THE FEMALE 71 



and placed in amphibian Ringer's solution and the ovulation process 

 may be observed directly. This will go on for several hours after all 

 connections with the nerve and blood supply are cut off. From the ini- 

 tial rupture of the theca externa until the egg drops free into the 

 body cavity there is a lapse of from 4 to 10 minutes at ordinary 

 laboratory temperatures. 



The first maturation division occurs at the time of ovulation. The 

 heterotypic chromosomes (i.e., of bizarre shapes) are placed on the 

 spindle of the amphiaster whose axis is at right angles to the egg sur- 

 face. Movement of the chromosomes is identical with that found in 

 ordinary mitosis. The outermost group of telophase chromosomes are 

 pinched off, with a small amount of cytoplasm and no yolk, to com- 

 prise the first polar body. The innermost telophasic group of chromo- 

 somes remain within a clear (i.e., yolk-free) area of the egg as the 

 nuclear mass of the secondary oocyte. These changes occur as the egg 

 leaves the ovary and before it reaches the oviduct. Possibly the same 

 forces which bring about follicular rupture also influence this mat- 

 uration process. 



The second maturation division begins without any intermediate 

 rest period for the chromosomes, at about the time the egg enters the 

 oviduct. There may be a variation in time up to 2 hours for eggs to 

 reach the ostium, depending upon the region of the body cavity into 

 which they are liberated. Thus the stage of maturation of different 

 eggs within the oviduct may vary considerably. There is a longitudi- 

 nal division of the chromosomes of the egg which are lined up in 

 metaphase on the second maturation spindle, the axis of which is at 

 right angles to the egg surface. Since the spindle is primarily proto- 

 plasmic, and is made up in part of fibers (which may be contractile), 

 the space occupied by the spindle will be free of yolk. Since it is 

 peripherally placed, and represents a slight inner movement after 

 the elimination of the first polar body, the surface layer of the egg is 

 slightly de-pigmented just above the spindle region. This situation is 

 exaggerated in aged eggs, a relatively large de-pigmented area of 

 the cortex appearing toward the center of the animal hemisphere. 



Maturation is not completed until or unless the egg is activated by 

 sperm or stimulated by parthenogenetic means. However, every egg 

 reaching the uterus is in metaphase of the second maturation division, 

 awaiting the stimulus of activation to complete the elimination of the 

 second polar body. 



