132 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS 



one prophase; that is, two metaphase-anaphase separations of chromosomes 

 preceded by a single, pecuUar prophase. The pecuHarities of this meiotic 

 prophase may be described as follows: As the prophase condition of the 

 nucleus is initiated, an odd type of behavior of the chromosomes becomes 

 evident — a behavior which is entirely absent from ordinary somatic mitosis: 

 namely, the homologous pairs or mates begin to show an attraction for each 

 other and they approach and form an intimate association. This association 

 is called synapsis (figs. 67, 69, zygotene stage). As a result, the two homolo- 

 gous chromosomes appear as one structure. As the homologous chromosomes 

 are now paired together and superficially appear as one chromosome, the 

 number of "chromosomes" visible at this time is reduced to one-half of the 

 ordinary somatic or diploid number. However, each "chromosome"' is in reality 

 two chromosomes and, therefore, is called a bivalent or twin chromosome. 



While the homologous chromosomes are intimately associated, each mate 

 reproduces itself longitudinally just as it would during an ordinary mitosis 

 (fig. 67, pachytene stage). (The possibility remains that this reproduction of 

 chromatin material may have occurred even before the synaptic union.) 

 Hence, each bivalent chromosome becomes transformed into four potential 

 chromosomes, each one of which is called a chromatid. This group of chroma- 

 tids is, collectively speaking, a tetrad chromosome. (As described below, 

 interchange of material or crossing over from one chromatid to another may 

 take place at this time.) As a result of these changes, the nucleus now con- 

 tains the haploid number of chromosomes, (i.e., half of the normal, diploid 

 number) in the form of tetrads (fig. 67, pachytene stage). However, as each 

 tetrad represents four chromosomes, actually there is at this time twice the 

 normal number of chromosomes present in the nucleus (fig. 67; compare 

 leptotene, pachytene, diplotene and diakinesis). 



The next step in meiosis brings about the separation of the tetrad chromo- 

 some into its respective chromatids and it involves two divisions of the cell. 

 These divisions are known as meiotic divisions. As the first of these two 

 divisions begins, the tetrad chromosomes become arranged in the mid- or 

 metaphase plane of the spindle. After this initial step, the first division of 

 the cell occurs, and half of each tetrad (i.e., a dyad) passes to each pole of 

 the mitotic spindle (fig. 67, first meiotic division). Each daughter cell (i.e., 

 secondary spermatocyte or oocyte) resulting from the first maturation 

 (meiotic) division thus contains the haploid or reduced number of chromo- 

 somes in the dyad condition, each dyad being composed of two chromatids. 

 A resting or interphase nuclear condition occurs in most spermatocytes, fol- 

 lowing the first maturation division, but in the oocyte it usually does not 

 occur (fig. 69, interkinesis). 



As the second maturation division is initiated, the dyads become arranged 

 on the metaphase plate of the mitotic spindle. As division of the cell proceeds, 

 half of each dyad (i.e., a monad) passes to the respective poles of the spindle 



