566 VERTEBRATE LIFE AND ORGANIZATION 



of moderate size, but have evolved segments of the tubule that take 

 back into the blood most of the water that is removed by the glomeruli. 

 Some terrestrial vertebrates (toads and many reptiles) also reabsorb 

 water from the urinary bladder, although ordinarily urine is not 

 changed after it leaves the nephron. 



.\s we pointed out in Chapter 5, animals can also save water by 

 converting ammonia into nitrogenous wastes that require less water 

 for their removal. Ammonia, which is produced by the deamination of 

 amino acids, is a very toxic compound, but it is highly soluble in water 

 and can be excreted rapidly if ample water is available to carry it away. 

 If an animal converts its ammonia to urea, some water can be con- 

 served, for each molecule of urea is formed from two molecules of am- 

 monia. If ammonia is converted to uric acid, more water can be saved, 

 for uric acid has a low toxicity, is relatively insoluble and can be ex- 

 creted as an insoluble paste. Ammonia is the primary nitrogenous waste 

 of fresh-water fishes whereas urea and uric acid are excreted by ter- 

 restrial vertebrates. 



240. The Gonads 



From a biological point of view, all of the structures and processes 

 that permit a species to survive are of no avail unless the species can 

 reproduce its kind. The general aspects of reproduction, including the 

 production of gametes in the gonads, fertilization, and the early de- 

 velopment of the embryo, were considered in Chapter 6. At this time 

 we shall be concerned more specifically with the reproductive organs 

 of vertebrates and their role in reproduction. 



Reproduction is sexual in vertebrates, and the sexes are separate. 

 The testes are paired organs of modest size, each consisting of numer- 

 ous, highly coiled seminiferous tubules (Fig. 28.4), whose total length 

 in man has been estimated at 250 meters! This provides an area large 

 enough for the production of billions of sperm. As the sperm mature, 

 they enter the lumen of the tubule and move toward the genital ducts. 

 The ovaries are more variable in size. They fill much of the body cavity 

 in primitive vertebrates that produce millions of eggs, but are much 

 smaller in higher vertebrates that produce fewer eggs and give more 

 care to those produced. The human ovary is little more than an inch 

 long (Fig. 28.5). The eggs are not free within the ovary for each one is 

 surrounded by a follicle of epithelial and connective tissue cells. When 

 the egg is ripe the follicle bursts and the egg is discharged into the 

 coelom, a process know as ovulation (Fig. 28.6). The accumulation of 

 fluid within the follicle causes it to burst in mammals, although, as we 

 have seen, muscular contraction produces ovulation in frogs. 



In the frog and most other vertebrates, the gonads are suspended 

 by mesenteries in the abdominal cavity, and they remain there through- 

 out life. But in the males of most mammals the testes undergo a 

 posterior migration, or descent, and move out of the main part of the 

 abdominal cavity into a sac of skin known as the scrotum (Fig. 28.4). 

 As they move into the scrotum, they carry a coelomic sac, the tunica 



