Meiosis 53 



logue may break and the broken pieces join up with broken 

 pieces of a chromatid of the homologous chromosome. All these 

 processes occur in the first prophase, and consequently this stage 

 is of longer duration than the prophase of ordinary mitotic 

 divisions. Cytologists have found it convenient to subdivide the 

 first meiotic prophase into five substages. 



First Projphase. At the beginning of the first prophase, the 

 chromosomes are present in the diploid number just as in a 

 mitotic prophase, but they are single throughout and not double. 

 This stage of the first meiotic prophase is known as leptotene. 

 In leptotene the chromosomes are very long, thin threads prob- 

 ably corresponding to the chromonemata of the anaphase chro- 

 mosomes of the preceding mitotic division. They have a more 

 granular appearance than the chromosomes in prophase of a 

 somatic mitosis, and they often appear as loosely strung strings 

 of beads of unequal size (Fig. 16). They are coiled loosely in 

 relic coils of the previous division. Other morphological dif- 

 ferentiations are not usually noticeable except that the centro- 

 mere is frequently stained more lightly and may at this stage 

 appear thicker than the rest of the chromosome. 



Shortly after the chromosomes have appeared, they begin to 

 pair up, each with its homologue. This pairing or synapsis, 

 which occurs during zygotene and continues until all the chromo- 

 somes are completely paired, is very precise, for each part of a 

 chromosome will lie exactly alongside the corresponding part of 

 its partner. This is true to such an extent that if pieces of one 

 chromosome are broken away or inverted, that chromosome and 

 its homologue will twist about so as to bring corresponding parts 

 together (Fig. 17). In most higher plants and in some animals 

 in which the chromosomes lie at random in the nucleus through- 

 out leptotene and zygotene, the homologues may begin to pair 

 at any place, but sometimes they do so at the centromere or 

 at the ends. In many animals and in some higher plants the 

 chromosomes may have the orientation of the preceding telo- 

 phase so that one or sometimes both ends are pointed towards 

 one region of the nucleus. In organisms with definitely oriented 

 or polarized chromosomes, pairing usually begins at the ends 

 nearest the nuclear membrane and continues along the chromo- 

 somes until they have completely paired. When the chromo- 



