78 MATURATION OF THE EGG [CH. 



lived species as much as fifty years may elapse between the 

 prophase of the division and its completion. 



While the growth of the oocyte and the deposition of yolk 

 are taking place the nucleus is very large, appearing as' a 

 conspicuous vesicle, usually with a large nucleolus (plasmo- 

 some). In the older works on embryology the nucleus at 

 this stage was called the "germinal vesicle," and the nucle- 

 olus the "germinal spot." When the egg becomes full-sized, 

 and approaches the final stages of maturation, the nucleus 

 shrinks to a very much smaller volume, sometimes becoming 

 so small as to be found only with difficulty. The chromo- 

 somes then reappear, often tightly packed together, and the 

 egg is ready for the "maturation-divisions," which prepare 

 its nucleus for conjugation with the spermatozoon. 



In spermatogenesis, as has been seen in the preceding 

 chapter, each primary spermatocyte divides twice, giving rise 

 to four spermatozoa. In the egg also two nuclear divisions 

 occur in rapid succession, but instead of the egg-cell dividing 

 into two and then four equal cells, the division is extremely 

 unequal, and the minute cells which are separated off from it 

 are called polar bodies (PI. IX). The first maturation or polar 

 mitosis may take place in the ovary or after the egg is dis- 

 charged; in some animals the act of laying seems to be 

 the stimulus, and in others the polar division only occurs 

 when a spermatozoon enters the egg. Typically the oocyte 

 nucleus comes to lie near the surface of the egg, usually 

 enclosed in 3 little patch of protoplasm free from yolk. A 

 spindle is then formed perpendicular to the egg-surface, 

 with its outer pole at the extreme margin ; the nuclear mem- 

 brane disappears and the bivalent chromosomes arrange 

 themselves in the equatorial plate. A heterotype division 

 follows, the bivalent chromosomes divide into their com- 

 ponent halves, and these travel to the poles of the spindle. 



