204 Symmetn/ of Egg and Symmetry of Embryo in the Frog 
the cytoplasmic organization of the egg and the symmetry of the future embryo 
may be taken then to be a well established fact. It does not, however, at all follow 
that the symmetry of segmentation will necessarily coincide with these other two 
symmetries. 
According to the terms of Roux's original " Mosaik-Theorie " the process of 
cell division in a segmenting ovum was qualitative, not, however, entirely because 
the parts of the cytoplasm were unlike, for the cytoplasm was imagined to be 
practically " isotropic " or equipotential in all its parts, but because the dissimilar 
units or determinants of the nucleus were supposed to be gradually sundered from 
one another by successive divisions. Later on, of course, these nuclear determinants 
were held to incite in the cytoplasm to which they were distributed the various 
processes of differentiation. 
This hypothesis of qualitative nuclear division has, on a number of grounds 
which need not now be recalled, been shown to be untenable, and has been 
given up by its author himself. It may, however, well be urged that since the 
" anisotropy " of the cytoplasm has now been demonstrated, cell, though not nuclear 
division is still the qualitative process of the " Mosaik-Theorie," that it is still an 
essential factor in differentiation. 
But even this view must be abandoned. 
In the Frog the First Furrow bears no necessary relation either to the Plane of 
Symmetry or to the Sagittal Plane. When outside interference is removed it is 
true that the tendency of all three towards coincidence increases, but by means of 
these same external agents it is possible experimentally to separate those internal 
factors which determine cell division from those which decide which meridian of 
the egg shall be occupied by the embryo. Further, by means of pressure 
(0. Hertwig, Born) the character of segmentation may be altered, but the embryos 
are nevertheless completely normal. 
In the Newt (Spemann) the First Furrow is sometimes in the Sagittal, some- 
times in the Transverse Plane. 
In Teleostei (Morgan) the First Furrow may make any angle with the median 
plane of the embryo. 
In Sea-urchins it is not easy to say what the exact relation of embryonic and 
egg-axes may be, since the egg is cofourless and almost isolecithal, and for that 
reason impossible to orient except either by the position of the germinal vesicle 
and polar bodies or by that of the segmentation axis (intersection of the first two 
divisions). As we have seen, these two axes do not coincide, and it is not certainly 
known which determines the embryonic axis, though it has been stated that the 
First Furrow lies in the Sagittal Plane. 
In Strong ylocentrohis lividus, however, the egg has a ring of pigment sub- 
equatorial in position (the axis as determined by the point of extrusion of the 
polar bodies coincides with the axis of segmentation). In ordinary development 
