Il6 CELL-DIVISION 



the formation of an amphiaster, the attraction-sphere remaining undivided mean- 

 while. Heidenhain showed in the following year, however, that in some cases 

 leucocytes containing two nuclei (doubtless formed by amitotic division) might 

 also contain two asters connected by a spindle. Both Heidenhain and Flemming 

 drew from this the conclusion that direct division of the nucleus is in this case inde- 

 pendent, of the centrosome, but that the latter might be concerned in the division 

 of the cell-body, though no such process was observed. A little later, however, 

 Meves published remarkable observations that seem to indicate a functional activity 

 of the attraction-sphere during amitotic nuclear division in the " spermatogonia " 

 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 ?1 of 

 Bellonci). These were, and still are by some writers, regarded as degenerating 

 nuclei. Meves, however, asserts and the accuracy of his observations 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 granular 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 ("archo- 

 plasm ") 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 " archoplasm " of the autumn-stage 

 arises by the breaking down of a definite spherical attraction-sphere, which is 

 re-formed again in the spring in the manner described, and in this condition the 

 cells may divide either mitotically or amitotically. He adds the interesting observa- 

 tion, 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 

 m 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 



1 '91, p. 628. 



2 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. 



