1 8 EMBRYOLOGY 



removed from the body and placed in extracts of the pituitary gland ovula- 

 tion will occur /';/ vitro. Therefore a substance from the pituitary gland must 

 come into direct contact with the ovary in order to release the eggs. 



These pituitary substances responsible for ovulation — there are two of 

 them in mammals — are called gonadotropic hormones. They are proteins 

 secreted by the cells of the pituitary and released into the blood stream. 

 The pituitary glands when injected into the frog liberate only eggs which 

 are fully developed; they do not liberate eggs which are not mature. From 

 all these experiments it would seem that the primary factor in the control 

 of ovulation is something which stimulates the pituitary to release its gona- 

 dotropic hormones. This leads us to ask what controls secretion of the 

 pituitary gland. We do not know the answer in all vertebrates. The mech- 

 anism is clear in the case of some birds and some mammals. 



The effect of light on breeding habits 



It is rather generally known that change in climatic conditions results in 

 variations in the time of ovulation in domestic mammals. Animals trans- 

 ported across the equator from one hemisphere to the other adjust their 

 breeding cycle to the new conditions, the reversal of summer and winter. 

 Therefore some environmental factor is operating to produce this change. 



Experimental control of environmental factors shows that variations in 

 the amount of light are extremely important in controlling the sexual cycle 

 of both plants and animals. We may take an example drawn from some 

 experiments on raccoons. These animals normally go into a partial hiber- 

 nation in November. In February they become active again and mate, and 

 the litters are born in April. This is the normal sexual cycle. If, however, in 

 October the animals are illuminated at night, starting with an exposure of 

 about one hour each night and ending with an exposure of about eight 

 hours each night, mating will occur in December instead of February, and 

 the litters will be born in February instead of April. Thus the breeding 

 season can be hastened by about two months by increasing the ratio of 

 the amount of light to darkness each day. This periodicity in light which 

 controls the sexual cycle is termed sexual photoperiodicity. 



The general interpretation of this phenomenon, of course, is that animals 

 that breed in the spring have been exposed from the winter solstice (Decern- 



