TOWARD A GENETICS OF STENTOR 329 



nuclei remain heavily chromatic and will form macronuclear 

 anlagen, while the other two undergo mitosis, but without showing 

 clear chromosomes, and produce four equal micronuclei. The 

 number of macronuclear anlagen then changes by combined 

 fusions and amitoses so that there may be i to 10 masses. 



Two to four days after separation the appearance is as follows : 

 macronuclear anlagen enlarge and show at first a chromatin net- 

 work resembling chromosomes, and nucleoli appear. The nodes 

 of the old macronucleus then begin absorbing. By mitotic division, 

 with appearance of definable chromosomes, the micronuclei 

 increase in number. Eight days after separation the macronuclear 

 anlagen separately constrict into nodes or chains of beads which 

 then attach together to form the definitive macronuclear chain 

 along which the micronuclei, now of their final size and number, 

 find their location. There is hence no sorting out of anlagen between 

 daughter cells, and when the first division occurs 10 days after 

 separation this is the fission of an animal which has in itself 

 regained the completely normal nuclear picture. Two variations 

 in macronuclear development were described but these may have 

 been pathological. 



S. polymorphus: Conjugation in this species is of course quite 

 similar. Multiplicative divisions of the micronuclei and concomi- 

 tant degenerations occur as in coeruleus. Cross-fertilization was 

 established, but the amphinucleus divides three times to produce 

 8 products before nuclear differentiation begins. Normally 6 of 

 these form macronuclear anlagen by increasing in size and pro- 

 ducing from the karyosome a spireme, later breaking into segments 

 or chromosomes which seem to be in the diploid number and split 

 in two longitudinally, like chromosomes, before they are reduced 

 to chromatin granules. The anlagen then nodulate and join to form 

 the definitive macronucleus. The two remaining products of the 

 amphinucleus form the micronuclei by repeated mitotic divisions, 

 during which the nuclei decrease in size. Because the number of 

 macronuclear and micronuclear anlagen may vary, it seems likely 

 that the 8 products of the third division of the amphinucleus are 

 still equivalent, and that differentiation is not predeterminedand 

 might even be guided by their location in the cell, as is the case in 

 other ciliates. 



