220 BIOLOGY OF THE PROTOZOA 



analogous to that of a plastid. Division is direct with only a few 

 isolated cases showing evidences of spindle formation or of indefinite 

 chromosomes. In preparation for division, however, there is evi- 

 dence in many forms of profound changes in the make-up of the 

 nucleus destined to divide and some of these afford evidence of a 

 clear-cut reorganization of this important element of the ciliate. 



In the less complicated types division of the macronucleus is 

 relatively simple. In Bileptus anser, for example, the nuclear 

 material is in the form of many scattered chromatin and plastin 

 spheres, each of which divides prior to cell division (Fig. 58, p. 116). 

 There is no equal distribution of this chromatin to the daughter cells 

 but the daughter halves may go together to the daughter cell in 

 whose protoplasm they happen to lie. Some of the granules, how- 

 even, those in the region of the division zone, may be represented in 

 each of the progeny. 



In forms with a single ellipsoidal macronucleus as in many of the 

 commoner types (e. g., Paramecium, Colpoda, Frontonia, Glavcoma, 

 etc.), the macronucleus simply elongates and constricts to form 

 two equal portions, one passing to each daughter cell (Fig. 21, p. 

 53). Band-form nuclei characteristic of Blepharisma, Spathi- 

 dium, Didinium, Vorticella, Enplofes, etc., condense into spheroidal 

 or ellipsoidal bodies before dividing. Where two macronuclei are 

 present in the usual vegetative cell, as in Oxytricha, STylonychia, 

 Gastrostyla, etc., each divides independently of the other but syn- 

 chronously. As with band-form nuclei the beaded macronuclei 

 likewise form short rods as in Stentor, Spirostomum ambigmmi, 

 etc., the beaded character in all cases being lost. Here the separate 

 beads are usually enclosed in a common nuclear membrane which 

 is constricted at intervals, the contained chromatin massing together 

 at the period of division. This is the condition in Urotiychia trans- 

 fufja, also, the twelve to fourteen apparently separate macronuclei 

 are all connected, and the chromatin fuses prior to division to form 

 a relatively short ellipsoidal nucleus (Fig. 107). 



In other types, however, the multiple macronuclei are independent 

 and entirely disconnected. They arise by division and retain their 

 independence during vegetative life. Thus in Urnleptus mobilis 

 the eight or more macronuclei are formed as a result of a fourth 

 division of the single parental nucleus from which they came. In 

 preparing for division of the cell each of these eight nuclei of Uro- 

 leptvs undergoes a remarkable transformation. A nuclear cleft 

 (Kernspalt) appears in each, and in the cleft is a single large granule 

 which reproduces by division. The major part of the nucleus lies 

 below the cleft and is filled with densely staining chromatin; the 

 other part lying above the cleft contains much less chromatin in 

 the form of fine granules (Fig. 104). This latter part, together with 

 the granules in the cleft, are thrown off and the chromatin contents 



