In normal hens, atresia of the ovarian follicles occurs when the laying cycle is interrupted by brooding or molting. Follicular atresia can be induced also by administration of progesterone( 1-3), by injection of peptone. casein, egg white, etc. (4-5) , or by abdominal surgical operations( 6-7). It has been suggested that these various factors operate by affecting the function of the pituitary( 4-5). Large doses of some gonadotropic hormone preparations induce hypertrophy of follicles, but inhibit rhythmic ovulation (8-10). I t was found that the appearance of serum vitellin (1 1-12) in circulating blood can be induced by administration of gonadotropic hormones, whole pituitary, or mare serum(l3), but not by chorionic gonadotropic hormones. In this laboratory it was observed that the decrease in serum vitellin and the follicular atresia are correlated. Furthermore, it has been recently suggested that the serum vitellin is formed in the liver and is released into the circulation in response to estrogenic stimulation ( 14). I t appears that a causal relationship exists between the stimulation of the ovary by the pituitary, ovarian production of estrogen, and serum vitellin production. Our experiments were undertaken to provide further evidence for such a relationship.Material and methods. Two-year-old, White Leghorns were maintained in a coop at an average temperature of 6°C: with the extremes reaching 2-' and 10°C. They were fed either the standard ration used in our laboratory. or given only water for a period of 7 to 10 days. GTH (bbGOKADOGEr\"', gonadotropic hormone of pregnant mare serum prepared by Upjohn Co.) was injected subcutaneously. Serum vitellin was determined in wing vein blood by the interfacial precipitin reaction ( 15 ) . Two-fold serial dilutions of the chicken serum were prepared with saline. 0.1 ml of each dilution was layered over an equal volume of rabbit anti-vitellin serum, in 2 mm -1 t OAYS FIG. 1. Effect of starratiou on lei-el of seruiii ritellin. Abscissa, da?-s ; ordinate, logarithm of titer of seruni vitellin.starred birds, ----fed control, 0 =-: Ovopositioii. + = Bird was sacrificed.tubes. The tubes were incubated one hour at room temperature. If a precipitate was visible a t the interface, the reaction was considered positive. The serum vitellin content was expressed in terms of the logarithm of the highest dilution giving a positive reaction. Fig. 1 illustrates the results of the experiment in which laying hens were subjected to starvation. Serum vitellin level in the blood began to decrease on the 2nd day of starvation and reached zero on the 7th day.
Results.A normally fed control bird showed but little fluctuation in serum vitellin level. Starving birds were periodically sacrificed and subjected to autopsy. A slight degree of follicut-
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