72 BIOLOGICAL LECTURES. 



with this differentiation is further shown by the fact that the 

 protoplasm which lies at the animal pole in early stages of 

 cleavage lies at the vegetal pole of the macromeres in later 

 stages ; while the yolk, which originally lay at the vegetal pole, 

 comes to lie at the animal pole of the macromeres ; in short, 

 the polarity of the macromeres is reversed during cleavage, and 

 this always happens in the same way, irrespective of the posi- 

 tions in which the eggs may lie. 



How far these mechanical principles fall short of explaining 

 the differentiations of cleavage has been pointed out by Mead 

 ('94), Kofoid ('94), Lillie ('95), McMurrich ('95), Wheeler ('95), 

 Zur Strassen ('95), Castle ('96), Jennings ('96), Conklin ('92, '94, 

 '97), and many others. Much has been said, and very justly, of 

 the difficulty of explaining by simple mechanical causes non- 

 alternating and unequal cleavages, and yet it ought not to be 

 forgotten that the causes of alternating and equal cleavages 

 have not been given. To say merely that cell divisions are typi- 

 cally alternating and equal affords no causal explanation. If we 

 knew why cleavages are usually alternating and equal, we should 

 probably be able to explain why they are sometimes neither. 

 How little alternating cleavage has to do with the mere diver- 

 gence of centrosomes in planes successively at right angles to 

 one another will be apparent further on, where we shall see 

 that the centrosomes do not preserve their original positions 

 in the daughter-cells, and that the direction of their divergence 

 bears no constant relation to the direction of the cell division ; 

 and how little yolk has to do with the inequality of division is 

 shown by the fact that in the formation of the polar bodies we 

 have two very unequal divisions of the egg, while the first and 

 second cleavages, are frequently equal ; the first, second, and 

 third divisions of the macromeres thus formed are usually very 

 unequal, while the fourth and fifth divisions of these cells are 

 again nearly equal ; finally, among the micromeres, which fre- 

 quently contain no yolk, the early divisions are more frequently 

 unequal than equal. These same considerations apply in the 

 main to the rate of division ; in most cases of determinate 

 cleavage it bears no constant relation to the presence or absence 

 of yolk in the cells. In every case of determinate cleavage 



