PERIODICAL EXCITEMENT OF GENITAL OKGANS. 279 



the corresponding part in birds namely, the oviduct is 

 single. 



The ovum, by the addition of a thick layer of albumen 

 within the oviduct, becomes capable, after its exit from the 

 mother, of supplying such an amount of material for the 

 development of the chick, as is supplied to the embryo of 

 mammals by the blood of the parent. 



The periodical excitement of the organs of reproduction is a 

 striking peculiarity observed throughout the higher orders of 

 the animal kingdom. It is observed in both sexes, but in the 

 female sex possesses more of a spontaneous character that is, 

 it takes place in the female of each species at a definite period 

 of the year, and consists in a general increased development 

 of activity throughout the parts subservient to reproduction. 

 This state of increased activity is doubtless connected with the 

 arrival of ovules in the ovary at their state of maturity, by 

 which the female is rendered fit for producing offspring, pro- 

 vided impregnation by the male is permitted. If such im- 

 pregnation does not take place, then the ovule or ovules 

 brought to maturity are thrown off as useless. The state of 

 heat, as it is termed, that accompanies these changes in the 

 female, ceases as soon as impregnation takes place, and does not 

 return, for the most part, during the period of gestation. If 

 impregnation is not permitted, it ceases in one or two days in 

 the cow and the ewe, and in three days in the goat, but in the 

 bitch it may last ten or twelve days. In this case that is, 

 when impregnation is prevented it returns in the cow every 

 three weeks or every month ; and the oftener it has returned 

 without being followed by impregnation, the shorter is its 

 period of duration. In the females of many animals the heat 

 does not recur after bringing forth till the period of lactation 

 is over ; but the cow, the mare, and the she-ass are ready for 

 the male a few days after they have brought forth their young. 



