177 
Pseudotsuga Douglasii. 
and although quite thin at first it increases in thickness with the growth of 
the pro thallium, and eventually becomes very conspicuous. In the mature 
stages it surrounds the prothallium except in the region of the archegonia. 
In this region it is entirely absent, and in this regard differs quite markedly 
from Tsuga. 
With the increase in the size of the central vacuole, and the consequent 
formation of the parietal layer of cytoplasm, free nuclear division continues 
for some time. 
The parietal layer now increases in thickness, and the primary 
prothallial cells are formed in the ordinary way. 
These latter structures elongate in an inward direction, and gradually 
close the central vacuole. Free nuclear division now takes place within the 
primary prothallial cells, before cross -walls are formed to organize per- 
manent prothallial tissue. 
The archegonia originate as superficial cells at the apex of the pro- 
thallium. 
They are generally four in number, and each is enveloped by a single 
layer of nourishing jacket-cells. There are generally two tiers, but frequently 
a single tier of neck-cells. 
The archegonia are separated from one another — especially in the 
region of the necks — by several layers of sterile prothallial cells, and each 
is provided with a separate archegonial chamber. 
A distinct ventral canal-cell is formed as a result of the division of the 
central cell. 
The membrane of the ventral canal-cell persists up to the time of 
fertilization. 
The fusion of the sex nuclei takes place in the middle of the arche- 
gonium. The female is many times the size of the male. 
The first segmentation-spindle is formed within the area bounded 
by the membrane of the fusion-nucleus. It is, however, of cytoplasmic 
origin. One or more dense masses of cytoplasm are carried into the 
egg-nucleus by the sperm-nucleus. 
The first division is very soon followed by a second, and the four free 
nuclei thus formed pass to the base of the archegonium. 
After the division that follows, cell-walls are formed separating the 
nuclei. 
Eventually the pro-embryo consists of three tiers of cells and one tier 
of free nuclei. The lowermost of these becomes the embryo proper. The 
middle one becomes the suspensor, and the next one the rosette. 
As the suspensors elongate, the cytoplasm in the base of the arche- 
gonium becomes replaced by a mass of mucilaginous substance which acts 
as a plug to prevent the suspensors from growing forward. 
The account here given of the gametophytes of Psetidotsuga makes it 
