REPRODUCTION 213 



B. Division in the Sarcodina. — It is questionable whether any 

 rhizopod divides in the very simple manner described by F. E. 

 Schultze for Amoeba polypodia. The "limax" types indeed approach 

 this simplicity (Fig. 106) but new discoveries are constantly at hand 

 to indicate that these are not as simple as they have been described. 

 Thus Arndt (1924) quite recently has given creditable evidence of 

 the existence in a simple ameba, Hartmannella klitzkei, of a definite 

 centrosome with centriole which is permanently extranuclear (Fig. 

 58, p. 106). At division of the cell the centrosome divides and the 

 daughter centers with their centrioles, take positions at the poles 

 of the nuclear spindle which originates within the nucleus. The 

 mitotic figure is thus made up of cytoplasmic elements, kinetic 

 elements derived from the nucleus, and chromatin. A similar 

 combination occurs in dividing Heliozoa. The original description 

 of division of Acanthocystis aculeata by Schaudinn, a form possessing 

 the characteristic central granule of the Heliozoa, has been consider- 

 ably modified by later observations. According to Schaudinn the 

 central granule or centroblepharoplast, which is the focal point in 

 the cell of the radiating axial filaments, divides to form an amphi- 

 aster (Fig. 50, p. 95) which becomes the central spindle of a typical 

 mitotic figure. The more recent observations of Stern (1924) 

 indicate that, as in the simpler ameba described above, the central 

 granule of Acanthocystis behaves as a cytoplasmic centrosome, 

 forming poles of a mitotic figure which is derived otherwise entirely 

 from the nucleus. Individuals which have been deprived of their 

 skeletons and membranes, which afford resistance to the activities 

 of the enclosed protoplasm, become "sprung," so to speak, and the 

 unusual freedom from restraint results in a separation of the eentro- 

 somes from the remainder of the spindle which completes its division 

 without further participation of the centrosomes (Fig. 0", p. 121). 



Schaudinn's description of division in Heliozoa was confirmed in 

 the main by Zuelzer (1908) in connection with the aberrant form 

 Wagnerella boreal is. Here the axopodia-bearing portion of the cell 

 is free from the silicious mantle which covers the remainder of the 

 animal, the nucleus being in an enlarged pedal portion attached to 

 the substratum. The central granule is in the geometrical center 

 of the "head" and is the focal point of the axopodial filaments. 

 Each of the latter bears a granular enlargement similar to a basal 

 body. In preparation for division these move centripetally toward 

 the central granule forming a zone about it which divides with the 

 division of the central granule. In the meantime the nucleus 

 migrates from the other end of the body and with the spindle formed 

 by the divided central granule forms the mitotic figure. 



Complications in the division process accompany the presence of 

 shells and tests. Where these are chitinous or pseudochitinous, 

 they may also divide with the cell body (Pseudodifflugia, Cochlio- 



