12 CYTOLOGY CHAP. 



Allium cepa (the onion) the number is i6. The species Ascaris megalo- 

 cephala contains two varieties, one of which, bivalens, has four chromo- 

 somes, the other, univalens, has only two. This last example has the 

 smallest number of chromosomes yet recorded, or indeed conceivable, 

 for an animal reproducing itself sexually, as will appear directly. The 

 largest number of chromosomes so far accurately determined for any 

 species is probably the 208 found in the crustacean Camharus immunis 

 (Fasten, 19 14). Often nearly related forms differ widely from each 

 other as regards the number of their chromosomes. 



Another extremely important fact is that in many species, both of 

 animals and plants, the chromosomes in any one nucleus are not all of 

 the same length, and, moreover, that the relative sizes of the chromosomes 

 are constant, in spite of the fact that the whole series of chromosomes may 

 be longer in some tissues than in others. The relative size differences 

 are also independent of the changes in length undergone by the chromo- 

 somes during mitosis, for these affect the whole series of chromosomes 

 alike and approximately simultaneously. Indeed, throughout the whole 

 mitosis, from the early prophase to the end of anaphase, the chromosomes 

 are almost continuously shortening and thickening. 



The constancy of the number of the chromosomes and of their relative 

 sizes in individual organisms and species, together with many other 

 considerations discussed in Chapter V., has led to an almost general 

 agreement that the chromosomes, though not recognizable as such in 

 the resting nucleus, nevertheless maintain their continuity from one 

 mitosis to another, so that the substance — at least the essential living 

 substance — of each chromosome, though diffused in the resting nucleus 

 and indistinguishably intermingled with the substance of the other 

 chromosomes, is nevertheless condensed together again in the next 

 prophase. The same series of chromosomes that entered into the resting 

 nucleus at telophase reappears therefore at the next prophase, each 

 single chromosome of the one stage being continuous with one of the 

 other. 



An examination of those species in which the size differences among 

 the chromosomes are strongly marked discloses at once the fact that 

 there are two chromosomes of each size. If the chromosomes of such a 

 form are designated, in order of magnitude, by the letters of the alphabet 

 A, B, C . . ., we find in each nucleus two chromosomes of each kind, 

 namely, A + A + B + B + C + C . . . We may here anticipate the later 

 chapters by explaining that this double supply of chromosomes is due 

 to the fact that in sexual reproduction the new individual is formed by 

 the union of two germ cells, one from each parent, and that each germ 

 cell has one complete set of chromosomes (A + B + C + . . .), so that the 

 fertilized egg has the double set (A + A + B + B + C + C + . . .). Owing 



