550 Trow . — On Fertilization in the Saprolegnieae . 
material revealed no new facts of interest. The examination of sections 
showed almost at once that the cytology of this species of Achlya corre- 
sponded closely with that of other species of Saprolegnieae which possess 
perfect sexual organs. 
There is no necessity for a detailed description of the observations. 
The essential points can readily be appreciated by reference to Figs, i to 
13. The oogonia and antheridia are multinucleate (Figs. 1, 2, and 3). 
The nuclei, although already much more numerous than is necessary to 
meet all the requirements of the sexual process, undergo an indirect division 
in both oogonia and antheridia (Figs. 1, 2, and 3). The oogonial nuclei are 
reduced in number by the degeneration and absorption of the excess before 
the oospheres are fully formed, and each oosphere as it rounds itself off is 
provided with a single, centrally-placed nucleus (Figs. 4, 5, and 6). The 
fertilization-tubes grow into the cavity of the oogonium, and when they 
reach the oospheres, each of these acquires a cell wall and a second nucleus 
(Fig. 7). The oospheres become, in fact, young oospores. That the entry 
of the second nucleus could not be traced seems to be of relatively small 
importance so far as the demonstration of the occurrence of fertilization is 
concerned. The two nuclei of the young oospore approach each other, but 
the actual fusion is delayed until the oospore wall has acquired a consider- 
able thickness (Figs. 8, 9, and 10). Ultimately the nuclei invariably fuse, as 
is proved by the fact that the old oospores are always uninucleate. The 
superfluous nuclei and protoplasm of the antheridia undergo very slow 
degeneration and eventually disappear. In the case of this species no 
single exception as to the number of nuclei has been discovered ; — the 
formula of succession, based on the examination of hundreds of normally 
developed sexual organs, is 1, 2, 1, and absolutely invariable. 
Several points in the cytology, however, deserve special attention. In 
the anaphases of some of the nuclei in the case of a series of sections 
through one oogonium, appearances were noted such as are represented 
in Fig. 1, a and b, which suggested that a second mitosis sometimes 
took place immediately after the first without the normal intervening 
resting stage. I spent much time and labour in following . up this 
clue, but failed to secure any better results in this species. It was 
partly this fact that impelled me to seek an additional species for 
further investigation. 
The degeneration of the nuclei during the formation of the oospheres 
was also very difficult to follow ; the experience gained in the study of 
degenerate nuclei in Pythium was of very little assistance. It was certainly 
not difficult to distinguish a single, central, deeply stained nuclear structure 
in each ‘origin’ and oosphere, but after Davis had published his paper 
I was obliged to admit that there would be ample justification for the 
criticism that in this case the c coenocentrum,’ if present, would be grouped 
