REPRODUCTION \2S 



ous tubules of the testis. Nurse cells are present in the walls of the 

 tubules to nourish the sperm as they develop from round spermatids 

 into mature, tailed spermatozoa. Each tubule is connected by a fine 

 tube, the vas eflFerens, with the complexly coiled epididymis, where 

 sperm are stored. From each epididymis a vas deferens passes to the 

 exterior either directly or through a copulatory organ or penis. The 

 seminal fluid, in which the sperm are suspended, protects, nourishes and 

 activates the sperm. It is secreted by glands associated with the repro- 

 ductive tract; in mammals these are the seminal vesicles, a pair of 

 glands whose ducts empty into the vas deferens, the prostate glands, 

 at the junction of the vas deferens and the urethra, and Cowper's glands, 

 which empty into the urethra at the base of the penis. Seminal fluid 

 contains glucose and fructose which the sperm metabolize, inorganic 

 salts which act as buffers to protect the sperm from the acids normally 

 present in the urethra and female tract, and mucous materials which 

 lubricate the passages through which the sperm travel. 



In many vertebrates the urinary and genital systems have one or 

 more structures in common and the two are sometimes considered to- 

 gether as the urogenital system. In the male mammal, for example, the 

 vasa deferentia empty into the urethra, which also carries urine from 

 the bladder to the outside. The urethra of mammals is surrounded by 

 the external reproductive organ, the penis. This consists of three col- 

 umns of erectile tissue— spongy venous spaces which become filled with 

 blood during sexual excitement to produce an erection of the penis. 



Eggs are produced in the ovaries of the female and are typically 

 surrounded and nourished by nurse cells during their development. At 

 the time of ovulation, the eggs are released from the ovary into the 

 abdominal cavity, whence they pass into the funnel-shaped end of the 

 oviduct. Eggs are moved along the oviduct by the peristaltic contractions 

 of its muscular wall or by the beating of cilia lining the lumen of the 

 duct. The yolk of the egg is formed while the egg is still within the 

 ovary, but the egg white and shell are added by glands in the wall of 

 the oviduct. The oviducts may open directly to the exterior or they may 

 expand into a terminal duct, the uterus, which is a thick-walled mus- 

 cular pouch in which the young develop. In mammals the uterus is 

 connected with the exterior by the vagina, which is adapted to receive 

 the penis of the male during copulation. Female mammals have a 

 clitoris, the homologue of the male penis, just anterior to the opening 

 of the vagina; it contains sense organs and erectile tissue which becomes 

 engorged with blood during sexual excitement. 



36. Fertilization 



The union of an egg and sperm is called fertilization. Most aquatic 

 animals deliver their eggs and sperm directly into the surrounding 

 water and the union of egg and sperm occurs there by chance meeting. 

 This primitive and rather uncertain method of uniting the gametes is 

 called external fertilization. Such animals usually have no accessory sex 

 structures. 



