292 Trow . — Observations on the Biology and 
although the nuclei in the antheridium are still undergoing 
division. 
In Figs. 16 a and j 6 b, representing adjacent sections of the 
same oogonium, the ‘ coenocentrum ’ is present and the egg 
fully differentiated. The periplasmic nuclei are relatively 
large, and the fertilization- tube just reaches to the egg. 
Sections like these when compared with those represented 
in the two preceding figures arouse the suspicion that there 
may be a double division of the nuclei. The weight of the 
evidence, based upon countings of the nuclei, is against a 
second division. Such differences as are presented here 
we must regard, at any rate provisionally, as due to simple 
variation. In Fig. 17 the egg is uninucleate but destitute 
of a ‘ coenocentrum, J yet the fertilization-tube has penetrated 
a considerable distance. The nucleus in the antheridium is 
in metaphase. In Fig. 18 the fertilization-tube is almost in 
contact with the female nucleus. Nuclear division in the 
antheridium is almost complete. One pair of daughter-nuclei 
are seen in the last stages of division. 
In Fig. 19 we have before us apparently the very act of 
fertilization. The egg has now separated from the periplasm, 
and the ‘ coenocentrum ’ is very obvious. The fertilization- 
tube can be traced almost as far as the f coenocentrum ’ and 
up to the posterior end of an elongated male nucleus. The 
wall of the fertilization-tube may be traced as a transparent 
membrane of measurable thickness through the periplasm 
up to the margin of the egg, but its continuation in the egg 
itself is no longer recognizable as a definite membrane. It 
must be remembered that such sections as these, stained to 
show the structure of the cytoplasm and nuclei and mounted 
in balsam, are not suitable for the study of the cell-membrane. 
The nature of the boundary line between the substance of 
the egg and the fertilization-tube which lies within it, must 
therefore, for the present at least, be left undecided. It is, 
however, abundantly clear from the preceding figures that the 
general view as to fertilization in Pythium , founded on De 
Bary’s observations on living material, is for this species 
