1008.] The, Cell Structure of the Cyanophycew. 407 



time connecting the two daughter nuclei together until complete 

 separation takes place (see fig. 2). In some cases, shortly after the 

 division is completed, delicate threads may be observed connecting the 

 daughter nuclei to the new cell wall, and we get then a resemblance 

 to the diaster stage of ordinary karyokinetic division. This is the 

 only indication of the existence of anything like a spindle figure 

 which I have observed in the Cyanophycese, and it is not by any means 

 a well-marked or constant feature of the division. 



The division of the cell appears to go on independently of the 

 nuclear division. It is quite common to find, long before" the first 

 division is complete, several new cell walls in various stages of develop- 

 ment in other parts of the cell. 



Theoretical. From a careful consideration of the observations con- 

 tained in this paper it seems clear that, although we cannot regard the 

 central body of the Cyanophycese as a nucleus of a normal type similar 

 to the nuclei of the higher plants, it certainly appears to possess a 

 sufficient number of the characteristics of such nuclei to warrant its 

 recognition as a nucleus of a simple or rudimentary type. If we take 

 the chief chemical and morphological characters of the nuclei of higher 

 organisms and compare them with those of the central body, we shall 

 find that it has as much claim to be regarded as a nucleus as some 

 of the structures which are accepted as nuclei in the protozoa. Out 

 of some twelve main characters, chemical and morphological, which 

 are found attaching to the nuclei of higher plants, we have seven, and 

 possibly nine, occurring in the nuclei of the Cyanophycese. 



These are : (1) the presence of a nuclear network ; (2) its reaction 

 towards nuclear stains ; (3) its behaviour towards digestive fluid ; 

 (4) the presence of phosphorus ; (5) the presence of masked iron ; 

 (6) the amitotic division, which resembles, in some respects, the 

 division in Euglena ; and (7) the presence of chromatin granules on 

 a linin framework. It differs from the nucleus of the higher plants 

 in the absence of a true mitosis with spindle fibres, and in the absence 

 of a nuclear membrane and nucleolus ; but under certain conditions 

 the deeply-stained substance of the central body is found condensed 

 into a deeply-stained granule suspended by delicate fibres in the centre 

 of the cell, and in young cells the central body is often limited towards 

 the cytoplasm by a vacuolar membrane, so that the presence, under 

 certain conditions, of a body resembling a nucleolus, and at least a 

 rudimentary nuclear membrane, is not excluded. 



If it is not a nucleus then it can only be (1) a specialised portion of 

 the cytoplasm ; (2) a body of the nature of a pyrenoid ; or (3) a 

 special organ of the cell of which we know not the function. 



There seems to me to be no doubt that it is among these plants and 

 in the bacteria and protozoa that we must seek for those types of 

 nuclei which will give us a clue to the most rudimentary condition 



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