IV. i INITIAL STRUCTURE OF THE GERM 265 



Again, by shaking the eggs it is possible to prevent the 

 division of one centre. A tripolar mitosis is the result and 

 a simultaneous division into three. Out of 695 such eggs 58 

 became normal Plutei. This is also in accordance with the 

 theoretical expectation, for the probability of each of three 

 receiving a complete set of chromosomes when there are 3 n x 2 

 to be distributed is much greater than when these chromosomes 

 have to be distributed amongst four cells. 



The abnormal larvae are of various types, some showing no 

 trace of the Pluteus organization, some a small trace, such as 

 a skeleton on one side. Others have no gut, or some of the 

 pigment cells are missing (Fig. 161). 



The distortion of development cannot be attributed to the 

 deficiency in the number of chromosomes. Each of the cells 



of a quadripartite ovum may have - = chromosomes, 



for the larvae reared from them may have all their nuclei of 

 the same size (containing the same number of chromosomes), and 

 it is known (artificial parthenogenesis- and merogony) that a 

 larva will develop quite well with only n. Nor is it need- 

 ful that all the parts should have the same number of chromo- 

 somes, for normal larvae may be experimentally produced, with 

 n chromosomes in the nuclei of one region of the body and 2 n in 

 the rest (see below). At the same time dispermy does not neces- 

 sarily involve abnormality : in such eggs the two pairs of centres 

 may remain apart and no quadripolar figure be produced : these 

 develop into ordinary Plutei (Fig. 162 B). Conversely, quadri- 

 polar mitoses may be found in monosperm eggs, but these produce 

 pathological blastulae. No explanation of the abnormality, 

 therefore, is possible, so Boveri concludes, except that it is due 

 to the irregularity in the distribution of the chromosomes. For 

 normal development it is essential that each cell should receive 

 not a definite number but a definite combination of chromosomes, 

 a complete set of the chromosomes of the species. It is the 

 lack of this complete combination in one or more cells that is 

 the direct cause of the derangement of development. 



Hence the chromosomes are not alike, but qualitatively 

 different. 



Their division, is, however, never (except in reduction) a 



