THE NUCLEI OF UEOLEPTUS MOBILIS 349 



each conjugant degenerates, and only a single fertilization nu- 

 cleus is formed. 



At the time of fusion the pronuclei may be either in the form of 

 spherical and vesicular nuclei or drawn out into more or less 

 pointed spindle form. We find the former type in Anoplophrya 

 branchiarum, Didinium nasutum, Spirostomum teres, Blepharisma 

 undulans, and Opercularia coarctata, while union of pronuclei in 

 the spindle form is reported in all species of Paramecium, Chilodon 

 uncinatus, Leucophrys patula, Onychodromus grandis, Stylony- 

 chia pustulata, Vorticella nebulifera, Ophrydium versatile and 

 Lionotus fasciola. In some cases it seems to be immaterial 

 whether the pronuclei are vesicular or spindle form at the time 

 of fusion (Loxophyllum meleagris, Carchesium polypinum). 

 Uroleptus mobilis appears to be an intermediate type, for here 

 the pronuclei approach and meet while in the spherical and 

 vesicular form, but before the fusion membrane dissolves both 

 pronuclei elongate and assume the spindle shape (figs. 77 and 

 78). The chromatin, however, is not aggregated into chromo- 

 somes at this period, but lies in diffuse granular form exactly as 

 in Paramecium caudatum. While the general form is that of a 

 nucleus in division, these fusing pronuclei can no more be re- 

 garded as mitotic spindles than can the fusing spherical nuclei; 

 in both types we have merely the antecedent phases of mitosis. 



g. Reco7istruction of the vegetative nuclei. The first division of 

 the fertilization nucleus occurs very quickly after fusion and is 

 not often seen. In Uroleptus I have found only one spindle 

 that can be identified as the first division spindle (fig. 81). In 

 this spindle the two sides are not symmetrical, indicating the di- 

 vision of chromosomes from one pronucleus earlier than in the 

 other. The further history of the first two nuclei after conjuga- 

 tion differs in different cases, and for some species the accounts 

 by different observers do not agree. According to Enriques ('08), 

 in Chilodon uncinatus one of these first two forms the new macro- 

 nucleus and the other the new micronucleus. This, perhaps, is 

 the simplest case on record. In all of the other cases described 

 by different observers the macronucleus is not differentiated 

 from the micronucleus until at least four nuclei have been formed 



