388 DONNELL BROOKS YOUNG 



meiits. If this were the case, it would appear that a cell cut 

 below the center would lead to division of the anterior part only, 

 the iwsterior part not having the division plane present. In 

 Uronychia, however, in some cases the smaller posterior piece 

 was the one to divide, while the anterior part with the so-called 

 division zone died without division after it had regenerated. 

 That a division zone does exist in Uronychia is indicated by 

 those cells which were cut from five hours after division up to 

 the time of the next division. In many cases the subsequent 

 division, after regeneration, of these individuals resulted in the 

 formation of two unequal cells, and in every case the smaller cell 

 was from the side which had been injured by the cutting. This 

 division zone is not as marked in Uronychia as that which Cal- 

 kins describes for Paramecium and it is not developed until a 

 few hours after division. This is shown by the fact that the 

 size of the daughter cells resulting from a regenerated indi- 

 vidual did not differ to any appreciable extent unless the cutting 

 had been done five or more hours after division. 



SUMMARY 



1. Three species of Uronychia are found at Woods Hole, one 

 of which has two micronuclei, while the others have but one. 



2. In Uronychia all the old cirri are absorbed during the divi- 

 sion of the animal and new ones are formed precociously. 



3. The power to regenerate parts lost by cutting or other 

 injury is not always dependent upon the presence of a micro- 

 nucleus. 



4. The ability to grow and divide is dependent on the presence 

 of a micronucleus. 



5. The powder to regenerate lost parts varies with the age of 

 the cell, being least shortly after division and increasing up to 

 the next division, being best developed at the start of division. 



6. In division, nuclear changes and cytoplasmic changes pro- 

 gress together, and it is difficult to tell which starts the process. 



7. In Uronychia the large cirri are highly differentiated, in 

 that they do not regenerate unless the body is injured. The 

 formative agency of the cirri lies in the body protoplasm, namely, 



