124 



CYTOLOGY CHAP. 



A. THE CONTINUITY OF THE CHROMOSOMES 



The first morphological hypothesis to be estabUshed is what has 

 come to be known as the individuality, or better, the genetic continuity 

 (Wilson) of the chromosomes. The meaning of this phrase is that the 

 material of the chromosomes is not resolved at telophase into a common 

 nuclear reticulum from which new chromosomes differentiate out in 

 the next prophase (as crystals might dissolve in a solvent and recrystalHze 

 out again), but that the substance of each telophase chromosome is 

 concentrated again into a corresponding chromosome in prophase. Thus 

 each individual chromosome is the direct descendant of the corresponding 

 chromosome in the previous cell generation as described on p. 128. 



This conclusion is at once suggested by the fact that the number of 

 chromosomes in any one species is constant (with certain, mostly well- 

 understood, exceptions) although it. may vary greatly in nearly alHed 

 species ; that the number is constant (with the same quahfication) in 

 different tissues though the total amount of chromatin may vary greatly 

 from tissue to tissue ; and that the number of chromosomes is halved at 

 gametogenesis and subsequently restored at syngamy. 



The truth of this hypothesis is indeed very generally accepted, being 

 supported by a great body of observations as well as by indirect evidence. 

 Indeed it would be exceedingly difficult to write a general treatise on 

 nuclear cytology without accepting the hypothesis as a basis, and the 

 reader will doubtless have noticed that it has frequently been implied 

 in this work. It will be necessary, however, to discuss briefly a few of 

 the problems which are raised thereby. 



While the earlier cytologists were content with demonstrating the 

 constancy of the number of chromosomes in a given species it became 

 evident, with the extension of the study to a wider range of forms, that 

 in many species the chromosomes are not all aUke, but differ from each 

 other in size, and especially in length ; as, except in the case of the meiotic 

 chromosomes, this is apparently the only dimension in which constant 

 differences occur, the thickness of all the chromosomes in a given nucleus 

 being approximately equal. Moreover, the length differences are constant, 

 so that in every nucleus (in mitosis) not only the same number of 

 chromosomes, but the same series, ranging from the largest to the smallest, 

 can be recognized. Again, it was found that there were in each diploid 

 nucleus two chromosomes of each size, so that if the chromosomes are 

 designated in order of size A, B, C, etc., the chromosome complex could 

 be designated thus : 



A+A + B+B+C+C+ . . . 



In the meiotic prophase the two chromosomes of each type, usually 



