694 HEREDITY AND CELL DIVISION 



diakinesis. The other features of prophase take place, and then 

 metaphase. In this there is no division of the centromeres, but 

 those of the paired chromosomes move farther apart in anaphase 

 and separate the chromosomes along their whole length. There is 

 usually no telophase in the ordinary sense of the word, and the 

 second division of meiosis may occur at once. Chromatids are 

 already present, so there is no further splitting, but after the 

 chromosomes are arranged on the metaphase plate the centromeres 

 divide and the chromatids, which have now become chromosomes, 

 are pulled apart. The result of the double division is four nuclei, 

 each of which has only half the number of chromosomes of the 

 original cell. The nucleus which has the full or double number 

 (2n) is said to be diploid, that which has the reduced number (n) 

 is called haploid, from a Greek word meaning single. 



The first or heterotype division of meiosis leads in the female 

 to the formation of the first polar body, in the male to the 

 formation of the secondary spermatocytes. The second or homo- 

 type division leads to the formation of the second polar body 

 and the spermatids, which grow to spermatozoa. In the female 

 the first division is often compHcated by cytoplasmic changes, 

 such as the deposition of yolk during the pachytene stage. While 

 this is going on the chromosomes may even disappear entirely, 

 and some special stimulus, which may be the entrance of the 

 sperm, is needed to complete the division. In mammals the end of 

 prophase is reached before or just after birth, but the division 

 is not finished until just before the egg is shed from the ovary 

 in ovulation ; in man and other long-lived mammals meiosis 

 may therefore take fifty years. 



AMITOSIS 



In some cells there is an amitosis, where the nucleus just 

 elongates and divides, without spHtting of individual chromo- 

 somes. Sometimes this is pathological, but it is normal in the 

 meganucleus of Ciliophora and in many adult tissues, for instance 

 in snails, insects, birds and mammals. 



LINKAGE 



We can now return to the later developments of genetics, 

 which we left on page 687 with the statement that separate 

 characters were not always independently inherited. Two types 



