lOS PROTOZOA 



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

 for oTowth, and for regenerating the lost parts. For a lime it can react to 

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

 tire necessarv elements, formed under the inllueuce of the nucleus, are 

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

 and death ensues. So it mav he said that the chemism of the coll needs 

 the participation of the nucleus. 



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

 primitive type is binary ilivision (figs, uo, 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 di^•ide amitotically, but it 

 can present the complicated phenonrena of mitosis (formation of spindle 

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

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

 complicateil division, are present in the phylum. 



\'ery frequently the nuclei multiply without a corresponding di\ision 

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

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

 nucleus and protoplasm may grow, without division, to e.xtraordinary 

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

 division into hundreds or tlrousands of reproducli\e particles; tire pro- 

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

 nuclei present; in the other following tire didsion of the mother nucleus 

 into a multitude of daughter iiuclei. jNIanv Protozoa di\ide in the free 

 state wliile swimnring or creeping about; others first ciicysl, that is, assume 

 a spherical shape and secrete a protective envelope. 



In the Protozoa mav occur a fusion of individuals — conjugation — 

 wlrich in many respects has iiruch similarity to the process of fertilization 

 in ]Metazoa and iir plants. In some (conjugation of many Rhizopods) 

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

 unites {pldsiiio(;a)tiy), while tlie fusion of nuclei (caryogaiiiy) necessarv to 

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

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

 the fusion of the nuclei, a process com])arable to the formation of the jiolar 

 globules in the egg, to tlris extent, th:it in each of the conjugating indi\ id- 

 uals the nucleus di\-ides twice and of the products of di^'ision onh' one, 

 the nucleus intended for caryogani)-, persists, while the others (jtolar 

 globules) degenerate. 



These cases of true fertilization may differ grealh". The conjugating 

 indixiduals ma}- be ecpial in size, isoi^iinuics (n-iost Infusoria, n-iany Rhizo- 



