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 ^^ith 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 Delia 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 greath' predominating. In Delia 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 iri the individiial, 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 })y Wilson ('09, '10), Carothers ('17), 



