SPECIAL MORPHOLOGY OF THE PROTOZOAN NUCLEUS 273 



F. General Considerations 



The mitotic figure in plants, higher animals, and in Protozoa is 

 composed of two parts, of which one, the chromatin, is widely held 

 to be the primary agent in heredity, while the other, forming the 

 "achromatic structures," is generally regarded as the agent by means 

 of which the chromatin is divided equally between the two daughter- 

 cells. The two parts have more or less independent antecedent 

 phases, and it is possible, therefore, to conceive the two portions 

 unequally developed. In Protozoa both portions are relatively simple 

 in structure, although complex chromosomes may accompany rela- 

 tively simple achromatic specializations, and vice versa. 



The stages in chromosome formation in different types of Protozoa 

 may be briefly summarized as follows : (1) The most primitive nucleus 

 is, apparently, in the form of a compact sphere of chromatin, the 

 multiple division of which is the prelude to reproduction of the cell. 

 (2) A higher type comprises nuclei with membranes, and with chro- 

 matin in one (Sporozoa) or in many (Noctiluca, etc.) karyosomes, 

 which break up by multiple division into granules. The granules 

 thus formed unite secondarily into lines forming primitive chromo- 

 somes. (3) In still higher forms the granules do not return to the 

 karyosome stage, but are widely distributed over the linin reticulum 

 (Flagellidia, Actinophrys, some stages of Actinosphcerium, Metazoa). 

 (4) The highest type contains chromatin granules embedded in a linin 

 reticulum, or aggregated to form net knots or karyosomes analogous 

 to the more primitive chromatin spheres. Like the primitive karyo- 

 somes, these net knots break up into granules which come together 

 in lines for division (spireme stage of some Protozoa, of plants, and 

 Metazoa), and these lines segment into chromosomes of definite 

 number and size (Metazoa and Metaphyta). 



In some Protozoa the nuclei remain permanently in one or the other 

 stage described. Thus in many of the Phytoflagellida they are per- 

 manently in stage 1 ; in the simple Monadida they are typically in 

 stage 3, in the Rhizopoda in stage 2, while the higher types of nuclei 

 pass through nearly all of these stages during preparation for division. 

 The net knots thus show a return to the primitive condition, the 

 chromatin granules to the permanent granular state in Flagellidia, or 

 to the disruption of the karyosomes of type 2 ; the fusion of the 

 granules into spiremes, to the primitive chromosome formation in 

 Noctiluca. The definite chromosomes, finally, represent the highest 

 grade of chromatin specialization. 



While it is quite probable that the chromatin of plant nuclei, of 

 metazoan nuclei, and of protozoan nuclei is everywhere essentially the 

 same substance, the similarity is not so obvious in case of the "achro- 



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