THE MITOTIC CYCLE 



begins, numerous fine granulations appear within the nucleus, and one 

 minute later the nucleolus disappears; the early occurrence of this 

 event is noteworthy. The intranuclear granulations increase in volume, 

 and migrate to the centre of the nucleus, where they form the chromo- 

 somes which become arranged in a regular equatorial plate. The nuclear 

 membrane, as judged from the living Acanthamoeba disappears during 

 metaphase. 



In telophase, the chromosomes are closely packed against the centro- 

 sphere. They become less and less visible; they appear to take up 

 water, and are then lost to sight. Although the observations of these 

 authors on the daughter nuclei are restricted to this single statement, 

 they also describe contemporary events of great interest within the 

 cytoplasm some of which will be mentioned later (p 133). 



Grasshopper embryonic neuroblasts — Carlson and his colleagues have 

 developed a method for the study of the dividing neuroblasts of the 

 embryo of the grasshopper, Chortophaga vividifasciata, which is dissected 

 out of the egg membrane and mounted as a hanging drop preparation 

 in a suitable medium, with the ventral side uppermost (Carlson;*^ 

 Carlson and Hollaender^^). These neuroblasts have several features 

 of great interest; at 38° C. they divide about every four hours, and 

 prophase occupies a full half of the whole mitotic cycle. Cell division 

 is unequal, and of the two daughter cells, one is a neuroblast, the other 

 a ganglion cell. The interphase nucleus is ring-shaped (Figure 25). 



So far, the only descriptions given of mitosis in this material have 

 been incidental to the experimental studies of these authors; a full and 

 illustrated description of the normal cycle would be of great value. 

 The interphase nucleus contains two nucleoli and is filled elsewhere 

 with highly refractile granules. In early prophase, chromatin threads 

 make their appearance which at first are recognizable only with diffi- 

 culty; the granules then disappear, and the threads become more 

 distinct. At mid-prophase, the chromatin threads are 'thick enough to 

 be traceable from one part of the nucleus to another by careful focus- 

 sing'. Late prophase is 'initiated as the chromatin threads become 

 sufficiently short and well spaced that about seven can be counted near 

 the nuclear membrane in one quarter of its circumference in mid- 

 optical section'. As soon as the course of the chromosomes can be 

 traced within the nucleus, they are found to be orientated round the 

 nuclear core at the polar side of the cell. At the end of prophase the 

 nucleoli and the nuclear membrane disappear. 



The duration of telophase at 38° C. is 57 minutes. Early in this period 

 the chromosomes lose their definite outlines; in mid-telophase the 

 nuclear membrane is formed, and the two nucleoli appear and 

 gradually increase in size. At first they are spherical in form, but later 

 their outline becomes uneven and 'finally they have the appearance of 



76 



