8 AKT. 10. — N. YATSÜ : 



seawater. Their early cleavage stages were studied in the water. 

 The eggs examined for this experiment were not dispermic, 

 as shown by the fact that the first two cleavages were not simul- 

 taneous. The blastomeres did not press together, l)ut remained 

 almost spherical (Fig. 12), as is always the case with eggs 

 under the same treatment. It is interesting to note that the 

 subsequent history is exactly alike to the type of dispermic 

 eggs, which cleave into four cells at once (" Simultanvierer " of 

 " ebener Tetrastertypus ") as first observed by Fol and later studied 

 by Driesch ('92, p. 31, figs. 69, 70), Morgan ('95, p. 73), Boveri 

 ('07, pp. 11-12) and others. The third cleavage planes are 

 vertical instead of- horizontal, giving rise to eight blastomeres^ 

 arranged in a ring (Fig. 15) or less frequently in two rows as 

 in the egg of Beroë (Fig. 14). One cannot hesitate to correlate 

 the horizontal position of the third division spindle with the 

 failure of the pressing together of the four blastomeres at the 

 end of the second cleavage, but at the same time this experiment 

 does not exclude the possibility that this is due to the shaking (cf. 

 Morgan '95, Boveri '07). But the cause of the abnormal appear- 

 ance of the third cleavage does not concern us here. What hns a 

 direct bearing upon the present problem is the further history of such 

 eggs. The fourth cleavage is horizontal, resulting, as in the third 

 division of the normal egg, in eight upper larger and eight lower 

 smaller blastomeres (Figs. 16 and 19). The fifth cleavage takes place 

 exactly like the fourth of the normal egg, i. c, the division of 

 the first quartet cells and the formation of the second quartet 

 (Fig. 18). So far as my observation went, each blastomere of the 

 8-cell stage behaved like that of the 4-cell stage of the normal egg. 



1 That the blastomeres isolated at the 4-cell stage ilo not divide vertically, may be due 

 to the fact that the position of cleavage centres was already settled at the time of the operation. 



