COMPLETION OF MATURATION OF THE EGG 77 



ing vitelline membrane now known as the fertilization membrane. This 

 is known as the perivitelline space, filled with a lluid, within which 

 the egg is free to rotate. Since the mature egg has a definite yolk- 

 cytoplasmic gradient, it slowly rotates within the gravitational field 

 (inside the fertilization membrane) until the black pigmented animal 

 hemisphere is uppermost and the heavier yolk-laden vegetal hemi- 

 sphere is lowermost. When eggs are artificially inseminated with frog 

 sperm in the laboratory, this rotation will be complete within an hour 

 after insemination and the egg mass will present a uniform black ap- 

 pearance from above, provided there is adequate water coverage. 



Completion of Maturation of the Egg 



At the time of insemination the frog's egg nucleus is in the metaphase 

 of the second maturation division, and the egg is provided with a 

 vitelline membrane and a covering of unswollen jelly. The activation 

 of the egg by the spermatozoon brings on a resumption of the matura- 

 tion process so that the suspended mitosis is completed, and the second 

 polar body is given off into the perivitelline space. 



Occasionally at the time of insemination eggs will show a minute 

 polar body pit or fovea, a surface marking indicating the point of 

 previous emergence of the first polar body. The first polar body can 

 be found generally as a translucent bead, floating freely within the 

 perivitelline space in the general vicinity of the animal hemisphere. 

 Within 7 to 10 minutes (at ordinary laboratory temperatures) a dis- 

 tinct pit will appear in the animal hemisphere, and this marks the 

 position of the activated amphiaster. Apparently sperm activation 

 causes the whole amphiaster to recede slightly into the egg, or such 

 slight movement might be the result of cortical changes following ac- 

 tivation, since the amphiaster is of quite different consistency from the 

 balance of the egg. In any case, there appears a small pit measuring 

 about 16 microns in diameter and often near the first polar body. 

 Within another 15 to 20 minutes (a total of 25 to 30 minutes from 

 insemination) the pit seems to disappear but only because there is seen 

 emerging from it the second, and also translucent, polar body. Within 

 about 1 minute this second body (nucleus) has emerged and its diam- 

 eter measures about 26 microns, indicating that it was squeezed 

 through an orifice somewhat smaller than its own diameter. There re- 

 mains no evidence of any pit and, even though the egg nucleus is just 

 below the surface, there is no superficial evidence of its presence. The 



