168 PROTOZOA 



live. The part without the nucleus loses the capacity for assimilation, 

 for growth, and for regenerating the lost parts. For a time it can react to 

 stimuli, move about. Sensibility and contractility persist only so long as 

 the necessary elements, formed under the influence of the nucleus, are 

 present. When they are used up the last manifestations of life are lost 

 and death ensues. So it may be said that the chemism of the cell needs 

 the participation of the nucleus. 



The nucleus is also concerned in reproduction, of which the most 

 primitive type is binary division (figs. 120, 150, 151). Budding is rarer, 

 its character being most evident when several buds are separated simul- 

 taneously from the mother animal (fig. 21). The nuclear division occurs 

 in different ways. Like the cell body, it may divide amitotically, but it 

 can present the complicated phenomena of mitosis (formation of spindle 

 and chromosomes). In not a few instances the specific organ of division, 

 the centrosome, "appears, so that all transitions from direct to extremely 

 complicated division are present in the phylum. 



Very frequently the nuclei multiply without a corresponding division 

 of the protoplasm, so that large masses of protoplasm, with hundreds or 

 even thousands of nuclei arise (multinucleate cells, syncytia); or both 

 nucleus and protoplasm may grow, without division, to extraordinary 

 size. In both instances, after an interval of time, there is a simultaneous 

 division into hundreds or thousands of reproductive particles; the pro- 

 toplasm, in the first case, dividing in accordance with the number of 

 nuclei present ; in the other following the division of the mother nucleus 

 into a multitude of daughter nuclei. Many Protozoa divide in the free 

 state while swimming or creeping about; others first encyst, that is, assume 

 a spherical shape and secrete a protective envelope. 



In the Protozoa may occur a fusion of individuals conjugation' 

 which in many respects has much similarity to the process of fertilization 

 in Metazoa and in plants. In some (conjugation of many Rhkcpods) 

 this does not correspond to true fertilization, since only the protoplasm 

 unites (plasmogamy), while the fusion of nuclei (car yoga my) necessary to 

 fertilization does not occur. In others a fusion of nuclei takes place. In 

 the cases which have been accurately studied there has been seen, before 

 the fusion of the nuclei, a process comparable to the formation of the polar 

 globules in the egg, to this extent, that in each of the conjugating individ- 

 uals the nucleus divides twice and of the products of division only one, 

 the nucleus intended for caryogamy, persists, while the others (polar 

 globules) degenerate. 



These cases of true fertilization may differ greatly. The conjugating 

 individuals may be equal in size, iso gametes (most Infusoria, many Rhizo- 



