,894. CELL-DIVISION. 419 



ova (10), and in a large number of cases has shown that the first segmen- 

 tation nucleus may entirely disappear, and all trace of chromatic or 

 achromatic substance vanish. The chromosomes appear to break up 

 into minute granules, and to become scattered throughout the proto- 

 plasm of the ovum, where, by fusion, they apparently, later on, form 

 the "free" or "yolk" nuclei. It must, however, be noticed that no 

 trace of division into blastomeres is to be found till normal karyo- 

 kinetic division sets in. The chromatin of the first nuclear spindle 

 appears to be the result of a fusion of some of these nuclear granules. 

 There appears, moreover, to be, in insects at any rate, every condi- 

 tion between a regular karyokinetic division of the first segmentation 

 nucleus and an irregular fragmentation of the same. The limits of 

 this article forbid further examination of the evidence in favour of 

 the fragmentation of the nucleus, but it should be mentioned that in 

 the spore-formation of many Protozoa a similar breaking-up of the 

 nucleus takes place, without any trace of karyokinesis. Hickson has, 

 in a recent paper (11), reviewed the literature on the subject, besides 

 giving details of his own observations, and the conclusions he draws 

 appear to be the most probable at the present state of our knowledge. 

 They are :- — 



1. Fragmentation of the nucleus is a normal method of nuclear 

 division, and is not always a sign of pathological change, or, indeed, 

 of anything abnormal. 



2. In many of the instances in which the nucleus is supposed 

 to disappear, there is, as a matter of fact, minute fragmentation of its 

 component parts. 



3. Fragmentation of the nucleus is never followed by any kind 

 of corresponding division in the body of the ovum. 



It is certain that no hard and fast line can be drawn between 

 mitosis and amitosis, since many recorded observat.ions prove how the 

 one passes almost imperceptibly into the other. There remains, 

 however, the question as to which of the two is the more primitive. 

 To give an answer in the least degree probable is a matter of con- 

 siderable difficulty, if not of impossibility. In the first place, it is 

 almost necessary to postulate the past existence of non-nucleate 

 organisms, even if it cannot be allowed that there are any such living 

 at the present day. It is equally essential to assume that they must 

 have reproduced themselves in some way or other, either by dividing 

 into equal halves, or by breaking up into irregular pieces. 

 Hence, before the nucleus was developed, there were already forces 

 residing in the cell-substance, tending either to pull it into equal 

 halves or into many pieces. After the appearance of the nucleus in 

 those individuals which had normally multiplied in the former 

 manner, regular karyokinesis would take place, while in the latter 

 case fragmentation of the nucleus could have been the only possible 

 cause. The latter method appears to have become rarer and rarer in 

 the Metazoa, probably becavise the nucleus took on a more definite 



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