CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 205 
extra chromosomes (dyads). This is a small variation in chro¬ 
mosome number in the individual of from twenty-three to twenty- 
five. In each of four individuals, including the above three, he 
finds that in from one to three of the primary spermatocytes 
observed in division, one of the eleven tetrads normally present 
is replaced by two separate dyads. Both of these dyads in the 
division of the cell may pass undivided to either daughter cell 
with or without the accessory chromosome. One of the secondary 
spermatocytes resulting from the division of the cell in which 
these two dyads accompany the accessory chromosomes receives 
twelve dyads plus the accessory and the other receives ten 
dyads. The other spermatocyte in which two dyads do not 
accompany the accessory chromosome gives rise to one secondary 
spermatocyte with ten dyads plus the accessory, and another 
with twelve dyads. This would make possible four classes of 
spermatozoa containing ten to thirteen chromosomes. 
If, similarly, in Salamandra maculosa one of the twelve 
tetrads normally present in the spermatocytes and in the oocytes 
should be replaced by two dyads, there would be produced 
gametes with eleven, twelve, and thirteen chromosomes. These 
gametes would produce zygotes (individuals) having twenty-two 
and twenty-six chromosomes, respectively, a variation com¬ 
parable to that claimed by Della Valle. Further, if extra chro¬ 
mosomes can thus appear in the germ cells of an occasional 
individual, the same might also occur in the somatic cells. But 
this variation should be expected in but few of the total indi¬ 
viduals, making the proportion of cells containing the normal 
number greatly predominating. In Della Valle’s counts about 
one-half contain the normal number which is far too small a 
portion unless such variation is much more common than is at 
present known. 
Although the above is a clear case of a small variation in chro¬ 
mosome number in the individual , it must be clearly understood 
that these cases are exceptional and do not represent the normal 
condition. The chromosome number may vary in the species, 
but it is usually constant for the individual, as has been 
especially pointed out by Wilson (’09, TO), Carothers (’17), 
