38 W. S. SUTTON. 



That," says Boveri, " konnte ich aus den verschiedenen Typen der 

 normalen Kontrolzuchten, durch Kombination der rechten Halfte 

 einer Larve mit der linken einer anderen, Bilder herstellen, die 

 den in Rede stehenden Dreierplutei fast genau entsprechen." 

 To these points is added the fact that while all the isolated blas- 

 tomeres of a normal four-cell stage develop exactly alike, those 

 of dispermic three- or four-cell stages rarely or never do so, even 

 when the numerical distribution of chromosomes appears equal ; 

 and, further, that in large numbers of larvae from double-fertilized 

 eggs all possible combinations of characters are to be found, just 

 as all possible combinations of chromosomes from the three parent 

 cells may enter into the composition of their nuclei. From these 

 and other data, Boveri draws the conclusion that " Nichteine be- 

 stimmte Zahl sondern eine bestimmte Kombination von Chromo- 

 somen zur normalen Entwicklung notwendig ist, und dieses 

 bedeutet nichts anders als dass die einzeln Chromosomen ver- 

 schiedenen Qualitaten besitzen miissen." 



Thus we are brought to recognize a physiological individuality 

 in a form in which the chromosomes are morphologically indis- 

 tinguishable and the nuclei of which, after the anaphases, offer no 

 mechanical hindrance to the free intermixture of the chromatin. 

 We have already reviewed the reasons for believing the accessory 

 chromosome in "the cells of Brachystola to be the possessor of 

 specific functions and it only remains again to call attention to the 

 likelihood that the constant morphological differences between the 

 ordinary chromosomes are the visible expression of physiological 

 or qualitative differences. 



In conclusion, from the point of view thus suggested, let us 

 again consider the phenomena of fertilization. In either sperm- 

 or egg-nucleus a complete series must be present since either 

 may produce a normal embryo without the other. Every normal 

 fertilized egg, therefore, as well as every cleavage-cell derived 

 from it, must have the field of each character covered by two 

 chromosomes-- one from each parent. The chromosome series 

 of the echinoderm cleavage-nucleus is thus shown to be physi- 

 ologically a double one just as in Brachystola we have seen it to 

 be morphologically double, and the doubling in both cases is 

 seen to be accomplished in an exactly similar way viz., by the 

 contribution of equivalent series by the two parents. 



