216 MONTGOMERY A STUDY OF THE CHROMOSOMES 



germ cells gradually acquired the same uneven number of chromosomes, then by conju- 

 gation of such cells, similar numbers of chromosomes being added together, a new even 

 number would result. But there is still another possibility by which the uneven number 

 could pass into an even one. The odd chromosome, at least in the cases here desrrilx !, 

 does not divide in the second maturation division, and so behaves abnormally. Now such 

 an abnormally behaving chromosome might in time become differentiated from the other 

 chromosomes, and I venture the view that such odd chromosomes are on the way to be- 

 come chromatin nucleoli. The main fact on which this conclusion is based is that in 

 Protenor belfrageiii is the odd, the eleventh chromosome the "chromosome x" which 

 in the spermatocytic growth period evinces the phenomena of a chromatin nucleolus. 

 Then another correspondence is that the chromatin nucleoli in most Hemiptera act like 

 the odd true chromosome in usually not dividing in the second maturation division. Here 

 we have an explanation for the origin of those peculiarly modified chromosomes, the chro- 

 matin nucleoli, thoroughly in accord with the facts I have described for them : the chro- 

 matin nucleoli are modified chromosomes, in point of origin the odd chromosomes which 

 appear in the period of transition from a higher (or a lower) to a lower (or higher) even 

 normal chromosomal number. And there are generally two chromatin nucleoli in the 

 spermatogonia, because the odd chromosome in those cases where there is an uneven nor- 

 mal number had probably been formed in most cases, as it certainly appears to have orig- 

 ignated in Protenor belfragei, as a union of two univalent, chromosomes which had failed 

 to separate from one another in the spirem stage of the spermatogonic mitosis. This also 

 explains why the two chromatin nucleoli are generally placed close together in the monas- 

 ter stage of the sperrnatogonium, they having been originally contiguous in the spirem 

 thread. 



Such an explanation of the origin of the chromatiu nucleoli from the odd chromosomes 

 seems to be in accord with all the facts, and so far may be considered a true explanation. 

 The chromatin nucleoli are modified chromosomes, and it is the odd chromosomes which 

 become thus modified. Conversely, we should expect that chromatin nucleoli would be 

 formed whenever the chromosomal number in changing from a higher to a lower one, or 

 the converse, passes through a transition period of an uneven number. Now, as has been 

 shown in the descriptive part of this paper and tabulated on page 207, all the Hemiptera 

 sxamined have two chromatin nucleoli, but some have a larger number. Wherever 

 there is a larger number we find generally that they are of different volumes, and the 

 question arises : why this difference in volume? The explanation might be that the largest 

 ones are those most recently formed ; the smallest those which had been evolved at earlier 

 periods, and which are smaller because they are perhaps diminishing through a gradual 

 degeneration. If the chromatin nucleoli when once formed should always preserve their 



