THE CELL AND CELL DIVISION 



61 



bodies, or karyosomes, within the nucleus is specialized as an organ of 

 division, called the central body and functionally equivalent to a centro- 

 some (Fig. 29). This body does not lose its chromatic character, may 

 be surrounded by a definite membrane, and appears to have functions 

 other than those of a centrosome which it exercises during the intervals 

 between divisions. In cell division this central body remains wholly 

 or in part intranuclear. Apparently this represents a very early stage 





FIG. 30. Nuclear division in the Rhizopod, Chlamydophrys stercorea. After 

 Schaudinn (Doflein). A. Nucleus with central body and chromatin threads. 



B. Elongation of central body and beginning of formation of equatorial plate. 



C. Equatorial plate. Central body spindle-shaped with polar centrosome-like 

 thickenings. D. Equatorial plate divided into two. Plasma radiations from 

 the "centrosomes." E. Fission of central body and chromatin masses. F. 

 Division completed. Daughter nuclei being reconstructed. 



in the differentiation from a chromatic body of the centrosome which 

 later becomes wholly achromatic and typically extranuclear. Several 

 other forms have a dynamic division center equivalent to the centrosome 

 but intranuclear; a nucleus of this type is known as a centronucleus (Fig. 

 30; see also Fig. 25, B) . In the division of such nuclei the nuclear mem- 

 brane may remain entirely intact (Euglena) or, as in Noctiluca, the 

 nuclear membrane may partly break down during mitosis. In several 

 forms possessing a definite extranuclear centrosome this body remains 

 undivided in cell fission, passing to one daughter cell alone, while a 

 new centrosome develops in the other cell; but this forms first as an 



