558 Trow. — On Fertilization in the Saprolegnieae . 
a happy one, for the structure in question is actually at the centre of an 
ovum. The term in question is not unsuitable for the case in which it was 
first used by Stevens — the oospheres of Albugo — for the numerous nuclei 
in these can be considered as having a common centre. Ovocentrum 
appears to be a more suitable term for general use, and will be used by me 
to denote the aggregation of granular protoplasm about the astrosphere 
and nucleus in the oospheres of this Achlya and the other members of the 
Phycomycetes investigated by me. Ruhland criticizes my comparison of 
such an ovocentrum to a ‘whirlpool in a river/ Now, as the simile was 
used to call attention to the view that the constancy of form was main- 
tained rather by a constant force than by a constant substance, there seems 
no real difference of opinion between us except as to choice of illustration. 
Surely, though, a whirlpool whose relatively permanent form depends pri- 
marily on the force of the rushing water, and whose actual particles, whether 
of swirling water or floating foam, are continually entering and leaving it, 
furnishes a not inappropriate simile. When the ovocentrum takes the form 
of a single well-defined granule or globule, with a definite limit, perhaps 
even a limiting membrane, as figured by Davis (’00) and Stevens (’ 99 ) for 
Albugo , the simile is no doubt an inadequate one. The fact, however, 
remains, that even here we are dealing primarily with force rather than 
with substance — the acting force is regular and necessary, the substance is 
more or less irregular and accidental. Figs. 22, 24, 25, 2 6, 28, 29, and 34 
lead one to infer that the ovocentrum is the result of forces emanating from 
the region of the centrosome. The question of the function of the centro- 
somes themselves has been the cause of much speculation, but it is one 
that can scarcely be attacked profitably in this connexion. We require 
more detailed observations on their origin and distribution in plants. My 
observations, indeed, lead me to doubt whether the centrosomes and astro- 
spheres are themselves anything more than manifestations of the forces 
acting at certain points in the oospheres, or better, at points in the imme- 
diate vicinity of the nuclei. To build up theories of chemiotaxis on such 
slender foundations as the juxtaposition of centrosome and nucleus, as has 
been done by Davis, appears to me to lead to no result of real value. 
The entry of the sperm nucleus into the egg. Fertilization. Three oogonia, 
containing respectively 4, 17, and 11 oospheres, were critically examined. 
Each oosphere had the structure represented in Fig. 22. It was provided 
with a single nucleus in the resting condition, a centrosome and astro- 
sphere, and an ovocentrum of dense protoplasm. Many other oospheres 
were casually examined, and if fertilization-tubes had not reached them, 
they always proved to be uninucleate, and to possess the structure shown 
in Fig. 22. The accurate demonstration of the entry of the sperm-nucleus 
into the egg is excessively difficult, and has only been satisfactorily carried 
out on one culture fixed in chromacetic acid. The fertilization-tubes, as 
