of the Tripyhan Radiolaria. 423 



served specimens that were engaged in direct division. 

 Nevertheless he aj^pears erroneously to have regarded them 

 as later stages of indirect nuclear division. 



The occurrence of mitotic nuclear division in Aulacantha 

 was first established by Karawaiew, wlio, however, observed 

 only a portion of the entire series of stages. 



In the resting nucleus the arrangement of the chromatin 

 resembles a coarse sponge. When the nucleus is preparing 

 to divide the framework gradually becomes finer and finer, 

 until at last there is formed in the cavity of the nucleus the 

 characteristic ball of thread. The thread is very fine and 

 extraordinarily closely coiled, so that one is unable to say 

 whether the ball consists of a single lonsr thread or of several 

 such threads. At a later stage the ball presents a looser 

 appearance, and at this period a longitudinal cleavage of the 

 chromatin thread takes place. Thus far we have been able 

 to follow Karavvaiew's series of pro-phrases ('' Prophasen"). 

 As a closer investigation teaches us, the thread in the stage 

 last described consists of a number of sections (chromosomes), 

 after the longitudinal cleavage of whicii a complete separation 

 of the daugiiter threads is effected. Now, after the disap- 

 pearance of the nuclear membrane also the segments of the 

 nucleus arrange themselves in the shape of a plate, which 

 passes from the main aperture towards the opposite side, and 

 so divides the contents of the central capsule into two parts. 

 At the same time the plate does not lie in one plane, but is 

 warped, so that in a certain position its edges form a figure 

 in the shape of an 8. Individual differences moreover prevail 

 with respect to the amount of the twisting. At this stage 

 another longitudinal cleavage of the segments takes place, 

 whereby the division of the equatorial plate into the two 

 daughter plates is ushered in. On the completion of this 

 process also the daughter plates move further apart, at the 

 same time becoming transformed into parallel plane disks 

 somewhat smaller in diameter. 



Since Karawaiew also observed this stage, but has not 

 described the structure of the plates properly, I must dwell 

 on it for a moment. 



According to Karawaiew each plate is formed of " two 

 different substances." " The bulk consists of a relatively 

 feebly stainable substance, \\ Inch, on the surface of the plates 

 that is turned towards the centre, projects into the endoplasm 

 in the form of numerous finger-shaped outgrowths ; the 

 surface directed towards the periphery is smooth. This 

 larger portion of the plates is permeated by numerous trans- 

 verse threads, which consist of ro^^■s of extremely small 



