THE GENERAL SCHEME 



Transportation of the egg from ovary to uterus, and of 

 the sperm cell from testis to the egg. 



The union of the germ cells, 



Development of the fertilized egg. 



The attachment of the embryo to the mother, and its 

 nutrition, 



Birth and the nursing of the young. 



These are complicated matters, which must be timed so that 

 each stage fits into the next. What goes on in one organ must 

 be coordinated with events in another. The chemical environ- 

 ment of the egg and the sperm cells must be kept in adjust- 

 ment to their needs ; muscle cells in the oviduct, uterus and 

 the male reproductive system must be ready to act when 

 required; the lining of the uterus must be prepared for the 

 embryo; in short, a whole complex system of organs and 

 tissues must work as a unit. 



The body has two important ways of linking the action of 

 its separate organs. One of these is the nervous system, 

 through which run innumerable signals connecting the organs 

 of sensation and motion and regulating many functions of 

 the internal organs. The other, with which we are much more 

 concerned in this book, is the system of the chemical messen- 

 gers or hormones. A hormone is a chemical substance made 

 in one of the special glands called ductless glands or glands 

 of internal secretion, among which are the pituitary, thyroid, 

 parathyroid, adrenal, and parts of the pancreas, which do 

 not discharge their product through a duct to the outside 

 of the body or into another organ, as for example do the 

 sweat glands, the salivary glands, the liver and the kidney. 

 Instead, these endocrine glands (for such they are also 

 called) put their respective secretions into the blood as it 

 courses through the blood vessels which pervade their sub- 

 stance. The hormones are thus carried all over the body and 



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