189G.] MICROSCOPICAL .TOUKlNAL. 279 



likeness to tlie resting nucleus. The chrotuatine is now 

 in the form of loops, of whicli there are two sets at op- 

 posite ends of the cell ; the numher of loops in each set 

 should be counted and their location shown ; the iiucJear 

 membrane has disappeared; there are fibres running 

 through the chromatine and converging beyond at a 

 point, nuclear spindle, at which in favorable cases a min- 

 ute particle, th6 centrosome, can be seen. Lines can be 

 seen to radiate from the centrosome into the cytoplasm 

 as well as into the nucleus proper. After these points 

 have been seen, you should examine other stages, you 

 will if successful be able to determine (1) that the nuc- 

 lear spindle forms very early, before the nucleus has 

 changed, (2) that the chromatine takes the form of loops 

 of a certain number, (3) that these are later separated into 

 the two sets already mentioned which form the founda- 

 tion of the new nuclei that are in process of formation, (4) 

 and pull more or more widely apart, ' Still later than 

 this, the spindle disappears and a nuclear membrane 

 again distinctly surrounds the two new nuclei, each of 

 which now contains an equal portion of the ori«;inal 

 chromatine. A large number of different dividing nuclei 

 should be examined and drawn and their relation in 

 point of time be carefully determined. (Besides this 

 "indirect" mode of nuclear division, the nuclei of certain 

 cells divide "directly," that is, there are no spindle or 

 chromatine loops, but the nuclear membrane simply con- 

 stricts in the middle and thus two are formed from one, 

 as in typical cell division.) 



PART YI. CONDITIONS OF CELL-LIFE*, (YEART.) 



The cell being a living object reacts directly to its surroimdiugs. By 

 studying this reaction the effects of various conditions upon cell life 

 can he inferred. Yea^t appears on the whole to furnish advantages 

 for experimentation, since it is always easy to get a supply through the 

 commercial use of the fresh yeast cake. The test oi its activity is the num- 

 ber of generations of buds produced in a given time, it being assumed 



