x 55 
Lloyd Williams . — Studies in the Dictyotaceae . 
‘ The significance of this multiple division could not be explained with 
only alcohol material ; it was too frequent, at least in the specimens I 
examined, to be an accident/ 
The parenchymatous mass may later on develop an apical cell and 
grow out into an elongated germling-like branch, and, as will be shown 
in another paper, it may remain alive when the rest of the thallus has 
decayed. 
The cytology of this stage has not been fully worked out, but what 
probably happens is that the conditions (temperature and light) being 
unfavourable to the development of the reduction stage (it is evidently 
very sensitive to external influence) the stalk-cell division is followed by 
5 ordinary vegetative divisions. As far as my observation goes this mode 
of cell-multiplication never follows the reduction division, and when this 
division is abnormal the death and disintegration of the cell inevitably 
ensues. 
(4) In Padina ‘ twin 5 tetrasporangia are of very frequent occurrence. 
After separation from the stalk-cell the mother-cell nucleus instead of 
going into synapsis divides vegetatively, there is a longitudinal division of 
the cell, then the two nuclei go through the various phases of the reduction 
stage, but they lag somewhat behind the neighbouring sporangia, as if 
they had lost time by the. extra division. 
Conclusions. 
1. Alternation of Generations. 
It has now been shown that in the thallus- and in the stalk-cell 
divisions of the tetraspore plant the nucleus has about thirty-two chromo- 
somes. The first division of the resulting mother-cell is different from 
all the others in its long preparation for division, its elaborate and distinc- 
tive spirem stages, the heterotype character of its chromosomes, and in the 
fact that here the number is reduced to sixteen. The succeeding division 
has the same number of chromosomes, and in the young plant produced 
from the tetraspore the earlier mitoses all show the reduced number. 
As will be shown in detail in a succeeding paper the thallus-cells of 
both male and female plants are probably characterized by the reduced 
number: it is difficult to be absolutely certain where the nuclei are so 
small. In the oogonial and antheridial divisions, however, there is no 
doubt about the number. Furthermore, in all these various mitoses there 
is not one that resembles the reducing division in its distinctive characters. 
In the interesting series of abnormal figures observed in the partheno- 
genesis of unfertilized eggs the chromosomes are always scattered, and 
consequently easily counted, and the number is invariably sixteen, whereas 
the fertilized oosphere in all its segmentations shows the full number. It 
