Meiosis 63 



a somatic mitosis. This stage between the end of the first telo- 

 phase and the beginning of the second prophase is known as 

 interkinesis. It is usually short, and may be entirely absent. 

 Sometimes the chromosomes seem to go into a partial but not 

 complete resting stage between the first and the second prophase. 



Second Prophase. If an interkinesis follows the first telophase 

 it is in turn followed by the prophase of the second meiotic 

 division. In the second prophase the chromosomes appear as 

 double structures, the result of the ''split" or separation of the 

 chromatids which took place at pachytene or one whole cell 

 division previous to their appearance here. The chromatids of 

 each chromosome are held together by the centromere, but the 

 arms repel one another instead of lying in close approximation 

 as in a somatic mitosis. This arrangement gives the chromo- 

 somes a very different appearance from the prophase chromo- 

 somes of a somatic mitosis for in the second meiotic division 

 they are X-shaped figures whereas in a somatic mitosis they are 

 two parallel threads. In a somatic mitosis there are 2n prophase 

 chromosomes, but in the second meiotic prophase the chromo- 

 somes are present in only the haploid number. During the 

 second prophase, the nucleoli, if they appeared during the first 

 telophase, disappear again, and finally the nuclear membranes 

 disappear. 



Second Prometaphase. When the two nuclear membranes 

 break down, two new spindles are formed in the position of the 

 former nuclei, and the chrom.osomes of each nucleus move on 

 to the equators of their respective spindles. If a new cell wall 

 formed at first telophase, each spindle is in a separate cell, but 

 if the wall did not form, as is normal in many organisms, both 

 spindles are in one cell. The two spindles may lie approximately 

 parallel and alongside one another, as in spermatogenesis in ani- 

 mals or the formation of the microspore in plants. Then the 

 four cells which form subsequently are arranged in the form of a 

 tetrahedron. In megasporogenesis in plants, however, the 

 spindles are oriented in the same direction and lie in the same 

 plane; the resulting four cells are in a linear row. 



Second Metaphase. As in somatic mitosis, the chromosomes 

 are lined up with the centromeres on the equator. The repul- 

 sion of the arms found at second prophase is now completely or 

 partially overcome, and the two chromatids of each chromosome 



