BROOKS AND RITTENHOUSE: ON TURRITOPSIS. 447 



In the case 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 2426 (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 blastomeivs 

 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 r 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 



