70 BIOLOGICAL LECTURES. 



larva or adult. This polar differentiation may appear at an 

 early stage in the ovarian egg, or it may be delayed until after 

 fertilization. In some cases (insects and cephalopods) not only 

 the primary axis of the egg, but all the axes and regions of the 

 future animal are marked out in the ovarian egg ; in other cases 

 these axes, except the primary one, are not apparent until the 

 end of cleavage or even after gastrulation. 



In the cleavage of the egg, differentiations occur in a 

 remarkable degree in certain cases, while they appear to be 

 absent in others. Typically, cell divisions are rhythmical, alter- 

 nating, quantitatively and qualitatively equal, and consequently 

 non-differential. The differentiations of cleavage cells are due 

 to departures from this typical condition in one or more partic- 

 ulars. In certain animals these departures are very notable, the 

 cleavages being from the first non-alternating, non-rythmical, 

 unequal, and qualitatively dissimilar. These differentiations of 

 cleavage have also a far-reaching prospective significance, since 

 in certain cases (polyclades, nematodes, rotifers, annelids, mol- 

 lusks) the principal axes and body regions of the future animal 

 are marked out by the cleavage planes, and the building mate- 

 rial of entire organs is segregated into a single cell or group 

 of cells. 



I have repeatedly observed these unequal, non-alternating, 

 and non-rhythmical cleavages with the feeling that the causes 

 of such differentiation were almost within sight, and with the 

 conviction that continued study could not fail to reveal them ; 

 and yet it must be said that these causes, which seem so near 

 at hand, generally elude one's grasp. Unequal cleavage is due 

 to the eccentricity of the mitotic spindle, but why is the spindle 

 eccentric? Non-alternating cleavage is due to the spindles hav- 

 ing approximately the same direction during successive cleav- 

 ages ; but why do the spindles take this peculiar position? 

 Non-rhythmical cleavage can be referred only to differences in 

 the substance of cells, but how these differences operate can- 

 not in most cases be explained. It is hopeless to look for an 

 answer to the last question that may be asked concerning the 

 cause of these or of any other phenomena; all that can reason- 

 ably be expected is that the many different phenomena and 



