PROCESS OF DIVISION. 261 



two parts, of which the posterior subsequently forms for itself a peri- 

 stome and an adoral zone of cilia. My observation was somewhat 

 different, and agrees with Stein's account of the division of Balanlidium 

 entozoon. The first step in the division of the almost cylmdrically 

 extended body (0*15 mm.) is the formation of a ciliated circle, which 

 surrounds the middle of the body, and comes short of the adoral cilia 

 neither in size nor in lively motion. On the dorsal surface this girdle 

 exhibits a considerable blank ; it is therefore primarily and especially 

 on the ventral surface that it originates, whether by new formation or 

 by outgrowth of former hairs I must leave undetermined. The two 

 vacuoles are widely separated, and the nucleus has grown com- 

 paratively little. 



FIG. 130. Balantidium colivn. various stages of division. 



The girdle of cilia is nothing but the first outline of the subse- 

 quent adoral zone, as the further course of the process shows. One 

 can very easily recognise, close beside the girdle of cilia, a constric- 

 tion which is at first shallow and easily overlooked, but which, after 

 a short time, separates the two halves from one another, so that the 

 connecting part is restricted to a small band. The two halves are 

 easily recognisable as independent organisms, the more so since the 

 granular inner substance is drawn back, especially posteriorly from 

 the line of constriction. The arch of cilia has gathered more and 

 more closely round the anterior pole of the posterior half, and has been 

 continued inwards into a short, flat, but already triangular protube- 

 rance, formed close behind the extremity on the ventral surface. At 

 this stage, the nucleus has usually divided into two parts, which lie 

 one in each half, and are extremely like one another, especially since 

 a second contractile vacuole has made its appearance. As the divi- 

 sion progresses, the posterior peristomial region gradually acquires 

 its more definitive structure, by the growth of the aforesaid protube- 



