148 Symmetry of Eyg and Symmetry of Embryo in the Frog 
part of the hj'pothesis of nuclear predetermination elaborated by Weismann out 
of Roux's Mosaiktheorie. Like many other of the facts upon which this theory is 
built, the universality at least of the coincidence of these two planes has been 
denied. Oscar Hertwig has stated — on the strength of observations made on eggs 
compressed between horizontal glass plates — that they may make any angle with 
one another. Schulze and Kopsch think it probable that they coincide in the 
majority of cases. 
Not one of these authors has, however, thought it worth while to examine 
a large — a statistically intelligible — number of cases, though it would appear that 
the magnitude in question is obviously a variable one and preeminently amenable 
to such treatment. It is by this method therefore that I have sought for a 
solution of the pi'oblem. 
In the meantime the centre of interest has shifted. The very numerous 
experiments that have been made on the behaviour of eggs segmenting under 
pressure and on the development of isolated blastomeres, have distinctly negatived 
the idea of the preexisteuce in the fertilized egg-cell of definite nuclear units for 
the determination of the inheritable characters of the organism, an idea which has 
now been abandoned by Roux himself, and less importance has come to be attached 
to segmentation as a mechanism for separating such units ; more attention is now 
paid to the initial structure of the ovum, and the presence in it — demonstrated 
by recent research in some cases — of definite cytoplasmic organ-forming substances 
as a cause of differentiation. 
In the frog's egg itself (i?.y»sca) Schulze has shown that though the symmetry 
of the unfertilized ovum is radial about the axis, a bilateral symmetry is acquired 
during fertilization by the formation of a crescentic band — at first grey, but after- 
wards white and added to the white area on the vegetative side of the egg — along 
the border of the pigmented area on one side. The grey crescent arises, according 
to Roux, by immigration of the pigment into the interior of the egg. Both Roux 
(1903) and Schulze agree that the point of entry of the sperm is in the plane of this 
bilateral symmetry, and on the side opposite to that on which the grey crescent 
appears; and Roux, following out his earlier idea of the causal connection between 
the sperm path and the first furrow, believes that it is the entry of the sperm that 
is responsible for the change of symmetry. It is further stated by both authors 
that the side of the grey crescent is postero-dorsal, since the dorsal lip of the 
blastopore is formed here. The plane of symmetry and the sagittal plane there- 
fore tend to coincide. 
Normally, according to Schulze, the first furrow also lies in this plane, but 
considerable deviations are possible, their frequency increasing with the length of 
time that the eggs have remained in the uterus before being laid. 
Morgan has investigated the relation between these three planes in R. tempor- 
aria and R. pahistris ; the first furrow lies in the plane of symmetry in 24 of 
the cases in the first species, in 50 % the second, and when this occurs the 
