MORPHOLOGY AND PHYSIOLOGY OF GENUS URONYCHIA 361 



posterior parts of the macronucleus, and the other between the 

 two anterior parts. 



In his study of U. setigera, Calkins ('02) did not recognize 

 more than one species, and therefore his description is a composite 

 one. As he states that no stained preparations were made, in 

 all probability the nuclear complex which he mentions, a spheri- 

 cal macronucleus with a micronucleus beside it, is not nucleus 

 at all but a food vacuole. 



NORMAL CELL DIVISION IN U. SETIGERA 



(Figs. 16 to 21) 



Normal cell division was worked out in detail in U. setigera, 

 both with living individuals and by means of stained specimens. 

 Unusually fine material can be obtained, for not only do the nu- 

 clear parts stain clearly, but the cirri, especially those just form- 

 ing, hold the stain and can therefore be studied in detail. The 

 results of this study confirm Wallengren's observations ('02) 

 in every respect. As is the case with all hypotrichs, all of the 

 old cirri are absorbed and new ones, precociously formed, take 

 their places. 



The following observations made in the laboratory show the 

 processes. 8.15 a.m. Individual isolated in a hanging-drop. 

 11.15. One of the right dorsal cirri of the anterior cell shows.. 

 12.45. Several of the other precocious cirri of the anterior cell 

 seen. 1.10 p.m. Some of the posterior precocious cirri show. 

 1.20. First sign of constriction noted. 2.10. Precocious pos- 

 terior cirri larger than the old. 2.30. Complete separation of 

 the cells. From this it will be seen that the whole process of 

 division after the first appearance of the precocious cirri lasts 

 about three hours. From eighteen to thirty-six hours elapse 

 normally between successive divisions. 



A study of stained individuals shows the process in more 

 detail. Figure 17 shows a stained individual in the first stage 

 of division. Precocious cirri can be seen just starting to develop 

 a little above the middle of the body. The micronucleus has 

 enlarged a little and is seen to be moving toward the outer edge 

 of the cell. This individual shows that the cytoplasmic changes, 



