26 ZOOLOGY 



7. Accompanying or following the last nuclear changes the 

 cytoplasm may have become constricted into two masses, or 

 separated by the formation of a wall perpendicular to the axis 

 of the spindle (Fig. 8, G, H) . The daughter cells may separate 

 or remain united. These final stages are known as the telo- 

 phase. There are other protoplasmic bodies in the cytoplasm 

 which seem to divide, though not so accurately as the nucleus, 

 into two sub-equal portions. Some observers claim, on the 

 contrary, that these bodies arise anew in the cytoplasm or 

 nucleus. These are variously named plastids, mitochondria, 

 etc. The important point is that there seems to be a definite 

 tendency to get equal division of the cell materials. 



Cell division is at the beginning of all the complexities 

 of structure found in the higher forms of animals. Each sexually 

 produced organism commences life as a single cell, from which 

 ■the adult is formed by cell-division, and the clinging together 

 of the daughter cells. 



42. Functions of the Nucleus and Centrosomes. — While we 

 can follow some of the externals of the various cell activities, 

 the manner of their occurrence and their causes are in the 

 greatest obscurity. We are not able to say just what part is 

 performed by the different structures involved. It is hazardous 

 to say that one structure is more important than another; yet 

 it seems to be proven that the nucleus is quite essential in 

 cells which possess nuclei, for the proper performance of even 

 the ordinary nutritive functions. Some of the unicellular 

 animals may be artificially mutilated in such a way that the 

 lost parts may be regenerated and the normal form restored. 

 A relatively small piece of the Protozoan, Stentor, for example, 

 can reproduce the whole, if a portion of the nucleus be present. 

 A much larger piece without nuclear material is wholly unable 

 to regenerate lost parts, and even seems unable to control or 

 exercise the ordinary assimilative functions. The phenomena 

 of indirect cell division suggest that activity on the part of the 

 centrosomes and nucleus precedes that of the cytoplasm 

 Experiments also show that the division of the cytoplasm may 

 be checked or interrupted by external influences without 

 interfering with the division of the nucleus. On the other hand 



