OF THE GERM CELLS OF METAZOA. 



183 



In the growth period of the spermatocytes there is oik 1 clearly bivalent chromatin 

 nucleolus, which is frequently apposed to the larger true nucleolus. 



In the monaster stage of the first maturation division (Fig. 150, pole view) there are 

 eight elements, the smallest of which is the chromatin nucleolus (K 2); on lateral view 

 all these elements appear dumbbell-shaped and hence are bivalent (Fig. 151). Of the 

 seven chromosomes of this stage, two are very small and five much larger, the two small 

 ones in Fig. 151 being designated as a. 2 and b. 2, while in the figure ofily three of the 

 large ones are shown. Apparently the two small chromosomes of the spermatocyte cor- 

 respond to the four small chromosomes of the spermatogonium ; thus the bivalent 

 chromosomes a. 2 and b. 2 of Fig. 151 would correspond respectively to the univalent 

 chromosomes a and b of Fig. 149, and the five large bivalent chromosomes of the 

 spermatocyte to the ten large univalent ones of the spermatogonium. It is very evident 

 thai, in the synapsis stage one of the small univalent chromosomes derived from the 

 spermatogonium never unites with one of the large, for the two univalent components of 

 each small bivalent chromosome of the spermatocyte have approximately the same vol- 

 ume. This speaks, of course, very strongly for the maintenance of the individuality of 

 the chromosomes during these generations. 



26. (Kdancala dorsalis Say 



Four testes of this species were studied. 



In the rest stage of the spermatogonium are present two chromatin nucleoli of 

 rounded form (N. 2, Fig. 152, H. IV); these are sometimes attached to one another, 

 sometimes to the true nucleolus (N). 



In the spermatogone monaster there are thirteen chromatin elements present, exactly 

 this number being found in all of nine clear cases. Two of these, which are rounded 

 and much smaller than the others, are the chromatin nucleoli (N. 2, Figs. 153, 154); 

 the remaining eleven elements are relatively large, elongated chromosomes. All these 

 elements are halved in the metakinesis. 



In the synapsis stage the two chromatin nucleoli combine to form one bivalent one, 

 and even up to the rest stage of the spermatocyte (Fig. 155) it remains dumbbell-shaped, 

 with a, bridge of linin connecting its univalent components; it is attached to the surface 

 cf the true nucleolus {N, Fig. 15;")), and the "double nucleolus" so formed usually lies 

 close to the nuclear membrane. 



In the first maturation division there are seven chromatin elements (Fig. 156, pole 

 view of monaster stage), of which the smallest, usually centrally placed one is the biva- 

 lent chromatin nucleolus (N. 2). Of the six chromosomes, five, when seen on lateral 

 view, are dumbbell-shaped, and so bivalent; but the sixth is oval in outline without any 



A. P. S. — VOL. XX. X. 



