RELEASE OF EGGS BY PITUITARY GLAND 17 



a membrane to the body wall. The eggs inside the ovary are composed of 

 black and white hemispheres, and are almost 2 mm. in diameter. They are 

 inside of membranes which form a follicle. After they have grown to their 

 maximum size, the eggs break out of the follicles which contain them and 

 fall into the general body cavity, which is completely lined with a thin layer 

 of cells, the peritoneum. The surface of the peritoneum is covered with cilia. 

 These cilia, by their beating, literally sweep the egg all the way from the 

 ovary forward to the opening of the oviduct, which is near the heart. Thus 

 the egg travels some distance through the body cavity before it enters the 

 ostium, the opening of the oviduct. It then passes down the oviduct, where 

 the various coats of jelly are deposited on it, and comes to rest in enlarge- 

 ments of the oviducts called uteri. The individual eggs go through this proc- 

 ess until about 2000 eggs finally accumulate in the uteri. They are then 

 liberated in one large batch at the time the male liberates sperm, and ferti- 

 lization takes place in water. (For a more complete description of mating 

 in frog see Chapter 9.) After ovulation the small germ cells begin to 

 grow, and the next year's crop of eggs is formed. 



Release of eggs by pituitary gland 



What controls this process? If the pituitary gland of the frog is removed, 

 ovulation does not occur. The cycle is completely stopped; no eggs are 

 produced or liberated. This is true of many other vertebrates. On the other 

 hand, if the pituitary gland is reimplanted (put back into the animal) or 

 if extracts of this gland are injected, ovulation will occur in these hypo- 

 physectomized animals. Thus, it is clear that the pituitary is necessary for 

 ovulation. But we still may ask whether it is the primary factor in the process. 



To go on with the analysis in the frog, the mature eggs are present 

 in the ovary in October, but no ovulation takes place until April. These eggs 

 are ready in October for ovulation, as can be shown by injecting a few whole 

 pituitary glands or an extract of the glands. Ovulation stimulated by pituitary 

 injection will occur within a few days after such injection at any time be- 

 tween October and April. And therefore in normal ovulation something 

 must be released by the pituitary gland in April which travels to the ovaries 

 and stimulates release of the egg. This stimulation is not nervous in character. 

 One can cut the nerves leading to the ovary and inject pituitary glands, and 

 ovulation will take place in the absence of nerves. If pieces of ovary are 



