TWINNING AS A MODE OF REPRODUCTION 209 



begins with those occupying the dynamic center of the 

 cell and proceeds from the center to the periphery, 

 culminating in a constriction of the cortex and the cell 

 membrane. In amitotic division the nucleolus, originally 

 single, divides into two nucleoli, which migrate apart 

 to opposite sides of the nucleolus, thus becoming physio- 

 logically isolated; the nucleus then divides and the two 

 halves migrate apart; and finally the cytoplasm becomes 

 at first physiologically, and later physically, separated 

 or isolated into two independent masses. In mitosis 

 the centrosome and astrophere seem to constitute the 

 dynamic center of the cell and to take the initiative in 

 cell-reproduction. Two centers arise instead of one and 

 these migrate apart, each organizing about itself a radial 

 system. The chromosomes, at first occupying a neutral 

 position between the two radial systems, divide longi- 

 tudinally so that half of each chromosome migrates to 

 each daughter-system; and finally the cytoplasm is 

 partitioned off into two independent or semi-independent 

 systems and cell- reproduction has been accomplished. 



Cell-division in both Protozoa and Metazoa may be 

 either equational or differential. When, as in the 

 fission of the Mastigophora and in the majority of cases 

 of early cleavage, the line of separation (cleavage 

 furrow) is meridianal, i.e., parallel to the axis of polarity 

 of the cell, the cell products are, as a rule, equivalent, 

 each being a mirror-image of the other. This may 

 justly be termed cellular twinning, for it strikingly 

 resembles the most characteristic types of twinning seen 

 in the higher organisms and differs only in that the 

 process is concerned with but one cell instead of many. 

 When, however, the line of separation is at right angles 



