davis: spermatogenesis. 93 



4. The Maturation Period. 



A. Dissosteira Carolina. 



1. Autosomes. 



At the end of the growth period the loops of the spireme, though still 

 showing their polar arrangement, have taken a peripheral position, 

 while the longitudinal split has become indistinct. In the next stage 

 (g), which marks the beginning of the maturation period (Plate 4, 

 Figs. 57, 58), the spireme loops have become detached from the nuclear 

 membrane and are irregularly distributed through the nucleus. The 

 chromatin becomes aggregated into larger granules, while the longi- 

 tudinal split is more distinct than at any previous stage, the space 

 between the two series of chromatin granules being wider than at any 

 other time. 



In stage h (Fig. 59) the loops have become converted Into definitive 

 tetrads. These vary greatly in shape, but may be roughly grou{)ed 

 into three types: (1) straight or curved rods, (2) crosses, and (3) rings, 

 or loops with their free ends crossed. These types are all, I believe, 

 modifications of a common fundamental form composed of two longi- 

 tudinally split threads or rods of equal length joined end to end. Each 

 tetrad is apparently formed from a single loop of the spireme in the 

 following manner: When the loops first become free they are appar- 

 ently of uniform appearance throughout their entire length, but a little 

 later the longitudinal split begins to widen at the middle of the loop 

 forming a diamond shaped opening (Plate 7, Figs. 160, 161). This 

 opening I believe to be at the point of union of two univalent autosomes 

 of the spermatogonia. These rod-shaped autosomes are usually 

 more or less curved, rarely straight. This is the simplest type of tet- 

 rad which I have found in this species, but in Hippiscus a still simpler 

 form occurs (Fig. 159). In this genus there is no widening of the 

 longitudinal split at the middle, but there is at this point a distinct 

 break in the chromatin, the intervening space being bridged over by 

 linin. 



The second type of tetrad has the form of a cross (Figs. 165-167) 

 in which the four arms may be approximately equal in length, or one 

 pair may be longer than the other. A further complication is caused 

 by the arms being usually more or less bent, the free ends of each pair 

 tending to approach each other. However, both pairs of arms never 



bend in the same direction. Figure 169 shows a case where this 







