THE UROGENITAL SYSTEM — EXCRETION AND REPRODUCTION 573 



placenta to the baby, and the umbilical cord can be cut and tied, al- 

 though tying is unnecessary for contraction of the uterine vessels would 

 prevent excessive bleeding of the infant. Other mammals simply bite 

 through the cord. W^ithin a week, the stump of the cord shrivels, drops 

 off, and leaves a scar known as the navel. 



Uterine contractions continue for a while after birth, and the 

 placenta and remaining extraembryonic membranes are expelled as 

 the "after-birth." Much of the uterine lining is lost at birth, for the 

 human placenta is an intimate union of fetal membranes and maternal 

 tissue. Uterine contractions prevent excessive bleeding at this time. 

 Following the birth the uterine lining is gradually reconstituted, and 

 the uterus decreases in size, though it does not become as small as it 

 was originally. 



Questions 



1. Describe the evolutionary sequence of kidneys. Compare this with the embryonic 

 sequence. 



2. What are the mammalian homologues of the cloaca of more primitive vertebrates? 



3. Describe a mammalian nephron and its blood supply, citing the functions of the 

 various parts. 



4. Define renal threshold. Of what significance is this in maintaining the constancy of 

 the internal environment? 



5. What changes have occurred in nephron structure and in the products of excretion 

 during the evolution of terrestrial vertebrates? 



6. With what is the descent of the mammalian testes correlated? 



7. How are the male genital ducts related to the kidney and excretory ducts? 



8. How are the reproductive organs of male and female mammals adapted for reproduc- 

 tion in a terrestrial environment? 



9. Why are millions of sperm necessary to insure fertilization in mammals? 

 10. Describe the birth process in man. 



Supplementary Reading 



Baldwin's Comparative Biochemistry contains an interesting account of the osmotic 

 and excretory problems that confronted the ancestors of vertebrates in moving from a 

 marine to a fresh-water environment, and of the problems vertebrates subsequently en- 

 countered when they entered other environments. H. W. Smith, a leading student of the 

 vertebrate kidney, deals with these same problems, as well as with kidney structure and 

 function, in a very readable book entitled From Fish to Philosopher, and in an article in 

 the Scientific American entitled The Kidney. He considers renal functions more thor- 

 oughly in his book, Principles of Renal Physiology. There is a very good account of the 

 biology of sex and reproduction in Turner's General Endocrinology. .Asdell's Patterns 

 of Mammalian Reproduction is an important source book on differences in reproduction 

 and reproductive cycles that occur in the various kind of mammals from the aardvark 

 to the zebu. 



