THE NUCLEI OF UROLEPTUS MOBILIS 339 



'06), and Anoplophrya branchiarum (Collin, '09). Here the nu- 

 clear membrane first swells up to form a nucleus two or three 

 times the diameter of the original micronucleus, while the com- 

 pact mass of chromatin, placed either centrally or peripherally, 

 fragments into numerous chromatin granules. In Uroleptus mo- 

 bilis there is an intranuclear centrosome which divides, one-half 

 passing to the periphery of the nucleus at the pole opposite the 

 chromatin mass, while the other half remains in this mass. The 

 two halves remain connected by a deeply staining strand (prim- 

 ary centrodesmus) throughout the prophase period, but none 

 of these structures can be demonstrated in the fully formed 

 spindle (figs. 28 to 34) . The distal centrosome is the focal point 

 of spindle fibers which spread out from it to the fragmenting 

 chromatin mass and forms one pole of the mitotic figure. It was 

 a similar stage in Anoplophrya that suggested the term 'cande- 

 labra' to Collin. I have called it the parachute nucleus. 



So far as I am aware, this is the first demonstration of the 

 centronucleus type of nucleus in ciliates. 



The majority of investigators show signs of embarrassment 

 when it comes to the description of the formation of the second 

 pole, or the full spindle, of the first maturation nucleus. In the 

 transformation of the crescent type, Maupas, Hertwig, and Ham- 

 burger all agree that the spindle is formed by the shortening of 

 the long axis of the crescent and that the tips of the crescent 

 form the poles of the spindle. Calkins and Cull, however, find 

 that the division center or kinoplasmic mass which forms the 

 poles of the spindle, migrates from its terminal position in the 

 crescent to the center of the convex side. This new position 

 becomes the first pole of the spindle. 



In the candelabra or parachute type, the second pole is formed 

 by the outgrowth from the chromatin mass, of a second pole 

 similar to the first, the chromatin granules thus being left in the 

 nuclear plate position or center of the spindle figure. In Uroleptus 

 mobilis this second pole is formed bj^ the migration of the proxi- 

 mal centrosome from the chromatin mass while the connecting 

 strand between the two centrosomes becomes thinner and im- 

 possible to distinguish, in the later stages, from the spindle fibers. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 27, NO. 3 



