7<3 McCLUNG. 



ties or for the possible exchange of mutual influence with the other 

 chromosomes. Montgomery assigns the origin of the accessory 

 chromosome to a single spermatogonial chromosome, in the 

 Hemiptera, and I have clearly traced it to the same source in the 

 Orthoptera. It must be regarded, therefore, as a single element 

 and as the possessor of two chromatids, after its longitudinal 

 division, and so differs from the other elements which are con- 

 stituted of four chromatids. This is a matter of considerable 

 importance as will appear when we consider the division of the 

 spermatocytes. 



The period which witnesses a breaking down of the sperma- 

 togonial chromosomes and the construction of a thin chromatin 

 thread from their chromomeres is marked by changes just the 

 reverse on the part of the accessory chromosome. It enters the 

 prophase as a definite body with a staining reaction that is con- 

 stant and marked. These characters it maintains until it be- 

 comes indistinguishable in the spermatid. 



Meanwhile the other chromosomes of the spermatogonia are 

 lost in the substance of the spireme, in which condition their 

 chromomeres exist in relations far removed from those prevailing 

 in the component individual chromosome. This spireme stage 

 is one of extreme importance to the structures it involves. In 

 many cases, as has frequently been pointed out, it is a phase oc- 

 cupying relatively, and sometimes actually, a long period of time. 

 During its continuance, profound changes take place in the 

 nucleus as a result of which the chromatin emerges in the form 

 of chromosomes the like of which we are unable to find in any 

 other cells of the body. Instead of having two chromatids at 

 the time of the metaphase, each of these has four. Instead of 

 being as numerous as those of the spermatogonia, there are but 

 half as many. Throughout all the time involved in the produc- 

 tion of these fundamental differences, the accessory chromosome 

 has existed quite apart from the field of mutual influence in 

 which the other chromosomes operate. It is thus apparent that 

 it has its characters fixed, not in the generation which witnesses 

 its division but in the previous one. In other words it is a sper- 

 matogonial chromosome which divides in the spermatocyte 

 mitosis. 



