160 Dr. M. Verworn's Biological Studies of Protista. 



unsuccessful, and only in one individual there was to be seen 

 in the protruded protoplasmic mass a single, very large, but 

 not particularly distinct nucleus ; so that from this it is not 

 possible for me to say anything about the behaviour of the 

 nuclei during division. As appears from the observations on 

 Dijjiugia, this genus therefore, as regards the shell-formation 

 during division, approximates directly to the other shelled 

 freshwater Ehizopoda. 



To ascertain whether and in what manner the Difflugia 

 repairs artificial injuries of its shell by means of the building- 

 material taken up into the protoplasm, I made a series of 

 experiments by division. When the individuals had com- 

 pletely withdrawn into their shells they were cut up with a 

 sharp lancet in definite directions under the microscope, and 

 then preserved in watch-glasses or upon object-slides with the 

 necessary shell-material. They all supported the operation 

 well, and could be kept alive for more than three weeks even 

 upon the object-slide if they were preserved in moist 

 chambers. 



First of all the Protista were divided into two portions of 

 nearly equal size by a cut through the middle of the animal 

 in the plane of the shell-aperture (fig. 3, a). The divided 

 pieces were carefully examined every day. They appeared 

 quite normal, extended their pseudopodia, and crept about. 

 When irritated they reacted as before, and took up glass- 

 granules, but they showed no regeneration of the shell ; the 

 cut margins remained about as irregular as soon after the 

 section. Other individuals were divided into two equal parts 

 by a cut made in the longitudinal direction of the shell (fig. 

 3, h). These portions also were quite normal in their be- 

 haviour, but without regenerating the shell. Next specimens 

 were cut into two unequal parts both transversely and longi- 

 tudinally ; but neither the larger nor the smaller divisions 

 showed any alteration at the cut portions of the shell. The 

 experiments were varied in the most multifarious manner 

 (fig. 3, c) in a very great number of specimens, but always 

 with the same result ; all the fragments still retained the same 

 vitality, but without regenerating the portions of shell which 

 remained to them. 



I then modified the experiment by inflicting only quite 

 small injuries, without injuring the protoplasmic body. 

 For this purpose I made small incisions in different parts 

 of the shell of many individuals,, and watched whether 

 these would be repaired. Every day a careful examination 

 was made to see whether any change had taken place ; but 

 even after the lapse of three weeks the cut parts showed 



