STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 21 



eggs capable of normal fertilization and development. Ovulated 

 eggs with two second maturation spindles have been described, but 

 these could have arisen through first-polar-body suppression (p. 23) ; 

 Dempsey (1939), however, records a binuclear (guinea-pig) oocyte 

 which appeared clearly to be undergoing maturation. Giant eggs 

 are known in several different groups of animals (Wilson, 1928, 

 p. 972) and their occurrence in mammals has already been referred 

 to (p. 15). In non-mammalian animals, giant eggs are generally 

 binuclear and the embryos resulting from their fertilization triploid. 

 In mammals, both binuclear and mononuclear giant eggs have been 

 found undergoing fertilization, and giant 2-cell eggs have been 

 reported (Fig. 11), but their ultimate fate is unknown. Binuclear 

 oocytes may arise during multiplication of oogonia, from nuclear 

 division unaccompanied by cytoplasmic division, or from fusion of 

 two oogonia. The former possibility seems to be the more likely, 

 but, in either case, the cells would probably be tetraploid. 



Maturation 



Before it takes part in fertilization, the oocyte undergoes ripening 

 or maturation. This involves a reduction of the chromosome 

 number to half, which is brought about in the course of two 

 maturation, reduction, polar or meiotic divisions, and the extrusion 

 of two polar bodies (Fig. 14). In the first meiotic division, the 

 nucleus passes out of the dictyate stage — the nucleolus fades and 

 vanishes, the chromosomes condense into small, rounded bodies 

 scattered through the nucleus, and the nuclear membrane disappears. 

 The chromosomes become arranged at the equator of the first 

 meiotic spindle, either directly from their scattered positions 

 (Makino, 1941) or first forming a dense mass of chromatin (Odor, 

 1955) (Figs. 15, 16 and 19). During the prophase, the chromosomes 

 are brought together in homologous pairs, chiasmata develop and 

 parts of corresponding chromatids are exchanged in the process 

 known as crossing-over. At the first meiotic anaphase, the mem- 

 bers of the homologous chromosome pairs are separated again, their 

 component chromatids now having a different constitution than 

 they had at the start of prophase. The division advances to telophase 

 and the chromosomes form compact groups at the poles of the 

 spindle. Since the oocyte nucleus was tetraploid in respect of 

 chromatids, each of these groups has a diploid number of chromo- 

 somes ; one group is expelled in the first polar body while the other 



