CHAPTER X 



OTHER MODES OF NUCLEAR DIVISION 



In accordance with the well established principle which states that 

 only through the simpler organisms can an adequate understanding of 

 those higher in the scale of complexity be approached, search has been 

 made for primitive modes of nuclear division with the hope that light 

 may thereby be thrown upon the origin and significance of the elaborate 

 karyokinetic process which is so universally found in the cells of higher 

 animals and plants. It is to be acknowledged that such a phylogenetic 

 explanation of mitosis is very far from being reached, but many of the 

 observations recorded are nevertheless of a very suggestive nature. To 

 botanists the most interesting of these have been made upon the Cyano- 

 phyceae, which have long been a subject of controversy in this connection. 



Cyanophyceae. — For many years the nature of the "central body" 

 of the cells of such blue-green algae as Oscillatoria remained very obscure. 

 Butschli (1890), Dangeard (1892), Scott (1888), and others believed it 

 to be a nucleus of a somewhat primitive type, whereas other investigators, 

 among them Zacharias (1892) and Chodat (1894), denied its nuclear 

 nature. Zukal (1892) held that the peripheral portion of the cell repre- 

 sents a chromatophore, the central body consisting of cytoplasm with a 

 number of minute nuclei imbedded in it. 



One of the first critical accounts based partly on the study of sections 

 was that of Fischer in 1897. Fischer concluded that the central body, 

 in which he found no chromatin, is the main portion of the cytoplasm, 

 and not to be regarded as the forerunner of the nucleus or indeed as an 

 independent organ at all. He also investigated the nature of the periph- 

 eral portion of the protoplast. By treating- the plants with 10 per 

 cent hydrofluoric acid he dissolved away the other parts of the cell, leav- 

 ing this portion intact; and as the result of comparative studies on other 

 plants he concluded, in harmony with Zukal, that it is a single large 

 chromatophore. 



Since Fischer's work the most important contributions are those of 



Hegler, Kohl, Olive, Phillips, Gardner, and Miss Acton. Contrary to 



the view of Fischer, all of these cytologists interpret the central body 



as a nucleus, and the first three regard its division as essentially mitotic. 



The opinions of these workers with respect to the organization of the cell 



of the Cyanophyceae and the behavior of its nucleus are summarized 



below. 



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