680 president's address. 



during segmentation the nuclei divide with caryokinesis, while 

 later, when the cells have definitely assumed the function of 

 taking up the nutrient matter of the yolk, divisioii is always 

 direct. 



In general Ziegler concludes that fragmentation (which term he 

 uses as the equivalent of direct nuclear division), takes place in the 

 case of cells which do not divide any more, or in protoplasm- 

 masses which have been produced by imperfect cell-division. Its 

 appearance is dependent on the fact that the cell is becoming 

 specialised — adapted to a particular function. The nuclei are 

 degenerate inasmuch as the cell is capable of no further division, 

 and, as a result, cannot take part mor[)hologically in the further 

 building up of the embryo, or in the phenomena of re-generation. 

 The appearance of amitotic nuclear division, according to Ziegler, 

 always indicates the end of the series of divisions. It is very 

 unlikely that nuclei which once come to divide amitotically will 

 ever again .divide with mitosis. As a rule, he holds, the amitotin 

 nuclear division is not followed by cell-division. 



The nuclei which divide amitotically are always characterised 

 by their specially large size, and in cells with such meganuclei 

 there always goes on a particularly active process of secretion or 

 assimilation. 



Ziegler adduces a number of cases of amitotic cell-division in 

 both vertebrates and invertebrates which, he maintains, tend to 

 bear out his view. 



Lowit* dissents altogether from Ziegler's view that dii'ect division 

 of nuclei indicates the termination of the series of divisions, and 

 states that in the blood-cells of the crayfish and the leucocyte 

 elements of the rabbit's lymph he has observed amitotic cell- 

 division leading to a true new formation of cells. He distinguishes 

 between a process of fragmentation which is degenerative, and a 

 process of direct nuclear division which is not. 



* " Ueber amitotische Kerntheilung.'" Biol. Centralbl. 1891. 



