364 Dr, H. E. Ziegler on Amitotic Nuclear Division 



division." In my previous paper I dilated upon the fact that 

 " in cases of a widely different character we find peculiar 

 forms of nuclei, which we may class together with the nuclei 

 of the periblast of Teleosteans, and that these phenomena 

 constitute an important chapter for the natural history of the 

 cell-nucleus in general." " It would seem fitting were we 

 to use the expression fragmentation in the animal kingdom 

 (and, indeed, in the first place only for the Metazoa) for those 

 morphologically and physiologically associated cases wliich 

 are characterized as follows. The nuclei are considerably 

 larger than the ordinary nuclei in the same animal, and 

 exhibit an abnormal poverty, or an abnormal distribution, of 

 chromatin. The nuclei multiply by direct nuclear division ; 

 it often happens that the division is not carried as far as the 

 separation of the segments, so that the nuclei show bud-like 

 processes and irregular prolongations, or appear divided by 

 constrictions. Fragmentation occurs in cells which no longer 

 undergo division, or in masses of protoplasm which have 

 arisen through incomplete cell-division [i. e. through nuclear 

 division without concomitant division of the cell). The 

 appearance of fragmentation is connected with the fact that 

 the cell has become specialized, has adapted itself to a definite 

 physiological function, that, for instance, it is harbouring and 

 assimilating food-yolk, is performing some process of secre- 

 tion or absorption, &c. The nuclei have degenerated, in so 

 far as the cell is no longer capable of division, and conse- 

 quently can no longer morphologically take part in the further 

 building-up of the embryo or in processes of regeneration ; if 

 in this sense we designate the nuclei as degenerate, this does 

 not preclude them from performing their physiological func- 

 tion for a longer or shorter time. There are simpler modes 

 of degeneration which lead to speedy destruction ; fragmen- 

 tation only occurs when the nuclei first undertake a specialized 

 function and then perish." 



" Within tlie blastoderm, scattered about in the yolk, are found, as is 

 well known, in theMuscidse, as well as in all Insects hitherto investigated, 

 cells, or at least nuclei, which we consequently very frequently term 

 yolk-cells (" Vitellophaga,' according to Nussbaum). Now as regards 

 the share which these niuch-discu^sed cells take in the building-up of 

 the embryo, at present far the most generally accepted view is that they 

 merely assist in the assiuiilation of the yolk, and that, although tbey and 

 the cells of the blastoderm have a common origin, the former elements 

 take no special part in the formation of tissues, aud are not to be included 

 in the category of the true germinal layers."' The vitellojthaga of the 

 Muscidee are nuclei without a plasma-envelope, and appear " as generally 

 very irregularly defined or amceboid structures of reluti^ely gigantic 

 eize." 



