164 N. M. 



4. The second >permatocytc> arc vi-ibly dimorphic in tin- n -i 

 stage, one containing a 1. IT-IT, tin- other a smaller heterochroino- 

 some. 



5. The spermatids and spermatozoa are not visibly dimorphic. 



I >IS< i 55 [ON. 



One or more unpaired heterochromosomes have been reported 

 by ('iiiyer ('<), *IO) for the guinea-chicken, dome-tic chicken 

 and man. and by Jordan (*Il) for the opossum. This is the 

 lirM case, so far as I am aware, in which an unequal pair of 

 heterochromosomes has been found in a vertebrate. Although 

 it has not been possible to follow the heterochromosomes through 

 the second maturation mitosis, there is no evidence aiiaiiM the 

 Mipposition that i-ver>- ( hromo-i mn di\'ides equally in that 

 mitoM's, ;^ivin^ spermalids one half of which contain a division 

 product of the larger heterochromosome, the other half one from 

 the smaller heterochromosome. If it can be shown that the 

 female guinea-pig has 56 chromosomes, two of which correspond 

 to the larger heterochromosome of the male, then the fertilization 

 formula will be like that for similar cases in inseci 3. 



Eggs. Spermatozoa. Xy^ote. 



\ 2-, +X + 27 +X 54 -f .V .V =9 



i \2 7 +X + 2 7 + y 54+Xl 



This is what is to be expected, but, in view of recent develop- 

 ments in echinoderm spermatogenesis, it is at any time possible 

 that we may find other conditions; for example, 



Eggs. Sjirrmatozoa. 



f 27 + Y + ->7 +A- 54 +x r =cf 



'1 2 7 + y 27H , 54 + 1 1 =9 



In this case the female would still be homo/yi;ous Inn 

 male would have the excess of chroma t in, rcali/ini; in thai 

 McClung's ('02) original sui;.^esiion in regard to the "accessor) 

 in the Orthoptera; and the s]ici-mato/( ( "ni containing tin- A chro- 

 mosome would be male-producing instead of female-producing. 



It has always seemed to me someuhai improbable that the 

 small heterochromosome of an ime(|ual pair o\\cs its smaller 

 size to gradual degeneration, and much more likely that the 



