476 Yasui . — On the Life-history of Salvinia natans . 
macrosporangium, I have observed in a few preparations that two macro- 
spores were becoming enlarged instead of one. It seems that even in 
those cases one of the two finally develops, at the expense of the other, into 
the mature macrospore. 
The mature macrospore is surrounded by three membranes. The 
outermost membrane, the epispore, shows a vacuolate structure, and is not 
of equal thickness, being thickest at the top, where it is divided into three 
folds. The folds are formed by three ridges at the sides (PL XLII, 
Figs. 84, 85). 
The structure and development of the spore-membrane have already 
been described by Mettenius (’46) and Pringsheim (’63). Some more 
details were later added by Juranyi (73), Prantl (79), and Heinricher (’82). 
The formation of the epispore by the vacuolization and the trans- 
formation of the protoplasmic mass surrounding the macrospore have 
already been described by Heinricher (’82). Fig. 83 shows an earlier stage 
of the epispore formation, in which the least differentiated cytoplasmic mass 
with one of the developing nuclei is found at the periphery, while the inner 
portion is transformed into a mass of vacuoles. A little later stage is 
shown in Fig. 84. The cytoplasmic mass is almost entirely transformed 
into the vacuoles. According to Heinricher (’82) the smaller vacuoles are 
found near the periphery, while the larger ones are arranged near the inner 
side. I have found, however, that the larger vacuoles are found near the 
periphery rather than towards the centre. In the young epispore a thin 
layer of protoplasm with a number of nuclei is found inserted between each 
episporic fold and the underlying portion of the epispore; Fig. 85 shows 
a tangential section of the epispore through the episporic folds at the apex 
of a macrospore. 
The next inner membrane, the exospore, is homogeneous in structure, 
and almost equal in thickness throughout. Although the origin of the 
exospore is not quite clear, I am inclined to think that it is formed from 
the protoplasmic mass surrounding the macrospore. The innermost endo- 
spore is very thin and directly lines the inner surface of the exospore. 
Female Gametophyte. 
The macrospores germinate on the surface of the water after the wall 
of the sporocarp has decayed or even when it is only partly decayed. 
Macrospores used for the present study germinated during January and 
February in our laboratory. When germination begins, the cytoplasm at 
the upper part of the spore becomes very dense, and soon the nucleus 
is found to divide. One of the division-figures is shown in Fig. 86. A faint 
membrane is formed between the two resulting nuclei, thus dividing the 
whole macrospore into a smaller upper cell and a considerably larger 
lower cell. 
