82 CELL-DIVISION 



togonia " of the salamander. 1 Krause and Flemming observed that in the autumn 

 many of these cells show peculiarly-lobed and irregular nuclei (the " polymorphic 

 nuclei" of Bellonci). These were, and still are by some writers, regarded as 

 degenerating nuclei. Meves, however, asserts and the accuracy of his obser- 

 vations is in the main vouched for by Flemming that in the ensuing spring 

 these nuclei become uniformly rounded, and may then divide amitotically. In 

 the autumn the attraction-sphere is represented by a diffused and irregular granu- 

 lar mass, which more or less completely surrounds the nucleus. In the spring, as 

 the nuclei become rounded, the granular substance draws together to form a definite 

 rounded sphere, in which, a distinct centrosome may sometimes be made out. 

 Division takes place in the following extraordinary manner : The nucleus assumes 

 a dumb-bell shape, while the attraction-sphere becomes drawn out into a band 

 which surrounds the central part of the nucleus, and finally forms a closed ring, 

 encircling the nucleus. After this the nucleus divides into two, while the ring- 

 shaped attraction-sphere (" archoplasm ") is again condensed into a sphere. The 

 appearances suggest that the ring-shaped sphere actually compresses the nucleus, 

 and cuts it through. In a later paper ('94), Meves shows that the diffused "archo- 

 plasm " of the autumn-stage arises by the breaking down of a definite spherical 

 attraction-sphere, which is reformed again in the spring in the manner described, 

 and in this condition the cells may divide either mitotically or amitotically. He 

 adds the interesting observation, since confirmed by Rawitz ('94), that in the 

 spermatocytes of the salamander, the attraction-spheres of adjoining cells are often 

 connected by intercellular bridges, but the meaning of this has not yet been 

 determined. 



It is certain that the remarkable transformation of the sphere into a ring during 

 amitosis is not of universal, or even of general, occurrence, as shown by the later 

 studies of vom Rath ('95, 3). In leucocytes, for example, the sphere persists in 

 its typical form, and contains a centrosome, during every stage of the division ; but 

 it is an interesting fact that during all these stages the sphere lies on the concave 

 side of the nucleus in the bay which finally cuts through the entire nucleus. Again, 

 in the liver-cells of the isopod Porcellio, the nucleus divides, not by constriction, as 

 in the leucocyte, but by the appearance of a nuclear plate, in the formation of which 

 the attraction-sphere is apparently not concerned. 2 The relations of the centro- 

 some and archoplasm in amitosis are, therefore, still in doubt ; but, on the whole, 

 the evidence goes to show that they take no essential part in the process. 



3. Biological Significance of Amitosis 



A survey of the known cases of amitosis brings out the following 

 significant facts. It is of extreme rarity, if indeed it ever occurs in 

 embryonic cells or such as are in the course of rapid and continued 

 multiplication. It is frequent in pathological growths and in cells 

 such as those of the vertebrate decidua, of the embryonic envelopes 

 of insects, or the yolk-nuclei (periblast, etc.), wJiich are on the way 

 towards degeneration. In many cases, moreover, direct nuclear divi- 

 sion is not followed by fission of the cell-body, so that multinuclear 



1 '91, p. 628. 



- Such a mode of amitotic division was first described by Sabatier in the Crustacea ('89), 

 and a similar mode has been observed by Carnoy and Van der Stricht. 



