BROOKS AND RITTENHOUSE: ON TURRITOPSIS. 
447 
In the ease referred to, the blastomeres are spread out so that the 
individuals, with three exceptions, touch only one of their fellows, thus 
resembling a string of beads somewhat coiled. 
With this separation and rolling apart, the regularity of arrangement 
of the cells in the segmenting egg is lost, and the stages from this point 
on become more and more irregular with each successive division up 
to the time when the readjustment takes place which is the beginning 
of the formation of the free-swimming embryo. 
It is possible to distinguish, during these early cleavage stages, a 
layer of ectosarc around each individual blastomere. Later as the 
cells increase in number and become smaller, the ectosarc covering 
becomes less conspicuous and finally is lost from sight entirely. 
After an interval of about one half an hour, the fourth segmentation 
begins. The divisions of the different cells no longer take place 
simultaneously; some occur a few minutes before others, but all are 
completed within a comparatively short time. So far as the cleavage 
itself is concerned, it is still equal and regular, but the arrangement 
of the blastomeres is no longer regular nor definite. They apparently 
follow no law of symmetry, and may come to lie in any position. 
Figures 24—26 (pi. 33) show three different forms which the cells of 
the sixteen-cell stage acquire, and various other arrangements of the 
blastomeres which could not be figured for want of space, were seen 
while studying the living eggs. However, the three figures are suffi¬ 
cient to show that the general form of the egg in this stage may be 
inconstant. In figure 24 of plate 33, it is possible to imagine a direct 
relationship to a preceding form just a little more irregular than is 
shown in figure 23 (pi. 32). In a form represented in figure 25 (pi. 
33) the descent of the different cells from the individual blastomeres 
of the eight-cell stage is less easily recognized. Figure 26 (pi. 33) 
shows an egg in which all sixteen blastomeres are spread out to form 
a flat plate one cell thick in the form of a quadrangle. One can easily 
conceive how this arrangement can have resulted from a regular eight¬ 
cell stage in which the rotation of the cells of the one quartet was 
greater than that shown in figure 22 (pi. 32). The flat, spread-out 
position of the cells at once suggests the idea that the egg may have 
been subjected to pressure. This might have been the case if the 
eggs had been studied on a slide under a cover glass; but there is no 
evidence that pressure was the cause of this plate-like arrangement, 
for these forms were occasionally found among a variety of other 
