182 BOTANY 



take up positions on the borders of the flattening matrix. When 

 prophase has been concluded the structure of the chromonemata 

 is difficult to see, but they appear to be a little contorted. At 

 metaphase each daughter chromosome can be seen to be composed 

 of a matrix containing two chromonemata, so that the entire 

 metaphase chromosome is double in respect to the matrix and 

 quadruple with respect to the more chromatic constituent. For 

 further details of this work the original should be consulted. 



As Sharp has very rightly pointed out : " the problem at present 

 confronting students of chromosome structure is to place a proper 

 evaluation upon these appearances presented by chromosomes 

 under the microscope. It seems clear that the chromatic substance 

 which has been called karyotin, and which is arranged in a slender 

 thread, with or without achromatic elements in the prophase, is a 

 constant nuclear constituent. It has been variously described as 

 having the form of granules, irregular short strands, a continuous 

 spiral chromonema, a spongy framework and a cortical sheath. 

 All of these aspects have been observed in sectioned material fixed 

 by the older methods, as well as in whole chromosomes by the 

 recently readopted acetocarmine process. Opinions differ widely 

 with respect to which are natural aspects and which are artifacts. 

 Probabilities seem to favour the naturalness of a regular type of 

 structure, such as is exemplified by chromonemata, as against the 

 irregularity characterising a spongy framework, arising through 

 alveolation or a matrix with scattered granules." 



Work on chromosomes from living cells has not progressed 

 sufficiently far as yet to determine wliich of the conflicting views 

 at present in vogue is correct. It is probably correct to say that 

 in all fixed material, by whatever method, a certain number of arti- 

 facts are bound to make their appearance, and it is the presence 

 of these artifacts that makes this particular problem of cytology 

 so difficult for investigation. 



THE NUCLEOLUS 



Practically all nuclei contain one or more bodies which are 

 termed nucleoli or are occasionally spoken of as plasmosomes. As 

 a general rule they are not present in the nuclei of male gametes. 



