92 DIFFERENTIATION III 



the dimensions (surfaces) of the nuclei are as 1:2:3; this is 

 accounted for by supposing only two of the centres to have 

 been united by spindles (this was actually observed) one of 

 which was occupied by one sperm-nucleus, the other by the 

 second male and the female nucleus: on division the centres 

 would receive only 2n and 3 n chromosomes. (3) By supposing 

 the numbers of chromosomes on the three spindles to have 



been -s and 2n, the centres, on the division of these, 



. X- 



would receive TI, 2J?i, and 2^ n. This would account for the 

 ratios of the dimensions of the nuclei observed in the three 

 regions. 



Lastly, amongst these larvae are asymmetrical forms. It 

 is suggested that this is due to some slight difference between 

 the sperms, such differences having been in fact observed 

 amongst larvae of the same monosperm culture. 



In the remainder of the embryos or larvae reared from 

 tripartite ova, more or less serious defects were found. Thus 

 the skeleton is incomplete or wanting in one part, the pigment- 

 cells may be entirely absent from some one region, but never 

 from the same part. The defective region is always charac- 

 terized also by the size of its nuclei, that is, is derived from 

 one of the original three cells. In other cases one-third, two- 

 thirds, or all three are pathological, that is, break up into cells 

 which pass into the blastocoel and there degenerate. The 

 remainder (if two-thirds) may develop into a normal larva 

 if the pathological one-third has been got rid of at an early 

 stage, but if not till later, then the two-thirds larva shows 

 irregularities in skeleton. These irregularities are supposed 

 to depend on (1) the relation of the triaster to the (assumed) 

 plane of bilateral symmetry in the ovum, and (2) the position 

 of the degenerate cell, whether dorsal, ventral, right, left, 

 anterolateral, or posterolateral. The nuclei in the two regions 

 of the sound part may be of different sizes. 



When two-thirds are degenerate only stereoblastulae or 

 stereogastrulae are produced. 



Of 1,600 quadripartite ova kept under observation, only 

 thirteen reached the pluteus stage, and only three of these 



