OF MICR O- OR GANISMS. 97 



that such a fragment is obtainable only by mere acci- 

 dent. 



In an experiment which we again cite as a type of 

 many others, the fragment containing the elements of 

 the old nucleus tends to reconstruct itself; this frag- 

 ment, which represented the posterior part of the 

 animal, presented, the day following, a rudimentary 

 peristome; the reconstruction did not go beyond this 

 point: it was left incomplete. Accordingly, the old nu- 

 cleus loses its power of regeneration. 



As to the phenomena presented in fragments con- 

 taining no nuclear substance, M. Balbiani has made 

 decided advances in the question; he has completed the 

 experiments of Gruber, he has also corrected them, 

 and he has reached conclusions essentially different. 



In order to understand more thoroughl)' the phe- 

 nomena connected with the absence of nuclear sub- 

 stance, the author has directed his attention to an- 

 other species, the Cyrtostomum leucas, which has the 

 advantage that it can be kept longer alive than the 

 Stentor can, on a glass slide holding a drop of water. 

 The Cyrtostomum is a large ciliated Infusory of more 

 than four-tenths of a millimeter in length. Its proto- 

 plasm is differentiated into two layers, one of which, 

 the cortical, encloses very heavy trichocysts; the other, 

 the endoplasm, holds alimentary substances. The an- 

 imal exhibits upon one of its faces a mouth, shaped 

 like a long narrow buttonhole, and upon the other 

 face a contractile vesicle, from which crooked and an- 

 astomosed passages radiate. It is easy, by making a 

 transversal division, to obtain fragments without nu- 

 clear matter; the nucleus of the Cyrtostomum being 

 formed of a single, round mass. But it is not easy, on 

 the other hand, to obtain fragments likely to live. 



