90 



BIOLOGICAL LECTURES. 



vortical movements, symmetrical in relation to this plane." If 

 these movements are violent, they lead to the complete sepa- 

 ration of the daughter-cells ; if not, ordinary cleavage results. 

 Later ('952) j^g observed in the segmenting eggs of Ctenola- 

 brus, droplets over the surface of the egg which collected in the 

 plane of the next succeeding cleavage ; this phenomenon he 

 considered a confirmation of his theory. 



The cell movements which occur during the maturation, fer- 

 tilization, and cleavage of Crepidula not only confirm Loeb's 

 view as to the mechanics of cleavage, but they throw light 

 upon the mechanics of differentiation. Of the four features of 

 differential cleavage, viz., differentiations in the (i) direction, 

 (2) size, (3) quality, and (4) rate of division, the first three are 

 due, in part at least, to these movements of the cytoplasm. Evi- 

 dently, varying rates of division must be attributed to some 

 other cause. 



If we go further and inquire what causes and directs these 

 movements, we cannot at present find any positive answer. It 

 seems probable that they are due to the appearance of unlike 

 substances in different parts of the cell, the movements being 

 of a chemotropic character. Further, these movements are 

 correlated with the growth and collapse of the nucleus and 

 centrosome,! ^j^^| especially with the escape of nuclear sap into 

 the cell. It is a priori improbable that any one cell constituent, 

 as distinguished from the whole, is responsible for these move- 

 ments. The centrosomes are not the sole dynamic or kinetic 

 centres of the cell, since the movements of the cytoplasm carry 

 the centrosomes where they will, and control the direction of 

 division and the relative size and quality of the daughter-cells. 

 In this, as in other phenomena, the cell acts as a whole, and in 

 the interaction of its various parts are to be found the causes 

 of all vital phenomena. 



1 My complete paper on the history of these structures durmg maturation, fer- 

 tilization, and cleavage, will appear shortly. 



