84 BIOLOGY OF THE PROTOZOA 



in all parts of the cell. The best known are those connected with 

 the nucleus— including its development and differentiation. The 

 changes in the nucleus, like changes in the cell, are brought about 

 through metabolic activity and the results of such changes belong 

 to the derived organization. The formation of nuclei, together 

 with chromatin changes, chromosome formation and spindle for- 

 mation, belong therefore to the derived and not to the fundamental 

 organization. 



A. Derived Nuclei and Derived Nuclear Structures. — 1. The For- 

 mation of a Nucleus.— The formation of the massive type of nucleus 

 during reorganization after conjugation is clearly shown in the 

 case of Uroleptus mobilis (Fig. 1, Frontispiece). The young macro- 

 nucleus is formed by a second division of a fertilization nucleus 

 after conjugation when it appears as a vesicular nucleus with a 

 fine linin reticulum which has no staining capacity. In life it 

 appears like a large, highly refractile vacuole (the so-called "pla- 

 centa"). It remains in this ghost-like condition for a period of 

 three or four days, enlarging meanwhile and becoming ellipsoidal 

 in form. Chromatin ultimately makes its appearance in the form 

 of minute granules on the nuclear reticulum. These granules in- 

 crease in number and in size until the characteristic dense nucleus 

 with intense staining capacity results and the nucleus is no longer 

 visible in life 1 (Fig. 27, p. 58). It then divides with the first post- 

 fertilization division of the cell, and each daughter nucleus divides 

 three times (see also p. 315). 



2. Multiple and Dimorphic Nuclei.— While a single nucleus is 

 characteristic of the vast majority of Protozoa, multiple nuclei are 

 not uncommon and may be found in every group. In some forms, 

 as in many Mycetozoa, the multinucleate condition may be due, 

 not only to repeated nuclear divisions as in Uroleptus described 

 above, but to the plastogamic union of originally independent 

 cells, the aggregate being called a plasmodium. In other cases, as 

 in Foraminifera, Radiolaria and Myxosporidia, the multiple nuclei 

 are due to the incomplete division of the cell body after the nuclei 

 have "divided; or no attempt at all is made by the cell body to 

 divide. Analogous multinucleate stages are frequently found dur- 

 ing certain phases of the life history of many types such as the ante- 

 cedent stages of sporulation and gamete formation in Rhizopoda 

 and Sporozoa. In still other, and in the typical cases, multiple 

 nuclei are present throughout the entire vegetative life, the num- 

 ber ranging from two to several hundred (c. g., Actinosphaerium). 

 Characteristic and familiar examples of binucleate cells amongst 

 rhizopods are Arcella vulgaris, Pelomyxa binucleata, etc.; amongst 

 flagellates, Giardia intestinalis and other species of the same genus. 



1 See also pp. 71 and 315 for development of nucleic acid. 



