Miyake, The development of the gametophytes etc. 
11 
endosporium. Tlie megaspore-membrane is tbicker at the lower 
part of the prothallium and becomes thinner toward the tip. Fig. 56 
shows a part of the base of the female prothallium where the 
membrane is thickest and measures about 3 y in thickness. 
Thomson (1905) has made an extensive comparative study 
on the distribution and character of the megaspore-membrane of 
Gymnosperms. The thickness and structure of the membrane is 
considered by him to have great phylogenetic significance. Thus 
the coat is thick and well developed in the Cycadales, the group 
which is recognizcd as the most primitive of the modern Gymno¬ 
sperms, while it is much thinner in the Gnetales, which is con¬ 
sidered as being the highly specialized of the Gymnosperms. 
Among the members of the Taxoclieae , Sciadopitys has the thickest 
megaspore-membrane and has more affinity to the Abietineae. Of 
two species of Sequoia, S. sempervirens has thicker membrane and 
measures about 2.5 y in comparatively young stage, while in the 
mature seed of S. gigantea the coat is only 1.5 to 2 y in thickness. 
According to Thomson (1905), in the mature seed of Cryptomeria 
the megaspore coat is not so thick as it is in S. gigantea, but 
otherwise is very similar to that of the latter”. Thomson has 
also examined the megaspore-membrane of the mature seed of 
Taxocliicm and gives its thickness as about 2.5 y. The megaspore- 
membrane of Cunnighamia seems to be as thick as that of any 
other mernber of the Taxoclieae , except Sciadopitys, if not much 
thicker. 
Development of the Archegonia. 
The archegonial Initials become apparent about the middle 
of June as a group of cells at the apex of the prothallium. They 
are the peripheral cells of the prothallial apex, and may be 
differentiated even before the prothallial tissue is thoroughly or- 
ganized. When the archegonial initials are first formed, they are 
scarcely larger than the other cells of the prothallium and can 
only be distinguished from the neighboring cells by the larger 
nuclei and denser cvtoplasm (figs. 58, 59). They soon become en- 
larged and elongate to three or four times of the original size 
before they divide. The fully formed archegonial initial has a 
prominent nucleus situated at the tip of the cell and most of the 
cytoplasm is collected around it. A very large vacuole occupies 
the greater part of the cell (figs. 60, 61). Fig. 62 shows the 
upper part of an archegonial initial whose nucleus is just dividing. 
This division results in the Organization of a smaller upper cell, 
the primary neck-cell, and a large lower cell, the central cell of 
the archegonium (fig. 63). 
The primary neck-cell soon divides into two cells by an 
anticlinal wall (figs. 64, 65). The two cells then divide again by 
walls which are perpendicular to the first and the four cells thus 
formed all lie in the same plane. The neck-cells usually divide 
no more, and the neck of the full-grown archegonium consists 
typically of a single^tier of four cells. The neck, tkerefore, shows 
