512 



point of view have appeared recently in various journals, I have not 

 thought it advisable to lengthen my paper by the addition of one here. 

 The present paper was completed at intervals between periods of 

 other work during my occupancy of the Smithsonian Table at the 

 Zoological Station of Naples and I wish to make grateful acknowledg- 

 ment to both of these institutions for their respective courtesies and 

 for the exceptional opportunities afforded me for work. 



Summary. 



1) Seventeen chromosomes differing considerably in size occur in 

 the spermatogonia. 



2) Nine chromosomes appear for division in the primary sperma- 

 tocytes. Of these, eight are presumably bivalent, the other, which is 

 comparable to the "accessory" or "odd" chromosome of Tracheata, 

 remains unpaired. 



3) The odd chromosome passes undivided to one pole of the 

 spindle considerably in advance of the other chromosomes. The result 

 of the division of the primary spermatocyte is, therefore, that half of 

 the daughter cells contain eight and half, nine chromosomes. 



4) The eight chromosomes which pass to the one secondary 

 spermatocyte pair again to form four which ultimately appear at the 

 time of division in this cell. Eight of the nine which pass to the 

 other secondary spermatocyte pair similarly, the odd remaining unpaired. 



5) At the division of the secondary spermatocyte the odd chromo- 

 some after lagging for some time divides longitudinally. 



6) The divisions of the secondary spermatocytes result in the 

 production in equal numbers of two classes of spermatids, those con- 

 taining the odd chromosome and those without it. The two kinds of 

 spermatids are visibly different and ultimately give rise to spermatozoa 

 which differ in size. 



7) The axial filament of the spermatozoon tail apparently arises 

 from the outset in contact with the nucleus. 



Naples, Italy, 31 May, 1909. 



Explanation of the Plates VII and VIII. 



All of the drawings were made with the aid of a camera lucida and as they 

 appear here their magnification is about 2000 diameters. While the chromosomes are 

 in every case represented as accurately as possible, no special attempt has been made 

 to show the details of the achromatic structures beyond general outline and relations. 



Fig. 1. Late pi-ophase of spermatogonial division showing seventeen chromosomes. 



Fig. 2. Metaphase of spermatogonial division. 



Some New Types of Chromosome Distribution and their Relation to 

 Sex. Biol. Bull., Vol. 16, March 1909, No. 4. 



