Pythium de Baryanum . 
661 
Maturation of the Oospore. 
Soon after the discharge of the antheridial contents into 
the oosphere, a thin membrane is formed around the latter 
(Figs. 20-22). This is the beginning of the exospore. After 
fertilization it gradually thickens until it reaches about the 
thickness of the oogonium-wall. Figs. 23 and 24 show the 
oospore in which the formation of the exospore is nearly 
completed. In this stage only a very small quantity of proto- 
plasm remains in the periplasm. It is very likely that the 
greater part of the periplasm has been used up in the forma- 
tion of the exospore. 
Later the endospore begins to be formed inside the exospore. 
Fig. 25 shows an exospore in which the endospore is about 
half way thickened, and in Fig. 27 we can see the endospore 
is nearly completed. The exospore is more deeply stained 
than the endospore. 
The ripe oospore is uninucleate, and in the nucleus several 
deeply-stained chromosome-like bodies are clearly visible 
(Figs. 26, 27). The protoplasm is vacuolate in structure, show- 
ing a regular net-work in the section, and it takes the stain 
more readily. The wall of the oogonium which thickens in 
the course of the development of the latter, but very slightly 
or scarcely at all after fertilization, persists as an additional 
protection of the oospore (Fig. 27, &c.). 
From the foregoing statements we can see that, in the mode 
of sexual reproduction, Pythium has more resemblance to the 
Peronosporeae than to the Saprolegnieae. In some species 
of Saprolegnieae, the oospore is formed parthenogenetically, 
the antheridia being very rare or never developed. In others, 
which have well-developed antheridia, the existence of fertiliza- 
tion is still in doubt. De Bary (’81) first expressed the doubt, 
and after careful observations came to the conclusion that the 
Saprolegnieae as a group are distinctly parthenogenetic. His 
conclusion was based on the failure to observe any passage 
of protoplasm from the fertilization-tube to the oosphere, 
or even any opening in the tube. Pringsheim (’82, ’83) 
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