33 2 C. E. MCCLUNG. 



this definiteness of separation and recombination of the chromo- 

 somes then there would be all conceivable combinations instead 

 of a definite and constant one such as is always found. An ex- 

 amination of the conditions in this genus cannot fail to put us in 

 possession of some facts that could not well be obtained in any 

 other way. It is necessary to go back to the early first sperma- 

 tocyte prophase to secure an understanding of the structure of 

 the chromosomes --a point that I have always insisted upon 

 since the beginning of my work upon spermatogenesis. Here it 

 is found that one of the tetrads has become united end to end with 

 the accessory chromosome so that the planes of their longitudinal 

 division are coincident. As is invariably the case in Orthopteran 

 cells the accessory chromosome is univalent, so that there is thus 

 produced a trivalent chromosome like that of Hesperotettix. 

 Throughout the length of the chromosome there is a clear longi- 

 tudinal split which, along the portion contributed by the acces- 

 sory chromosome, is sharply marked because of the homogeneous 

 character of its elements, while in the remainder of its length it 

 appears somewhat interrupted on account of the granular struc- 

 ture of the tetrad region. 



Some time near the dissolution of the nuclear membrane there 

 i*s joined to this trivalent chromosome, by end to end union, one 

 of the tetrads, thus producing a pentivalent element or decad. 

 This element, we know, has a division running its length and cor- 

 responding to the longitudinal split of the chromatic thread, 

 although in its homogeneous condition it is, like that of all the 

 other chromosomes, obscured. In the metaphase of the first 

 spermatocyte, as I have before stated, the separation of the chro- 

 mosomes takes place in such a way that to one of the daughter 

 cells there goes a tetrad and to the other a tetrad plus the acces- 

 sory chromosome. For these elements the first maturation 

 mitosis represents a reduction division and one of a very unusual 

 character. At the same time, however, the other chromosomes 

 are dividing longitudinally. Of this there can be no doubt for 

 in the Tryxalines the ring chromosomes are numerous and in 

 Meruiiria as many as five or six may be found in one equatorial 

 plate. This evidence is unmistakable to one who has a knowledge 

 of their structure gained from a study of their formation in the 



