348 GARY N. CALKINS 



These differences are too uncertain to permit any conclusion 

 as to the size relations between wandering and stationary pro- 

 nuclei. In some cases the wandering pronucleus is smaller, in 

 other cases larger than the stationary pronucleus. 



Apart from size differences, we occasionally find structural 

 differences between the wandering and the stationary pronuclei. 

 Maupas ('88) was the first to note the presence of a dense aggre- 

 gate of cytoplasmic granules at the forward end of the advancing 

 pronucleus of Euplotes patella, while no such aggregate was seen 

 in connection with the stationary pronucleus. Hoyer ('99) ob- 

 served 'astral rays' about each of the pronuclei of Colpidium 

 colpoda, but without distinctive differences; similar radiations, 

 more pronounced about the wandering pronucleus, were de- 

 scribed by Prandtl for Didinium nasutum. In Uroleptus mobilis 

 there are no radiations such as occur in Didinium, but the granu- 

 lar aggregate at the forward end of the wandering pronucleus is 

 highly characteristic and its presence confirmed in material fixed 

 in sublimate acetic, in Flemming's fluid, in Bouin's fluid and in 

 Schaudinn's fluid. It appears to be a directive center, possibly 

 analogous to the centrosphere of Noctiluca. It cannot be inter- 

 preted as a collection of granules due to pressure of an advancing 

 solid for the advanced end of the mass curves around the anterior 

 end in a definite way (figs. 71 to 75). With the approach and 

 union of the pronuclei this mass disappears. 



/. Fusion of the pronuclei. The outcome' of the interchange of 

 pronuclei is the fusion of migrating and stationary pronuclei. 

 In all ciliates which have been carefully examined, with the ex- 

 ception of the vorticellidae, this interchange and fusion is mutual. 

 Hoyer ('99), however, holds a different view. He, like Maupas 

 before him, failed to find evidence of the union of pronuclei iu 

 Colpidium colpoda, and concluded that no fusion occurs, the 

 foreign pronucleus in each individual forming the functional 

 micronucleus. In the face of the overwhelming evidence in other, 

 and probably more favorable ciliates, this peculiar view cannot 

 be admitted. In the vorticellidae the microgamete fuses with 

 the macrogamete and loses its identity in the protoplasm of the 

 larger conjugant. Here one of the two pronuclei . formed by 



