282 KANSAS UNIVERSITY SCIENCE BULLETIN. 



ZOOLOGICAL LABORATORY. 



of this element. To the contrary, we have found it always and 

 under all conditions an independent, odd, unpaired and uni- 

 valent member of the chromosome complex. 



THE CHROMOSOMES IN THE SPERMATOCYTES. 



It is at this stage in the course of the development of the 

 germ-cells that the characteristics of the various chromosomes 

 of the complex stand out most prominently. The drawings 

 were made principally from polar views of the cell-plate. A 

 few lateral views were drawn to show in particular the habit 

 of the accessory. To understand the shapes assumed by the 

 various chromosomes during the mitotic stages attention 

 should be called to the forms they assume in the prophase just 

 before the formation of the mitotic figure. Tetrad formation 

 has already received such a thorough discussion by McClung 

 ('00 and '02) that it seems unnecessary to say anything further 

 about it, but the mistakes of some recent workers along this 

 line make it again necessary to bring up the subject. In fig- 

 ures 26 and 29 are shown the different forms of rods, rings, 

 crosses, V's, Y's, etc., that may be seen in the late prophase 

 stages. As a study of the figures will show, "all of these 

 shapes may be reduced to a single type, that of a rod with one 

 longitudinal and one cross division." Every rod is formed 

 by the end-to-end union of two homologous spermatogonial 

 chromosomes. In this union like ends always unite with like 

 ends, i. e., proximal with proximal and distal with distal, if 

 the latter unite. By "proximal" is meant that end of the chro- 

 mosome which is always turned towards the center of the 

 cell-plate and to which the spindle fibers are attached, whether 

 in spermatogonia or spermatocytes (figs. 1-6 and 30-36). By 

 "distal" of course is meant the opposite or outer end. In this 

 conjugation of the spermatogonial pairs to form the tetrads the 

 proximal ends always unite, but the distal may or may not. 

 If they do there are formed rings, figure 8's, etc. If they bend 

 toward each other but do not unite there occur partially closed 

 rings, V's, bent rods, or kidney shapes. If the members of 

 the pair lie fully extended there is produced simply a straight 

 rod. 



At the point where the members of the pair always unite, 

 the proximal end, the chromatids (halves of the spermato- 

 gonial chromosomes) may quite often have started to move 

 outward in the plane of the cross-division (XX in fig. 29). 



