280 
F. Cavers. 
cells (antheridia), though the actual passage of protoplasm from 
the latter into the former was not seen. Other forms are found 
with resting sporangia quite like those of Olpidiopsis but without 
an antheridium, and Schroter (in Engler and Prantl) restricted the 
genus Olpidiopsis (in Olpidiaceae) to these species with (presumably) 
asexually produced resting sporangia, transferring to a new genus 
Diplophysa (in Oochytriaceae) those species with sexually produced 
sporangia ( Olpidiopsis spp. and Pleocystidiuni). Barrett (1912) 
rejects the name Diplophysa in favour of Olpidiopsis , using the 
name Pseudolpidium for the forms with Olpidiopsis- like but asexually- 
formed resting-sporangia (Fig, 6, C). Barrett studied three species 
of Olpidiopsis (Fig. 6, E) in detail, and found two cilia of equal 
length attached at or near the anterior end of the elongated 
zoospore, which showed two motile stages separated by a brief 
rest-period ; the individuality of the zoospore after entering the 
host is maintained throughout development, no plasmodium being 
formed, and apart from slight amoeboid movement immediately 
after entrance the parasite does not undergo any noticeable changes 
of form ; segmentation of the sporangial contents occurs at least 
in part before the sporangium enters into a resting period and is 
simultaneous throughout; true sexuality probably exists and takes 
place by fusion of two sexually differentiated individuals and the 
subsequent passage of the protoplasm of the smaller male into the 
larger female cell, followed by fusion of nuclei; the gametangia, 
like the sporangia, are multinucleate, as is the oospore ; germination 
of the oospore was unfortunately not observed in any of the species. 
As Barrett points out, these Olpidiopsis forms may be regarded 
as primitive sexual organisms of the Oomycete type, as indicated 
by the influence of external conditions on the development of the 
sexual stage, the mode of fertilisation, the unequal size of the two 
gametes, and the apparent morphological equivalence of these 
gametes with the sporangia. The cytology of the Pseudolpidiutn 
forms is not known, but in view of the conditions found in Reessia, 
Monochytrium, Olpidium and the closely related Olpidiopsis, it seems 
probable that a sexual process occurs also in these forms. In any 
case, we have in the Olpidiaceas a remarkably interesting series 
leading from primitive types like Reessia and Monochytrium up to 
definitely oogamous forms and thence, as we shall see later, to 
the Peronosporaceae, probably through the Ancylistineae. 
(To be continued). 
