THE REPRODUCTION OF ANIMALS 219 



matid segments by crossing over is a precisely equal exchange, and that 

 the crossed-over chromatids are now composites, made up of segments 

 from both of the original homologues. 



Meta phase. Toward the end of prophase the nuclear membrane 

 disappears and a spindle begins to form, quite as in mitosis; but as the 

 nucleus passes into metaphase, the centromeres of the bivalents do not 

 move into the equatorial plate but come to lie on either side of it, one 

 centromere of a bivalent just above, the other just below the plate. There 

 is no splitting of centromeres at the first meiotic division. 



Anaphase. The centromeres now move toward the poles, drawing the 

 crossed-over strands apart and toward the opposite ends of the spindle. 



Telophase. This is essentially the same as in mitosis, although in 

 certain organisms the chromosomes do not return to a resting stage but 

 pass directly to the prophase of the second meiotic division. 



The "daughter nuclei." Each of the two nuclei produced by the first 

 meiotic division has just half as many chromosomes as were present in 

 the mother cell — one chromosome from each homologous pair. This is 

 termed the haploid number in contrast to the diploid number found in 

 soma cells and in the germ cells before meiosis. Note that no chromosome 

 has yet undergone division and that each of the haploid chromosomes con- 

 sists of two chromatids united by an undivided centromere. 



The Second Meiotic Division 



The second meiotic division, which usually follows closely upon the 

 first, is essentially a mitotic division. Here at metaphase the centromeres 

 finally split into two, a.nd the single chromatids are carried to opposite 

 poles at anaphase. At the conclusion of this division each of the daughter 

 nuclei contains a single chromatid from each of the bivalents that were 

 formed at the pachytene substage of the first division, and meiosis is 

 completed. 



OTHER MATURATION PROCESSES 



We have seen that the nuclear processes of maturation (meiosis) are 

 the same for egg and sperm, but eggs and sperms are specialized for 

 complementary roles in fertilization, and in consequence the other proc- 

 esses of maturation are markedly different for the two kinds of cells 

 (Fig. 15.4). 



Maturation of the egg (oogenesis). The unmatured eggs (oogonia) 

 multiply by mitosis in the ovary to form a large stock of potential eggs 

 before the beginning of sexual maturity of the individual. With the 

 arrival of sexual maturity some (or all) of the oogonia begin to grow, 

 both by an increase in the amount of their cytoplasm and by the storing 

 up of food material in the form of minute oil droplets (yolk). This growth 



