Miyake, The development of the gametophytes etc. H 



endosporium. Tlie megaspore-membrane is thicker at the lower 

 part of the prothallium aud becomes tliinner toward the tip. Fig. 56 

 shows a part of the base of the female prothallium where the 

 menibraiie is thickest and measures about 3 ,« 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 raost primitive of the modern Gymno- 

 sperms, while it is much thinner in the Gnetales, which is con- 

 sidered as being the highly s])ecialized of the Gymnosperms. 

 Among the members of the Taxodieae, Sdaclopitys has the thickest 

 megaspore-membrane and has more affinity to the Ahietineae. Of 

 two species of Sequoia, S. sempervirens has thicker membrane and 

 measures about 2.5 a in comparatively young stage, while in the 

 matiire seed of S. gigautca the coat is only 1.5 to 2 /./ in thickness. 

 According to Thomson (1905), in the mature seed of Cnjptomeria 

 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 examiued the megaspore-membrane of the mature seed of 

 Taxodhiin and gives its thickness as about 2.5 /<. The megaspore- 

 membrane of Cimnighamiü seems to be as thick as that of any 

 other member of the Taxodieae, except Sciadopitys, if not much 

 thicker. 



Development of the Archegonia. 



The archegonial Initials become apparent about the middle 

 of June as a group ofcells 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 distingiiished fi-om the neighboring cells by the larger 

 nuclei and denser cytoplasm (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 perj)endicular 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, therefore, shows 



