106 bulletin: museum of comparative zoology. 



the same pole. Using this as a basis, he was able to find satisfactory 

 explanation of the variation in number, and one which served to sup- 

 port the theory of the individuality of the chromosomes. Stevens 

 ('12b) found in Diabrotica supernumerary chromosomes varying in 

 number from 1 to 5, and believed that they had their origin in trans- 

 verse and longitudinal divisions of the X-chromosome, which normally 

 divides only longitudinally. These anomalies can therefore be 

 explained on the basis of some unusual method of distribution of the 

 chromosomes in mitosis; the fact that such extra cliromosomes persist 

 in all the cells of the animal in which they are found is a striking bit 

 of evidence in favor of the belief that they maintain their individuality. 



Delia Valle ('09, '12) has attacked the theory of individuality, 

 declaring that the chromosomes are temporary and variable structures, 

 which form in the prophase and dissolve in the telophase. He thinks 

 their number is the quotient of the quantity of chromatin divided by 

 the average size of the clu-omosomes. The quantity of clu-omatin 

 is said to vary with conditions of nutrition, and the number of cln-omo- 

 somes with variations in external conditions. He made counts of 

 chromosomes from cells of the peritoneum of salamander larvae and 

 obtained numbers varying from 19 to 27. Montgomery ('10) points 

 to the following grounds for doubting the accuracy of Delia Valle's 

 conclusions : — " 1. The chromosomes counted are long, sinuous 

 ribbons, that overlap and interlace, the most difficult kind to count 

 with accuracy. 2. He included in the counts some cells in prophases, 

 where one cannot be certain that all the clu'omosomes have fully 

 separated. 3. The total number of the chromosomes is so large, 

 about 24, that the chance of error in enumeration is great. It is but 

 fair to conclude that while his technique was excellent, his choice of 

 material was bad, consequently a degree of scepticism might well be 

 maintained toward his results." Delia Valle in his latest paper ('12) 

 argues that the chromosomes are variable structures, because he has 

 been able to find transition stages between mitotic and amitotic 

 methods of cell-division in the erythrocytes of young salamanders. 

 It is a well-known fact that amitosis frequently accompanies degenera- 

 tion, and the figures of Delia Valle present strong indications of being 

 those of degenerating cells. It is precisely in degenerating cells that 

 one would look for inconstancy in the behavior of the chromosomes. 



2. Constancy in size and shape. — It will be convenient to consider 

 the subjects of size and shape together. As to shape, we may dis- 

 tinguish spheres, rods, and V-shaped elements. Spheres are invariably 

 small and may be regarded as short rods. It will be convenient to 



