.■)14 I. IKEDA : 



cloiivngo ])lanf' passes at riglit angles to the first (fig. 2 h). It 

 is a remarkable peculiarity of Phoroivh eggs that two sister ])las- 

 toiueres derived hy the division of a mother blastomere, never 

 undergo the next division siinultaneonsly, so that l)etween any two 

 consecutive stages having an even mnnl^er of blastomeres there 

 intervenes an intermediate stage with an odd number of the same. 

 This ])henoinenon oecui's even at the second cleavage ; thus just 

 before the egi;' attaiiis the four-cell stage, there exists a stage of 

 tlircc cells, siicli as is seen in fig. 'Id. Among tlie later stages, 



lli(tse of ?">, 7. *> cells are of constant occurrence. C<mseqn- 



ently it is scarcely admissible to say that the segmentation ])ro- 

 ceeds with considerable regulai'itv. 



Caldwell ('82) has assei'ted that the first differentation of the 

 future blastoderm into the ectoblast and entoblast is observable 

 as early as in the four- cell stage. He says: " ^/ the stage of 4 

 sera/tientafion-spherrs a dirhion into tiro >iiiiallfr clear and 

 two larger opa(j}ie celh Indicate'^ the future ectoderm and endo- 

 derrn " (/.r., ]>. ?uA). At the corresponding stage of riioronis 

 ijiniai I have not been able to discover any appreciable difterence 

 in the size of its cells (see fig. 2 />). Following the 4-cell stage, 

 the division of the blastomeres in the ecpiatorial ]>lane puts the egg 

 on the way to the 8-cell stage. According to my own observations, 

 the alxn^e mentioned difference in size of the blastomeres becomes 

 iii'st ]X'rceptible at this stage. Fig. 3 shows a side view of an egg 

 with S blastomeres ; it will be seen that the up])er four Idastom- 

 eres are veiy slightly smaller than the lower four. I could not 

 liowever, at that period, recognize any difference in the cell-con- 

 tents of the two classes. 



The iri-egularity of division, which, as 1)efore mentioned, be- 

 comes more and more pronounced as segmentation advances, tends 



