CELLS IN DIVISION 



be seen that the nucleoH have already begun to change in form when 

 this stage has been reached. Their movement, together with that of the 

 whole nucleus, may continue throughout prophase. The developing 

 chromosomes gradually acquire their slowly increasing contrast which 

 we see both in the living state and in Feulgen preparations. This 

 phase lasts for half an hour or perhaps more, for the process is usually 

 already under way when observation begins. The phase-contrast 

 microscope with its extreme shallowness of focus reveals the fact that 

 the chromosome threads are closely applied to the inner surface of the 

 nuclear membrane; when one focuses into the interior of the nucleus, 

 nucleoli and chromocentres alone come into view. 



Here also, the whole nucleus often revolves during prophase, and the 

 nuclear outline may repeatedly change. The end of prophase comes 

 with remarkable suddenness. The nuclear membrane contracts and 

 vanishes; the nucleoli breaks up equally abruptly, and the chromo- 

 somes begin to move freely in the cell. Often the chromosomes congre- 

 gate at either end of the nucleus round the poles of the developing 

 spindle, and thus momentarily give a spurious appearance of anaphase 

 (Plate XII (i6b) ). It seems that the dissolution of the nuclear mem- 

 brane and of the nucleoli begins at the same instant. As the nuclear 

 membrane contracts, so do the chromosome threads which line its inner 

 surface. Sometimes the developing poles of the spindle then momen- 

 tarily indent the shrinking membrane at opposite ends of a diameter 

 (Plate XI (14b) ). The spindle quickly joins across the line between the 

 two poles, and sometimes the chromosomes can then be seen moving 

 towards it. 



In Feulgen preparations the split in the chromosomes can be seen 

 in mid-prophase, behind the apparently terminal heterochromatic 

 granules. In the living cells, the chromosomes are not seen to be double 

 until they approach metaphase. 



In cells of the chick, the general course of prophase is similar. 

 Jacobson and Webb^^ have shown that as the nucleoli disappear, the 

 chromosomes then first become able to take up basic dyes, as for in- 

 stance methylene blue in the May-Grunwald-Giemsa stain. This 

 suggests that the ribonucleoproteins of the nucleoli are rapidly trans- 

 ferred to the chromosomes at the end of prophase. This change precedes 

 the final dissolution of the nucleoli, remains of which can sometimes be 

 seen among the chromosomes in metaphase. Very early in anaphase, 

 the basophilic material begins to leave the chromosomes (Plate XII 



(17))- 



In the mouse, reconstruction of the daughter nuclei from late ana- 

 phase onwards proceeds in the following manner. At first the daughter 

 chromosomes are closely packed together in apparent fusion; then the 

 whole mass begins to swell, and as its texture becomes looser, fine 



79 



