100 



bulletin: museum of comparative zoology. 



2. Monosome. 



Having followed the history of the autosomes during the matura- 

 tion period, we will now trace the monosome through the same period. 

 At the beginning of the maturation period the monosome can be easily 

 distinguished as a deeply staining U-shaped element applied to the 



Q. 



R. 



a. 



Figs. Pa, Pb., Pc. — Successive sections of spermatocyte of Arphia tenebrosa 

 during tiie early anaphase of the first division. All the chromosomes are 

 shown. In Fig. Pb are two monosomes, neither of which is dividing. X 966. 



Fig. Q. — Polar view, metaphase of second maturation division of Dissosteira 

 Carolina. The monosome is present, as the total number of chromosomes (12) 

 indicates. X 966. 



Fig. R. — Polar view, metaphase of second maturation division of Dissosteira 

 Carolina. The monosome is lacking as there are only eleven chromosomes. 

 X 966. 



Fig. S. — Polar view, metaphase of second maturation division of Arphia tene- 

 brosa. Two monosomes are present. X 966. 



nuclear membrane (Plate 4, Figs. 57, 58; Plate 7, Figs. 108, 109). 

 During the prophase of the first maturation division the arms of the 

 U shorten and become apposed to each other, so that the monosome 

 appears as a longitudinally spHt rod (Figs. 110, 111). During the 



