120 



PROTOPLASMIC AGE OF PROTOZOA 



breaks up into ameboid gametes, in which the nuclei are formed, as in 

 centropyxis, by fusion of the idiochromidia granules (Figs. 46 and 47). 



Not only in rhizopods, but in flagellated protozoa as well, the idio- 

 chromidia arise in this manner. Thus, in the case of Mastigina 

 setosa, Goldschmidt ('07) has shown that the idiochromidia accumu- 

 late in heaps about the nuclear membrane, as in arcella or centropyxis, 

 before being scattered throughout the cytoplasm, where they ultimately 

 form the nuclei of gametes (Fig. 4S). 



Nuclear Dissolution. — There is probably no great difference 

 between the above-described method of idiochromidia formation by 

 transfusion, whereby the chromatin materials percolate through the 

 nuclear membrane in fluid form, and that by nuclear dissohition, 

 whereby the peripheral portion of the nucleus becomes scattered in 

 granular form throughout the cell body. Nor is this second method 



Fig. 49 



Ameba Umax (group of five on left) and Chilomonas Paramecium to show alveolar 

 structure of protoijlasni prior to idiochromidia formation. Two of the aiuebae are in process 

 of di\'ision. 



different, save in degree, from the third, which I have called nuclear 

 fragmentation. The distinctions have, at best, only a descriptive 

 value. 



Nuclear dissolution, in substance, was described more than thirty 

 years ago by Hertwig ('76) in connection with the radiolarian acan- 

 thometra. In this form there is a great increase in the thickness of 

 the chromatin at the periphery of the nucleus and at the expense of 

 the karyosome, and this cortex ultimately breaks down to form quan- 

 tities of minute secondary nuclei of the macro- and microgametes 

 (see Hertwig, 1907). Here, then, the peripheral rind of chromatin is 

 little more than a condensed zone of idiochromidia, and is closely 

 associated with the karyosome. In Ameba Umax (Fig. 49) there is no 



