512 JOHNSON. [Vol. VIII. 



the working-out of the impulse thus given is only partially 

 dependent upon temperature, food, the size of the individual, 

 or even, as Balbiani's ('92) and my own experiments in 

 merotomy (see p. 550) show, upon the intact condition of the 

 organism. 



2. Nuclear Phases. 



Meganucletis. — The best observations hitherto upon the 

 meganucleus at time of fission are those of Balbiani ('60) and 

 of Stein ('67), later observers of fission in Stentor having paid 

 little or no attention to the nucleus. 



The most satisfactory method of studying the meganuclear 

 changes is to select a specimen at an early stage of fission in 

 which the meganucleus is clearly visible, and then watch the 

 nucleus throughout its changes until the two daughter-nuclei 

 have attained their definitive shape. This method I have 

 supplemented and controlled by the examination of stained 

 preparations, but these can by no means supplant the study of 

 the living meganucleus. 



At the beginning of fission the meganucleus has its usual 

 spiral disposition in the body. The first alteration, just 

 previous to the formation of the new pharynx, is a straightening- 

 of the nucleus and disappearance of the commissures, the nodes 

 becoming appressed. (Fig. 26, mgn. ; Fig. 47 a) The next 

 step is coalescence of the nodes into a solid mass, which 

 shortens rapidly until it assumes a nearly spherical form 

 (Fig. 41 a). The time required for coalescence of the nodes 

 is about one hour, but it varies in different individuals. 

 Usually the meganucleus has assumed the spherical shape 

 when the pharyngeal funnel has begun to form. Its shape at 

 this time is rarely a perfect sphere and often it is jagged and 

 irregular. In fact, when observed in the living state it is seen 

 to be a plastic, fluctuating mass, almost amoeboid in its rapid 

 change of form. (Figs. 46 c, 47 d, e) These changes of form 

 are the outward expression of an internal commotion that may 

 be considered as a continuation of the forces that produced the 

 coalescence of the nodes. In a few minutes the movements 

 take a definite direction, producing an elongation of the 



