NUCLEAR REGENERATION IN STYLONYCHIA 



MYTILUS. 



DONNELL B. YOUNG, 

 UNIVERSITY OF ARIZONA. 



In 1919 while working on the regeneration of lost parts in 

 Uronychia which had been cut in various ways (cf. Young, 

 Jour. Exp. Zool., Vol. 36, No. 3, Oct. 1922) it was discovered 

 that one species, U. binudeata had two micronuclei. When this 

 form was cut in such a way that the two micronuclei were in 

 separate halves each part regenerated completely and sub- 

 sequently divided. At that time, however, no stained prepara- 

 tions were made to show the method of nuclear reorganization. 

 In those species which had but a single micronucleus, U. setigera 

 and U. transfuga, division never took place in the amicronucleate 

 fragment although regeneration might do so. The question 

 naturally arose, does the nuclear complex in those forms having 

 two micronuclei regenerate at the same time that the cytoplasmic 

 structures are being replaced and if so, how? 



Uronychia binudeata has not appeared in any of my cultures 

 recently but as Stylonychia mytilus has two micronuclei so placed 

 that a cut in the median transverse plane passes between them, 

 they have been selected for this study. A wild mass culture was 

 used and individuals selected from it at different times as needed. 

 Several of the normal individuals were stained in order to record 

 their nuclear complex. In every case two macro- and two 

 micronuclei were found. Animals which have just completed 

 division can usually be distinguished by their smaller size. In 

 the following experiments individuals were selected which were 

 of average size, indicating that they had not divided for four or 

 five hours. As the formation of precocious cirri can be readily 

 seen and as animals do not show any nuclear changes until such 

 cirri do form, it is clear that individuals were not used which had 

 already begun the process of division. The cutting was done 

 free hand with a small sharp scalpel. The fragments were care- 

 ts 



