170 Lawson . — The Gametopkytes and Embryo of 
These are the first or primary cells of the prothallium which were first 
described by Mile. Sokolowa (’90) as alveoli, and which have since been found 
in many other Gymnosperms. The nuclei of the primary cells were nearly 
always found occupying a position at the surface of the cytoplasm nearest 
the central vacuole. In Fig. 15 is represented a group of these cells as seen 
in a section taken at right angles to the dividing walls. It was observed 
that these walls were nearly always six in number for each cell, and were 
arranged about the respective nuclei in such a fashion as to give the appear- 
ance of a regular symmetrical mosaic. 
Rapid development immediately follows the organization of the primary 
prothallial cells. These structures elongate in an inward direction, and 
encroach upon the central vacuole in the manner already described for 
other Conifers (Sokolowa, ’90; Arnoldi, ’01; Coker, ’04; Ferguson, ’05 ; 
Lawson, ’04 and ’07). This is represented in Fig. 16. The later stages in 
the formation of the permanent prothallial tissue were not found, but it was 
observed that free nuclear division may proceed in the primary cells for 
some time before cross- walls are formed. Some cross- walls were, however, 
found at quite an early period. The multinucleate condition of the primary 
prothallial cells may be seen in Fig. 16. 
During these stages in the development of the prothallium the mega- 
spore membrane becomes decidedly thicker and much more conspicuous. 
Its structure in the mature condition is very similar to that described for 
Pinus (Thomson, ’05). The fibrillae of the exosporium, however, seemed to 
be longer, finer, and more regularly arranged than that of Pinus . From 
measurements taken it was found that the membrane varied in thickness — 
being thicker at the base of the prothallium than along the sides, its 
approximate average thickness being 4-5 /x (Fig. 14). At a plane almost 
level with the base of the archegonia the membrane thins out rather 
abruptly, and from this region to the very apex of the prothallium no trace 
of the membrane could be detected. A section of the prothallium showing 
this is represented in Fig. 17. It will be seen from this that the archegonial 
region extends beyond the limits of the spore. In its distribution the 
megaspore membrane suggests that of Larix (Thomson, ’05), but is quite 
unlike that of Tsuga , where, according to my own observations, the 
membrane completely surrounds the archegonial region. 
The Archegonia. 
The young archegonia make their appearance during the second and 
third week of May. The earliest stage observed was that where the primary 
neck-cell had already been cut off from the central cell, and the latter had 
enlarged to several times the size of the neighbouring sterile prothallial cells. 
From the position of these two cells it was quite evident that the archegonial 
initials originate as superficial cells at the apex of the prothallium, as in 
