216 THE CONTINUITY OF THE RACE 



adaptations that fit the mature eggs and sperms for their respective 

 roles. Although much of the nuclear and cytoplasmic change takes place 

 concurrently, it will be simpler to consider first what happens in the 

 nucleus and afterwards the cytoplasmic occurrences. 



Meiosis 



The nuclear divisions known as meiosis are identical in egg and sperm 

 formation (and also in the formation of haploid spores in plants, Figs. 

 17.4, 17.5, 17.15). The meiotic divisions show many close similarities 

 to mitosis (Fig. 2.2 and text) — the same chromosomes, the same forma- 

 tion of spindles, in animal cells the same appearance of amphiasters, and 

 the same sequence of prophase, metaphase, anaphase, and telophase. 

 However, there are a number of striking differences between meiosis and 

 mitosis that have great significance not only for reproduction but also 

 for the fields of genetics and evolution. 



The account of meiosis, like that of mitosis, is illustrated by a series of 

 diagrammatic drawings of successive stages (Fig. 15.3). Considerable 

 detail is given, in the hope that this description will be adequate both for 

 an understanding of reproduction and for an appreciation of the role 

 played by meiosis in genetics and evolution. 



We have already seen in mitosis that the chromosomes consist of two 

 like sets, or in other words that there are two of each sort of chromosome. 

 Such paired chromosomes are spoken of as homologous, or as being 

 homologues, for their resemblance is due to the fact that they could have 

 been derived from the same parent chromosome two or more generations 

 removed, and, as we shall see, one member of each pair came from the 

 individual's mother (the maternal chromosome) and the other from the 

 father (the paternal chromosome). 



The First Meiotic Division 



Prophase. When the chromosomes first become discernable, they are 

 in the form of extremely long, thin threads, even longer than in ordinary 

 mitosis. Unlike mitosis, where each chromosome consists of two chro- 

 matids, the chromosomes in meiosis appear to be single — not divided into 

 chromatids. This, the leptotene substage of prophase, is followed by a 

 zygotene substage in which the two homologous chromosomes that 

 constitute each chromosome pair come to lie closely side by side. The 

 pairing takes place gradually, starting at one or more points and spread- 

 ing along the whole length of the two chromosomes as though they were 

 drawn together by a "zipper." At the completion of zygotene the paired 

 homologous chromosomes are ordinarily so close together that it is diffi- 

 cult or impossible to see that there are two. Such closely applied pairs 



