MAMMALIAN SPERMATOGENESIS — OPOSSUM 19 



other (fig. 4) or it may even appear as two unequal egg-shaped 

 chromosomes joined as in figure 1. This inequaUty in the two 

 ends of the X-element persists throughout maturation. 



The autosomes are all more or less rod-like, but in some cases 

 there is a tendency for the inner end of a chromosome to be smaller 

 than the outer; the inner end may even be bent at an angle to the 

 rest of the chromosome. 



The cells figured (1 to 6) are taken from three different males; 

 there is no appearance of any fusion of the chromosomes and 

 little overlapping (fig. 1, 2, or 4). In some cells there is a tend- 

 ency for certain chromosomes to occur together, either lying side 

 by side (fig. 6) or one partly on top of the other (fig. 5). In 

 poorly preserved material such chromosomes would doubtless 

 appear as one element. 



In addition to the counts figured in plate I a great number of 

 other drawings of dividing spermatogonial cells were made by 

 the aid of a camera lucida, and counts then verified. In every 

 case 22 was found to be the number of chromosomes present. 



The division of spermatogonia offers no point of especial 

 interest, as it proceeds in the usual way. No lagging behind of 

 any elements has been seen. 



FIRST MATURATION DIVISION 



The author has made no attempt to follow in minute detail 

 the changes which the chromosomes undergo from the early 

 growth period to the time when they enter the first maturation 

 spindle. Jordan has clearly figured and described the essential 

 facts; my own preparations show stages similar, in the main, to 

 those found by him. A few points illustrated in text figure 2 

 will be touched upon. The so-called 'diffuse stage' (text fig. 

 2, A) is characterized by three nucleoli, which lie scattered among 

 the faintly staining chromatin knots. The two smaller nucleoli 

 are unequal in size. With the appearance of the leptotene 

 threads, some deeply staining areas are found in the nucleus, 

 which probably represent one or more of these nucleoli. The 

 pairing of the leptotene threads is seen with almost schematic 

 clearness in the opossum. In text figure 2, B are three pairs 



