THE THEORY OF CONTINUITY 41 



interfere with the generaHsation that each species has a characteristic 

 chromosome complement seen at mitosis (v. Plate I). This com- 

 plement is a constant property of the nucleus, undergoing a 

 constantly recurring series of changes in each cycle of mitosis. 

 Hence each chromosome that appears at the prophase of one 

 mitosis corresponds with a chromatid that entered into the nucleus 

 at the preceding telophase in all its visible properties except 

 that it is double : it has reproduced itself identically. It was on 

 the assumption of this essential constancy of behaviour that Weis- 

 man correctly predicted the universal occurrence of a reduction that 

 would compensate for the addition that occurs when nuclei conjugate 

 in fertilisation. 



The correspondence may be attributed to one of two conditions. 

 Either it is due to conditions that are external to the chromosomes 

 in the same sense as a correspondence in form between the walls of 

 two cells is due to conditions external to the cell- walls, or it is 

 due to conditions inherent in the chromosomes themselves. This 

 would mean that the chromosomes, or essential parts of them, are, 

 unlike the cell-wall, permanent structures, arising, like the nucleus 

 as a whole, the cell as a whole, or the organism as a whole, only from 

 pre-existing structures of the same kind. This hypothesis, which 

 is expressed by saying that the chromosomes have the same 

 permanence as nuclei, cells and organisms, was verified by the 

 work of Boveri (1892-1909) on Ascaris. Boveri found embryos 

 that developed from eggs in which only one polar body had been 

 formed, owing to the failure of separation of two nuclei at the 

 second meiotic division. Clearly the original failure of the 

 separation would give egg nuclei with two chromosomes instead of 

 one. Fertilised by a sperm with one chromosome the first 

 segmentation nuclei should have three chromosomes. All the 

 nuclei of embryos arising in this way (still recognisable from the 

 single degenerated polar body) had three chromosomes at mitosis. 

 Thus, if it is assumed that external conditions determine the 

 number of chromosomes, it must also be assumed in this case that 

 these conditions are permanently altered by an accident of separation 

 which, on the hypothesis of permanence, would lead directly and 

 simply to the observed results. 



