132 



FUNDAMENTALS OF CYTOLOGY 



multinucleate conditions can be found. In the infusorian cell there are 

 commonly nuclei of two kinds: one or more micronuclei, which divide 

 mitotically and are concerned chiefly with reproduction, and a large 

 macronucleus, which divides by constriction and is concerned in the 

 physiological activities of the cell, including the mating reaction (Fig. 96). 

 A comjjonent of many nuclei in Protozoa and flagellates is the endosome, 

 a large, compact central mass which in different cases appears to contain 

 nucleolar matter, chromatin, or both of these substances. At the time 



Fig. 96. — Fission in Paramecium caudatum. 1, individual with single macroniicleus and 

 .single uiicronucleus above it. 2, the macronucleus is elongating and the inioronucleus has 

 divided, constriction beginning at middle of cell. 3, the macronucleus has divided, and 

 fission is nearly completed. (Courtesy of General Biological Supply House, Inc., Chicago.) 



of nuclear division it elongates and constricts into two (Fig. 97). Its 

 function is not understood. 



The aspects presented by protozoan nuclei at the time of division 

 are often very difficult to interpret. The nucleus may be very compact 

 and seem to undergo a simple mass division, or it may show a cloud of 

 small chromatic granules instead of chromosomes with obvious indi- 

 viduality. In many cases, however, chromosomes in definite numbers 

 and essentially like those in Metazoa have been demonstrated, ^\dth 



Fig. 97. — Mitosis in a flagellate (Heteronema) , showing endosome 

 chromosomes is longitudinal; late separation at one end makes 

 (After J. B. Loefer.) 



The division of the 

 it appear transverse. 



kinetochorcs, spiral chromonemata, longitudinal division, the association 

 of nucleoli with certain members of the complement, meiosis, and even 

 polyploidy. To what extent the less definite types of structure and 

 behavior should be regarded as primitive or as special modifications of 

 the condition generally prevalent in organisms nobody can say. The 

 achromatic figure also presents itself in many forms, with and without 

 asters and centrioles. Centrioles in some groups attain an astonishing 

 size and form (Fig. 99). 



