366 EDWIN G. CONKLIN 



into one of the first two blastomeres and the lighter substances 

 together with most of the cytoplasm into the other one, each of 

 these blastomeres continues to segment in a normal manner, the 

 second cleavage being approximately equal and the subsequent 

 ones giving rise to macromeres and micromeres as in normal eggs. 

 The formation of macromeres and micromeres therefore does not 

 depend upon the presence of yolk in the former but upon some 

 other factor. 



But if the pattern of cleavage is not determined by the cell 

 inclusions it is equally clear that it is not determined by any 

 fixed and unalterable localization of the protoplasm with respect 

 to the cell axes, for if the first cleavage plane is forced to take 

 an equatorial position the second cleavage is meridional and equal 

 and from each of the four cells thus formed micromeres are cut 

 off on the animal pole side as in normal eggs. In short the 

 macromeres which lie at the original animal pole are not the 

 only ones which form micromeres, but even those which were cut 

 off below the equator of the egg also form micromeres. This 

 shows thht the pattern of cleavage is not predetermined with 

 reference to the original polarity of the egg. On the other 

 hand eggs which have been subjected to pressure in the direc- 

 tion of the chief axis of the egg at the time of the third cleavage 

 may divide so as to form five, six, seven or eight macromeres 

 and in such cases each of these macromeres gives rise later to 

 three sets of micromeres as if it were a normal macromere. 

 Consequently it cannot be said that the character of the cleav- 

 age is determined by an inherited and wholly definite orienta- 

 tion of each succeeding cleavage spindle, for if this were the 

 case, when the third cleavage is rendered equal by pressure, 

 subsequent cleavages should give rise only to second and third 

 sets of micromeres; indeed when the third cleavage is forced 

 to be an equal one the subsequent cleavages occur as if this 

 were an entirely new cleavage which had been intercalated be- 

 tween the typical second and third cleavages. 



In conclusion, then, the pattern of the cleavage is dependent 

 upon the position and direction of the spindles and this is deter- 

 mined, not by inclusions, but by the spongioplasm which holds 



