REPRODUCTION, PHYSIOLOGY OF. It is common knowledge that the majority of animals and plants have more or less definite times at which they breed, though the ova (esti mated at ioo,000 in man) are probably preformed at birth. These depend often upon seasonal or environmental conditions and is well known that spring and summer are the times for repro ductive activity among birds, insects and a host of other animals. Unusual warmth or cold may hasten or check the periodic devel opment of sexual activity and the accompanying internal and external changes which take place in the body. The connection between breeding and food supply is also generally realized. Moreover, where climatic and nutritive conditions are approxi mately uniform throughout the year, periodicity in the breeding habits of animals is often obliterated. Thus, Semper states that sexual periodicity is absent among molluscs, insects, and other land animals in the Philippine Islands. On the other hand, the regularity of the migratory movements, which directly relates to changes in the reproductive organs and the instincts for breed ing, occurs to a great extent independently of temporary climatic conditions, though not wholly SO. (See MIGRATION OF BIRDS.) It is clear, however, that, broadly speaking, the factors which control the periodic changes in the generative system in association with breeding are of two kinds, the external ones referred to above, and internal factors inherent in the animals themselves, and par ticularly in the essential reproductive organs. Before attempting to describe these changes and the manner in which they occur it will be well, briefly, to describe the reproductive organs, referring more particularly to the higher animals.
The usual arrangement, however, is for each individual to have its own characteristic sexual organs, those of the other sex, if represented at all, failing to develop or undergoing early degenera tion. In the male of all lower vertebrates (including birds) the testes lie dorsally inside the body cavity and discharge their products, the spermatozoa, along with fluid secretions, into ducts communicating with the exterior by a passage (the cloaca) com mon to the urogenital and alimentary systems. In most mammals
on the other hand, the testes lie outside the main body cavity in a double sac (the scrotum) between the anus (or opening of the gut) and the penis. The testes are largely composed of tubules whose walls give rise to the spermatozoa and these latter are budded off into the interior as in other animals. Between the seminiferous tubules are interstitial cells. These give rise to chem ical substances (hormones, q.v.) which pass internally into the blood. (See ENDOCRINOLOGY.) There is strong evidence that these internal secretions by their power of stimulation are respon sible for the growth and development of the distinctively male characters and instincts. Thus, the presence of the testes is commonly regarded as the test for maleness.
In all mammals the spermatozoa pass out from the testis by a number of short ducts (the vasa efterentia) into a coiled tube lying alongside it (the epididymis). This acts as a storehouse for the spermatozoa until they are ejaculated. Spermatozoa may remain alive within the epididymis and still be capable of fertiliz ing ova for 3o days (rabbit). The epididymis is a long coiled tube with muscular walls and the coils lie in juxtaposition so that the whole forms one discrete body closely applied to the testis. From each epididymis a duct (the vas deferens) passes back through the inguinal canal (a passage connecting the scrotum with the body cavity). The two vasa deferentia open close together in the common channel with which the urinary bladder also com municates. This passage (the urethra) is continued within the erectile copulatory organ or penis, at the end of which it opens to the exterior. In addition to these organs there are several acces sory glands communicating with the common urogenital passage. These are the seminal vesicles, the prostate gland and Cowper's glands, all of which contribute fluid substances to the semen in which the spermatozoa swim ; the secretions are believed also to cleanse the urethra of urine prior to the ejaculation of semen. The above description applies more especially to man, but in the majority of the lower mammals the organs are similarly arranged.