244 GENERAL ZOOLOGY 



The color of the dark portion fades out gradually at its edge 

 toward the light area. The cream color of the light area is the 

 color of the yolk seen through the egg membrane. Since the 

 yolk is concentrated in the light-colored or vegetal half of the egg, 

 the dark or animal half is relatively free of yolk. The egg is 

 radially symmetrical, its principal axis being a line connecting 

 the animal pole, a point in the center of the dark area, with the 

 vegetal pole, a point in the opposite half in the center of the light 

 area (Fig. 151, A). 



When the egg is released from the ovary, it is in the primary 

 oocyte stage, the nucleus lying in the animal half of the egg near 

 the animal pole. The first polocyte is formed after the egg 

 reaches the oviduct and then, without an intervening resting 

 stage, the mitotic spindle of the second maturation division 

 develops as far as metaphase, beyond which under normal condi- 

 tions it does not proceed until the egg is fertilized. The first 

 polocyte is extruded near the animal pole. Just before the 

 formation of the spindle, the pigment thins out in the region of 

 the animal pole to form what is known as the fovea of the egg. 

 The fovea is also marked by a slight flattening in the surface 

 of the egg. 



Fertilization of the Frog's Egg. — During amplexus, eggs and 

 sperm are deposited in the water simultaneously and the eggs 

 are at once fertilized. In this process a single spermatozoon 

 makes its way through the jelly of the capsule and penetrates 

 the egg at a point about 40 degrees from the animal pole. The 

 entrance of the sperm acts as a stimulus for the secretion of fluid 

 from the egg which raises a fertilization or vitelline membrane 

 from the egg surface and creates a perivitelline space within 

 which the egg orients itself with the dark animal pole up. A 

 second result of the entrance of the sperm is the formation of a 

 gray crescentic area on the surface of the egg in the equatorial 

 region opposite the point of penetration (Fig. 151). A third 

 result is the completion of the second maturation division and 

 the formation of an egg nucleus. 



Usually the entire spermatozoon enters the egg, but the tail 

 quickly disintegrates and plays no further part in fertilization. 

 As the head and middle piece move through the egg, with the 

 middle piece foremost, the penetration path is marked by a trail 

 of pigment, which enables one to determine that the entrance 



