54 - The Ceil 



ible and individually identifiable. Second, in 

 the prophase of the first meiotic division, the 

 two homologous chromosomes of every pair 

 always lie side by side, intimately entwined 

 one around the other one (Fig. 3-10). This 

 pairing and entwining of homologous chro- 

 mosomes, which is called synapsis, has very 

 real consequences in heredity because, while 

 the pairs of chromosomes are in the synaptic 

 condition, they may mutually exchange a 

 certain number of their genes (p. 491). Syn- 

 apsis occurs only during the prophase of the 

 first meiotic division. Usually during ordi- 

 nary mitosis, the homologous chromosomes 

 behave as independent units and show little 

 or no attraction for each other. 



Before metaphase, in the first meiotic divi- 

 sion, the pairs of chromosomes have un- 

 raveled from synapsis, but still the mem- 

 bers of each pair lie side by side. By this time 

 each chromosome has divided lengthwise into 

 two daughters. Consequently at the meta- 

 phase of the first meiotic division one sees 

 the chromosomes assembled on the spindle 

 in characteristic quadruple groupings, which 

 are called the tetrads (Fig. 3-10). 



The postsynaptic division of each chromo- 

 some is the only occasion when the chromo- 

 somes divide during the whole process of 

 meiosis. In fact the two cell divisions, which 

 follow shortly, serve merely to distribute the 

 chromosomes that exist as soon as the tetrads 

 have been formed (Fig. 3-10). And if later 

 events proceed in a normal fashion, each of 

 the four sperm cells produced by any one 

 spermatocyte is destined finally to receive 

 just one chromosome from every tetrad (Fig. 

 3-10). 



After the first metaphase, the remaining 

 events of meiosis usually proceed quite rap- 

 idly. The anaphase and telophase of the first 

 division lead to the formation of two cells, 

 the secondary spermatocytes, each receiving 

 half of each tetrad. In other words each sec- 

 ondary spermatocyte receives one of the two 

 diads into which each tetrad has become 

 separated (Fig. 3-10). But the precise man- 

 ner in which a tetrad separates into diads 



G© 



Fig. 3-11. Diagram showing how the four chromo- 

 somes of any given tetrad may separate during the first 

 meiotic division. Regardless of which of the two pos- 

 sibilities (A or B) occurs, one member of each tetrad 

 eventually reaches every sperm, when the second 

 meiotic division occurs. 



appears to be a matter of chance. The two 

 possibilities indicated in Figure 3-11 seem to 

 occur with equal frequency. 



Without pause, the secondary spermato- 

 cytes usually launch into the second meiotic 

 division. In fact the diad chromosomes re- 

 main visible, as they quickly pass through 

 prophase and assume the metaphase arrange- 

 ment on the spindle (Fig. 3-10). Then 

 the anaphases and telophases are completed, 

 and a single chromosome from each of the 

 diads is delivered to each of the four final 

 cells. But here again it is a matter of chance 

 as to which member of a diad goes to a 

 particular one of the resulting cells. 



The four resulting cells are called sper- 

 matids. But shortly thereafter, when each has 

 developed an elongate motile flagellum, thev 

 are called the sperm. As may be seen by refer- 

 ring to Figure 3-10, each sperm possesses the 

 haploid set of chromosomes that later will 

 be carried to the egg, when fertilization oc- 

 curs. 



Similar processes of meiosis are encount- 

 ered in the development of the egg cells in 

 female animals, and of spore cells in plants; 

 the special features of these developments 

 will be discussed in Chapter 26. 



