MONOGRAPH OF THE EXISTING CRINOIDS. 407 



protoplasm free from the metaplasic substances with which the rest of the cyto- 

 plasm is loaded. The formation of such a layer of hyaline protoplasm at the 

 junction of the cytoplasm with the caryolymph is to be explained as due to 

 physical causes and is comparable with the hyaline layer of Amceba. 



Such a conception of the nuclear membrane not only allows its formation to be 

 readily explained as due to physical causes, but also offers a ready explanation of 

 the ease with which this structure can be absorbed and reformed at different periods 

 in many cells. Moreover, it would certainly seem that the living protoplasm of the 

 cell is the most natural material of which the membrane, through which all solu- 

 tions must diffuse before entering the nucleus, would be composed. 



There are two very characteristic features in which the germinal vesicle differs 

 from the nucleus of actively secreting somatic cells; these are (a) the relative 

 paucity and fine subdivision of its chromatin, and (6) its large size and (at all 

 events usually) spherical form. 



Professor Chubb believes himself able to offer an explanation of both these 

 features of the germinal vesicle which will be in complete harmony with the inter- 

 pretations and conclusions to which he has been led as a result of the study of the 

 oocyte of Antedon, and which will, moreover, bring the behavior of the latter into 

 line with that, not only of somatic gland cells, but also of abortive ova functioning 

 as nurse cells. 



It is generally assumed that the chromatin of the germ cell constitutes the 

 mam, if not the only, carrier of specific characters. If this assumption be accepted, 

 then it is not unreasonable to suppose that this consideration must place a very 

 considerable limitation on the adaptive changes which the chromatin may undergo 

 during the increase of the cell activity which accompanies the process of growth 

 and yolk secretion. The somatic gland cell, on the other hand, as also the abortive 

 egg or nurse cell, are not destined to give rise to a long line of descendents; still 

 less are they responsible for the unimpaired preservation of the specific characters 

 of the race. Their chromatin, therefore, is free from the limitations which Pro- 

 fessor Chubb has assumed to exist in the case of the oocyte, and is thus capable of 

 very considerable adaptation to the requirements of the cell's activity. We thus 

 find that, while the nucleus of the gland cell and nurse cell speedily becomes richly 

 chromatic as the cell activity increases, that of the egg remains relatively poor in 

 chromatin. 



If such a limitation to the growth of the chromatin in the oocyte does exist, 

 then we have a ready explanation of the fine subdivision and equal distribution of 

 this material so frequently found in the germinal vesicle which will permit of our 

 following Riickert in interpreting this condition as correlated with intense nuclear 

 activity, although it is absent in the equally active nuclei of gland and nurse cells. 

 For it is obvious that this limitation will necessitate what chromatin there is being 

 used to the greatest possible advantage, and no clearer indication of such an 

 economical utilization of the chromatin could be offered than a fine subdivision and 

 equal distribution of this material. 



Professor Chubb has suggested that the spherical form of the germinal vesicle 

 is due to the latter constituting an active osmotic system, and that the occasional 

 142140 21 Bull. 82 28 



