the nucleus probably represents a highly specialized tj^e and 



