302 KANSAS UNIVERSITY SCIENCE BULLETIN. 



ZOOLOGICAL LABORATORY. 



to receive due consideration, is not to be held as a first essen- 

 tial in the determination of individuality in the chromosomes 

 but rather as a secondary character dependent to a large ex- 

 tent upon size. 



SUMMARY. 



1. There are twenty-three chromosomes in the spermato- 

 gonia. They may be arranged in a graduated series of eleven 

 pairs plus one odd unpaired member, which is most likely the 

 accessory. In this series there may readly be recognized three 

 extremely large pairs and three extremely small pairs. The 

 members of the pairs do not always lie together in the equa- 

 torial plate. 



2. The accessory chromosome is seen for the first time in 

 the prophase of the spermatogonia, where it may be recog- 

 nized by the peculiar vesicle within which it lies, located at 

 the periphery of the nucleus. It is at this time reticular in 

 structure, like the other chromosomes. After the last sper- 

 matogonial division it remains condensed, while the ordinary 

 chromosomes become reticular and diffuse. As usual in Ac- 

 rididse it lies in close contact with the nuclear wall, and it 

 continues to maintain this peripheral position even in the spin- 

 dles of the two maturation divisions that follow. In the growth 

 period it passes from the telophase condition of the last sper- 

 matogonial division to a lump condition. Out of this lump is 

 evolved later a long, slender, much-coiled spireme. This spi- 

 reme continues to shorten and thicken, and the chromosome 

 finally enters the metaphase as a short, thick, almost straight 

 rod. During all this time this chromosome has shown no signs 

 of being a paired or bivalent structure. In the prophase of the 

 spermatogonia there was no spireme segmented into two parts. 

 In the spermatogonial metaphase it was not paired, for the 

 spermatogonial number of chromosomes was odd, not even. 

 In the telophase of the last spermatogonial division, and in the 

 resting and growth period following, it was not paired, for the 

 second dense chromatin-like body present there was found to 

 be a nucleolus, not a heterochromosome. In the latter part of 

 the growth period and in the prophase of the first spermato- 

 cyte, when the chromosome had assumed the spireme condi- 

 tion, no constriction or bending at any particular place (the 

 middle) was met to indicate that the element had already 

 been paired before leaving the spermatogonia. On the con- 



