10 ORGAXIZATIOX AND LIFE-HISTORY OF PROTOZOA 



dividing nucleus, though the chromosomes are no longer visible as 

 individual units, they still exist as separate entities. During syngamy, 

 when two gametes unite and their nuclei fuse, the chromosomes of the two 

 uniting nuclei enter the zygote nucleus, so that, unless a reduction is made 

 in the number of chromosomes, at each succeeding union the chromosome 

 number would be doubled. Usually the number of chromosomes in the 

 gamete nuclei is only half that of the nuclei of other cells of the body, and 

 the process by which this reduction is brought about is known as the 

 reducing division, or meiosis. 



Though in the vast majority of cases it is recognized that the nuclei 

 of daughter cells are the products of division of the nucleus of a parent 

 cell, it is supposed that occasionally amongst the Protozoa nuclei may be 

 formed from extra-nuclear chromatin granules which appear in the 



A B 



Fig. 2. — Formation of Nuclei from the Chromidial Body in Arcella vulgaris 



( X ca. 300). (After E. Hertwig, 1899.) 



A. Normal individual with two nuclei and mass of chromidial substance. 



B. The chromidial substance is breaking up and nuclei are being formed from the fragments. 



cytoplasm (Fig. 2). It seems to be an undoubted fact that chromatin 

 material in the form of granules may leave the nucleus and take up a 

 position in the cytoplasm. This has been described as taking place, not 

 only in Metazoan cells, but also in the Protozoa. Such granules of 

 chromatin, which occur in the cytoplasm, are known as cTiromidia. It 

 is not, however, an easy matter to determine the true nature of granules 

 which occur in the cytoplasm, and it has not infrequently happened that 

 identical granules or material have been described as chromatin by one 

 observer, and as some other substance by another. There seems little 

 doubt that both in the case of Metazoan cells and Protozoa, chromidia 

 do not occur so frequently as some have supposed. When the question 

 of the origin of nuclei from these chromidia is considered there is still 



