chap, xvi.] SYMMETRY IN TRI ASTERS. 431 



different in its nature, or indeed that it is anything but a visible 

 expression of the equality of the strains tending to part each 

 segment from its neighbours. (The case of the triaster is taken 

 as the simplest and most plainly symmetrical, but examples of 

 cells with greater numbers of centrosomes, sometimes dividing 

 symmetrically, have also been seen.) 



For our purpose this fact is first of use as a demonstration of 

 the absurdity of an appeal to " Reversion " as a mode of escape 

 from the admission that variations in Radial Symmetry may be 

 total and perfect though the new number of segments is one 

 which presumably never occurred in the phylogeny ; for we need 

 scarcely expect that even conspicuous defenders of the doctrine 

 that all perfection must have been continuously evolved, will 

 plead that the cells of every tissue in which a triaster is found 

 did once normally divide with three poles. Yet if it be once 

 granted that the symmetry of these abnormal forms is a sudden 

 and new departure from the normal, it will not be easy to put the 

 other cases on a different footing. 



Though we have repudiated all concern with the causes of ab- 

 normality, mention may be made of the fact that multipolar figures, 

 both regular and irregular, have been observed to result from the 

 action of reagents {e.g. quinine, Hertwig 1 ). Such figures are of 

 course well known especially in the case of carcinomatous growths, and 

 as Hertwig observes, from the resemblance of these figures to those 

 artificially induced by chemical means it seems possible that these 

 pathological appearances may also be the result of some chemical 

 stimulus. But whatever be the immediate or directing causes of 

 abnormalities in cell-division, or of those other abnormalities in the 

 segmentation of Radial Series of larger parts, and whether any of the 

 causes in the several cases be similar or different, we can scarcely 

 avoid recognition that the resulting phenomena are closely alike 2 . 



1 0. Hertwig, Die Zelle u. d. Geivebe, 1893, pp. 192—198. 



2 See also a case of the presence of triasters in two bilaterally symmetrical 

 tracts of the blastoderm of Loligo [v. infra). 



