vi] INTERPRETATION OF MEIOSIS 83 



(embryonic fission) occurs. In some of these it has been 

 shown by SILVESTRI that the united polar nuclei persist and 

 later divide and give origin to the nuclei of the envelope 

 ("trophamnion") which encloses the mass of embryos into 

 which the egg divides. In most species of the same families 

 in which there is no embryonic fission, the polar nuclei 

 degenerate, but SILVESTRI finds that Platygaster is an ex- 

 ception to this rule in having a trophamnion with nuclei 

 derived from the polar nuclei (Text-fig. 10). 



In all eggs, therefore, which are destined to be fertilised 

 the maturation phenomena are closely comparable yvith 

 those found in developing spermatozoa, and a similar pro- 

 cess is found in the development of the spores of the Vas- 

 cular Cryptogams and of the pollen-grains and embryo sac 

 of the Flowering Plants. In all these cases the essential 

 feature is the formation of double or bivalent chromosomes 

 which appear in half the somatic number, followed by two 

 rapidly succeeding divisions, in one of which the bivalents 

 separate into their constituent univalent halves, and in the 

 other the univalents divide by a longitudinal split. Each 

 mother cell thus gives rise to four daughter cells containing 

 half the somatic number of chromosomes; in the male all 

 these cells develop into gametes, while in the female three 

 are small and degenerate and only one mature gamete is 

 formed. 



It remains to consider some points in connection with the 

 theoretical interpretation of the maturation-process or 

 meiosis. Of these perhaps the most important is the constitu- 

 tion of the bivalents. The somatic number of chromosomes, 

 sometimes called the diploid or "2" number, is produced 

 in the fertilised egg by the union of the egg and sperm 

 nuclei each of which contains the reduced (baploid or "w") 

 number, and, as has been said above, the object of the 



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