igo CYTOLOGY 



CHAP. 



Finally, we may point out that the local differentiation, or amsotropy, 

 of the e^g cytoplasm belongs to the class of exceptions which prove 

 the rule. Not only is there no mechanism for an equal partition of these 

 substances among the daughter cells at cell division, but their mode of 

 action depends upon the fact that they are not so distributed. It is 

 therefore very improbable that they can retain their continuity through 

 the numerous cell divisions leading from the unfertihzed eg^ through 

 the cleavage divisions and all the divisions in the female germ track 

 till the cycle is complete with the formation of the next generation of 

 oocytes. It would appear therefore that they must be formed anew 

 in these cells in each generation, and all the arguments in favour of 

 the general morphogenetic activity of the nucleus in moulding the 

 cytoplasm apply equally in favour of the view that these substances also 

 are formed by the agency of the nucleus. Whatever view therefore is 

 taken of the " organ-forming substances " of the egg cytoplasm, their 

 presence does not affect the question of the monopoly by the nucleus 

 of the hereditary substance, which stands or falls on other grounds. 



G. CHROMIDIA AND CHONDRIOSOMES 



We now come to a very difficult chapter of cytology, in which state- 

 ments of fact and theory are so contradictory that it is at present 

 scarcely possible to do more than give an abstract of the work done 

 and the interpretations put upon it ; the questions involved are, however, 

 too important to pass over, in spite of the necessity of reserving judge- 

 ment on the issues. 



(i) Chromidia 



As we have already pointed out, we are in almost complete ignorance 

 as to the way in which the nucleus exerts its regulative and morpho- 

 genetic functions, but these appear to be usually exerted on the cyto- 

 plasm through the nuclear membrane. Occasionally, however, the 

 chromatin comes to he naked in the cytoplasm instead of forming part 

 of a nucleus with a definite architecture enclosed by a membrane. This 

 is very commonly the case in Protista, where granules of chromatin 

 called chromidia often He in the cytoplasm. These may be in addition 

 to the formed nucleus, or may take the place of this at definite stages 

 of the Ufe history, or again may constitute the whole chromatic garni- 

 ture of the animal, a formed nucleus being absent. Sometimes these 

 chromidia are destined to take part in the reproduction of the organism 

 (generative chromidia) ; in other cases they are finally absorbed by the 

 cytoplasm without playing the part in reproduction usually allotted to 



