212 BIOLOGICAL LECTURES. 



amphiaster are present, the suspicion is warranted that in the 

 ripe egg of other forms the sea-urchin, for example the 

 mitosis is not inhibited merely on account of the lack of a cen- 

 trosome^ nor is it incited merely because a new centrosome is 

 introduced to organize the mitotic figure. 



Other observations on fertilization strengthen this supposi- 

 tion. Wheeler has shown in Myzostoma that no centrosomes 

 or asters are developed in connection with the male pronucleus, 

 and that the centrosomes, which are left in the egg after the 

 formation of the polar globules, probably form the poles of the 

 cleavage-spindle. According to Lillie, the sperm-centrosomes 

 in the egg of Unio degenerate, and the centrosomes which 

 participate in the first cleavage mitosis are egg-derivatives. 

 The well-known researches of Fol, Guignard, and Conklin, 

 even if they are not complete enough to prove the theory of 

 the "quadrille," certainly indicate that the egg-centrosomes 

 have a considerable degree of persistence. 



Furthermore, it is difficult to demonstrate that the " sperm- 

 centrosomes " are actually brought into the egg by the sper- 

 matozoon, and caution must be exercised in referring the 

 origin of the sperm-centrosomes to this source. 



Just how the entrance of the sperm revives the latent activ- 

 ity of the oocyte is not yet fully understood, but the phenome- 

 non is suggestive in that it shows that it is neither the mass of 

 the cell, nor the abundance of yolk, nor the position of the cell, 

 nor the presence of the centrosome that determines the time 

 or rate of cell-division, but that a stimulus is required analogous, 

 perhaps, to that which starts into activity the motor apparatus 

 of pigment-cells, leucocytes, or muscle-cells. Following out 

 this suggestion, I have made some experiments upon the un- 

 fertilized egg of Chaetopterus. 1 



Watase has pointed out that the " mechanism of protoplasmic 

 motion " in the leucocyte, pigment-cell, and muscle-cell is 

 similar in its essential features to that in a blastomere during 

 mitotic division (Fig. 5). The aster in the leucocyte and the 

 fibrils, contraction-bands, etc., in the muscle-cell are, most of 



1 I am glad to acknowledge the valuable suggestions and kind assistance of 

 my friend, Mr. C. W. Green, of Johns Hopkins University. 



