1911.] 



Infusorian Micronucleus in Regeneration. 



838 



spite of the continued presence of the micronucleus, the evidence only 

 indicates the importance of leaving part of the oral disc, and leaves the 

 question of the possible influence of the micronucleus quite undecided, as, 

 indeed, the authoress states. 



Of late, however, Calkins ('11) has given evidence which goes some way to 

 show that in ZTronychia the micronucleus, though not always necessary to 

 regeneration, exerts an influence on the process. Cutting this hypotrieh 

 when division had taken place less than an hour previously, he found that in 

 2 cases out of 13 neither merozoon regenerated, and that in the remaining 

 11 one merozoon only in each case produced a normal animal, this being 

 invariably the one with the single micronucleus characteristic of the species. 



Also, in cells cut between 8 and 16 hours after division (the period between 

 two divisions being 36 hours), out of nine recorded experiments in which 

 both merozoa were preserved, seven showed a regeneration of one, and 

 that the micronucleate, merozoon only. In the other two, both pieces 

 regenerated. 



These experiments favour the view that regeneration cannot occur in the 

 absence of the micronucleus, unless the cell operated upon is at least from 

 8 to 16 hours old. This conclusion must, I think, be accepted with great 

 caution, especially as but little information is vouchsafed as to how the 

 highly differentiated soma of Ur -onychia was divided. In the cases figured 

 there is only one (fig. 6) in which the micronucleus fell to the portion of the 

 anterior merozoon. 



It must also be pointed out that Calkins does not consider the possibility 

 of the micronucleus being reformed from the meganucleus in the cases where 

 regeneration occurs. It is impossible to determine the presence of the 

 organella in the life of the animal, and from Calkins' statements it is not 

 clear how he found out which merozoon had received the micronucleus. In 

 only two cases does he expressly say that the unregenerate was killed and 

 stained (Calkins' Expts. 28 and 34, and his figs. 7 and 4b). 



His fig. 4b shows a trilobed meganucleus, and a spherical fragment which 

 from the sketch appears as likely to be a micronucleus as to be a detached 

 piece of the meganucleus. In an ordinary stained preparation of the animal 

 there is but a slight difference in texture to be distinguished between the 

 two nuclei. I reproduce Calkins' fig. 4b in my fig. A, together with a second 

 drawing (X) of the unregenerate, with the doubtful body represented as a 

 micronucleus ; Calkins gives no proof of its meganuclear nature. By the 

 supposition that the micronucleus could be re-formed in regeneration, all the 

 results reported by him could be explained, at least, as regards the supposed 

 influence of the micronucleus. 



