Ill DIFFERENTIATION 93 



really deserved the name of pluteus. Of these three one had 

 to be removed to the category of double-spindled eggs, another 

 was probably a J egg, while the third had an abnormal 

 skeleton. In this last there were three sizes of nuclei, small 

 in one quarter, medium in one quarter, and large in one half 

 of the larva, and a supposed distribution of chromosomes in 

 the tetraster is suggested to account for this. 



In the remaining ten, which did not deserve the name and 

 style of pluteus, there were disintegrated cells in the blastocoel, 

 imagined to be derived from one of the four blastomeres, while 

 three areas characterized by the different size of their nuclei, 

 and each one-third of the whole, were found in the larva. 



Usually, however, only one half or one quarter of the egg 

 develops. 



The abnormality of development is considered by Boveri to 

 be directly due to the irregular distribution of chromosomes, 

 which is such that the chance of each cell in a quadripartite 

 tetraster egg receiving a full set at least of the n chromosomes 

 is practically negligible. The chance of each cell of a tripartite 

 triaster egg receiving a complete set is however appreciable, 

 and a certain percentage of these develop normally. The 

 chance of one cell in either case receiving the total comple- 

 ment is greater still, and these when isolated give rise to 

 normal plutei in a fair proportion of the cases. 



It is concluded, therefore, that the chromosomes are different, 

 and that at least one complete set of the n chromosomes of the 

 species is necessary, not merely in the ovum, but in every cell 

 into which that ovum divides in order that its development 

 may be normal. 



Such alternative hypotheses, as that the abnormality 

 observed is due merely to irregularities in the number of 

 chromosomes, are negatived by the known normal develop- 

 ment of plutei with nuclei of different sizes in the different 

 regions of the body, the sizes being determined (see above) by 

 the number of the chromosomes, and being such that the 

 number may have been less than n, or greater than n, and in 

 the latter case either a multiple of n or not. 



The chromosomes are therefore different, and probably them- 



