NUCLEI AND KINETIC ELEMENTS 123 



similar chromosomes in Monocystis ovata and in Stylorhynchus 

 longicoUis. In the latter case, also, there is a peculiar lagging hetero- 

 chromosome ("axial chromosome") of unknown significance. 



Finally, in the maturation divisions of many ciliates, a small 

 number of chromosomes, and the reduction to half the normal 

 number, have been described by several different observers. Two 

 fairly definite types of chromosome formation occur, according as 

 the resting nucleus is vesicular or massive in structure. The 

 majority of massive micronuclei behave more or less like the micro- 

 nucleus of Paramecium cavdatum, forming a crescent or some 

 other equality characteristic figure during the prophase of nuclear 

 division. Thus in Chilodon uncinatus according to Maupas and 

 later Enriques, the chromatin is drawn out first in the form of an 

 elongate cross-shaped band, while in Vorticella monilata and in 

 V. nehiilifera, according to Maupas, and in Opercidaria coarctata, 

 according to Enriques, a similar chromatin rod extends the entire 

 length of the cell. The characteristic type of nuclear figure formed 

 by the resting vesicular nuclei is represented by Onyclwdrunms 

 grandis (Maupas), Bursaria truncatella (Prowazek), Didiniwn 

 nasutum (Prandtl), Anoijlophrya hranchiarinn (Collin), A. circitlans 

 (Brumpt, 1913), and Uroleptvs mohilis (Calkins). In all of these 

 cases the two poles of the spindle are not formed simultaneously. 

 The chromatin granules into which the compact chromatin mass 

 fragments are retained at one end of the micronucleus, where they 

 form the chromosomes. The first pole of the later spindle is 

 formed by the migration (in Uroleptus mohUis) of a centriole from 

 this aggregate of chromomeres to the opposite pole of the nucleus 

 (Fig. 36, p. 78). After the chromosomes are established the 

 second pole is formed by the migration of the remaining centriole 

 to the opposite part of the nucleus. The chromosomes in all cases 

 are compact, granular aggregates of chromomeres, and, being 

 spheroidal, afford no evidence of either longitudinal or transverse 

 division. Their number, in many cases, is sufficiently small to 

 permit of exact counting; 20 ( ?) in Ophrydiinn versatile (Kaltenbach) ; 

 16 in Carchesiumpolypinum (Popoft"), Chilodon uncinatus (Enriques), 

 Didiniuni nasutum (Prandtl) and Opercularia coarctata (Enriques); 

 12 (?) in Bursaria truncatella (Prowazek); 8 in Uroleptus mohilis 

 (Calkins) ; 6 in Anoplophrya branchiarum (Collin) and in Stylonychia 

 pustulata Prowazek); and 4 in Boveria subcylindrica (Stevens). 

 In the majority of cases where reduction in number to one-half 

 has been made out, viz.: in Carchesium, Chilodon, Didinium, 

 Opercularia, Uroleptus, and in Anoplophrya, the reduction in number 

 of chromosomes occurs with the second meiotic division. Between 

 the two maturation divisions the micronuclei rarely return to the 

 massive structure characteristic of the resting micronuclei. The 

 phenomenon of synapsis or pseudo-synapsis is represented in a large 



