200 



MONTGOMERY — A STUDY OF THE CHROMOSOMES 



divisions; and in that paper 1 bave referred to the observations of oil ier workers cor- 

 roborative of tbe individuality of the chromosomes, so that they need not be recalled here. 

 In such H&miptera as tbe Coreidce there is no true rest stage in the growth period, so that 

 definite chromosomal outlines can be determined throughout the; growth period. And 

 even in those Hemiptera where a. rest stage does occur in this period, in which the 

 chromosomes become reticular and practically indistinguishable from one another, all 

 evidence renders it probable that the chromosomal individuality is retained through this 

 period — i. e., that a particular univalent chromosome of the maturation mitosis represents 

 a particular one of the spermatogonia. This evidence is as follows: The chromatin 

 nucleoli, which axe only modified chromosomes, retain their compact form and so are 

 readily distinguishable throughout all periods of these generations. Then in those cases 

 where there is an uneven number of chromosomes in the spermatogonia, and the odd oik; 

 remains univalent in the spermatocytes, this odd one can always be distinguished in the 

 maturation mitoses. Then where two of the spermatogonic chromosomes are particularly 

 large, there is always found in the first maturation mitosis one particularly large bivalent 

 chromosome which can only correspond to those two. Further, after a rest stage there is 

 always the same number of chromosomes as were present before that rest stage. All this 

 evidence speaks for a chromosome of a spermatogonium corresponding to a chromosome 

 of a spermatocyte ; and if in these generations there is a, maintenance of chromosomal 

 individuality, there is a probability that, there is such a, maintenance through all 

 generations of the germinal cycle. But this conclusion by no means implies that a 

 chromosome of one generation is actually the same as a chromosome of another. For 

 we know that each chromosome is halved in an equation division, and that each daughter 

 chromosome so produced must increase to a volume equal to that of the mother chromo- 

 some before it, enters upon a second mitosis. Thus new substance must continually 

 be elaborated by the chromosomes during the rest stages, and in the course of this 

 elaboration the old substance of the chromosome and its physical form are correspond- 

 ingly changed. There is no evidence that chromosomal substance remains unchanged 

 from generation to generation, for all evidence shows that, it undergoes metabolic change 

 and growth in the rest stages. But nevertheless it seems very probable that, a chromo- 

 some of one generation is a, derivative of a particular chromosome of the preceding genera- 

 tion, and that the chromosomes may thus Ik; said to maintain themselves as entities 

 through successive generations. 



There are many cases where chromosomal boundaries are indistinguishable in the 

 rest stages, so that in such cases it has been argued that the chromosomes show no indi- 

 viduality. But these are negative examples, and positive cases that speak for the main- 

 tenance of the chromosomal individuality must be considered as the decisive ones. No 



