494 



JOHNSON. [Vol. VIII. 



later stages of fission. The other instance of abnormal mega- 

 nucleus is shown in Fig. i8. We here have the rare anomaly 

 of a nucleus with a side-branch. The animal was killed at a 

 late stage of fission, just as the nucleus was returning to the 

 moniliform condition. If the process of nodulation had been 

 completed, we should have had a noded branch such as Stein 

 has figured ('67, Taf . V, Fig. 8) for 5. polymorpJms. 



The number of nodes present in the meganucleus of one and 

 the same species is well-known to be highly variable, but 

 within definite limits. Thus in 5. polyniorphus I found only 

 two meganuclei out of fifty which had fewer than 8 nodes 

 (these had 6 and 7 respectively) ; and only two that had as 

 many as 18. The average of the fifty examples was 12.42 

 nodes. 1 This agrees with Stein's ('67, p. 231) statement that 

 the nucleus of S. polytnorpJms most frequently has 11-13 

 nodes, and that he only once found a nucleus that had as many 

 as 20 nodes. The meganucleus of S. cceniletts, according to my 

 experience, regularly has more nodes than that of vS". polytnor- 

 pJms ; but Stein (p. 242) found, on the contrary, a smaller 

 number, and never counted in any meganucleus more than 1 3 

 nodes. Later observers, however, have frequently found 20 or 

 more nodes in European specimens of S. cceriileits. I have 

 counted the nodes in 75 stained specimens of this species, and 

 have found the maximum number to be 20, which occurred but 

 twice.2 The minimum number was 9, found only once; but 

 occasionally a smaller number is found in this species (see 

 p. 524). The average number of nodes for the 75 examples 

 was 14,16. 



The finer structure of the infusorian meganucleus has been 

 frequently described. It is well known that its appearance is 

 strikingly different from that of the typical nucleus, and it has 

 been appropriately termed "massive" in contradistinction to 

 the "vesicular" nuclei of Metazoa and of plants. The nuclear 

 substance of Stentor is dense, granular and viscous. It is 

 enclosed by a firm membrane, very conspicuous in isolated 



1 The specimens in which the nodes were counted were taken at random and 

 came from both Williamstown and Worcester. 



' I have, however, seen as many as 22 nodes in this species (see p. 524). 



