22 THEOPHILUS S. PAINTER 



for himself. Once one learns to know the shape of the individual 

 tetrads, then polar views of equatorial plates are intelligible and 

 in all complete spindles 11 chromosomes are found. ^ 



The history of the sex-chromosome complex is given in the 

 adjoining text figure 4. A close inspection of this complex 

 (labeled XY, figs. 7, 10, 12 and text figs. 3 and 4) shows that it 

 is made up apparently of three elements arranged end to end. 

 (Only two spermatogonial chromosomes are involved.) There 

 are two egg-shaped masses, the X-element and a blunt rod which 

 is the Y-chromosome. The egg-shaped mass lying distal to the 

 equatorial plate is somewhat larger than the other similar body. 



Text fig. 3 The chromosomes of the first maturation spindle. All are tetrads 

 except the X-Y elements. 



The Y-chromosome may appear as bulged at one end (figs. 7 

 and 10), but more frequently it is simply an elon|^!|^^^ss of 

 chromatin (text fig, 4, A to E). :9liillB^ 



The behavior of the X-Y chromosome complJPfl^ltistrated 

 in figures 7, 8, 10, 12 and in text fig. 4. The time oF#^oion may 

 be early (text fig. 4, A, B, and C) or late when these elements are 

 found lagging behind in the spindle (text fig. 4, D and E). In 

 either case the plane of division is such that the egg-shaped ele- 



* The fact that mammals have elongated tetrads in the first maturation divi- 

 sion (the author has observed them also in the bat and the striped skunk) un- 

 doubtedly explains the great difficulty which has been experienced in making 

 counts at this period. 



