692 HEREDITY AND CELL DIVISION 



and unstainable, and the nucleoli, which have disappeared in 

 prophase, reappear. 



The division of the cell takes place by the formation of a new 

 cytoplasmic dividing wall in various ways. In amoeboid cells 

 in tissue culture the two daughter cells move actively away from 

 each other, pulling out the cytoplasm between them to a narrow 

 waist which finally breaks ; in eggs and early blastulae there is 

 a sudden rearrangement of the protoplasm and its surface, Hke 

 the reverse of the way in which two soap bubbles in contact 

 will suddenly become one ; in plants there is a slow laying down 

 of a new dividing wall, and presumably something similar happens 



in epithelia. 



The process of mitosis clearly provides for the maintenance 

 of constancy of number and shape of the chromosomes, but it is 

 to be noted that the really essential part, the division of chromo- 

 somes into chromatids, has never been seen, and must take place 

 in the resting nucleus. We assume that the chromosomes continue 

 their existence and identity while they are invisible ; there is 

 no direct evidence for this, but much that is circumstantial. 



MEIOSIS 



There is another type of nuclear division, called meiosis 

 (Fig. 541), or reduction division, in which the chromosomes spHt 

 once for two divisions of the nucleus ; in animals, except for some 

 Protozoa, it occurs in the formation of the germ cells. When the 

 chromosomes appear in prophase they are not spht into chroma- 

 tids ; instead, and perhaps because of this, homologous or similar 

 ones come together in pairs, sometimes for a short distance, 

 sometimes for their whole length, and shorten and coil round 

 each other to give the pachytene stage. Next comes the diplotene 

 stage, in which the pairs fall apart except at certain points called 

 chiasmata (singular chiasma). Each chromosome is now seen to 

 be divided into two chromatids, and it is claimed that in a few 

 instances it can be seen that at the chiasmata the chromatids 

 exchange partners, a segment of one changing places with the 

 corresponding segment of another, so that the separating chromo- 

 somes are not identical with those which come together. This 

 phenomenon is called crossing-over. There follows uncoihng and 

 shortening, and further repulsion of all parts of the chromosomes, 

 especially the centromeres, except at the chiasmata, a stage called 



