158 PRINCIPLES OF ANIMAL BIOLOGY 



that no follicle-stimulating hormone is forthcoming. If the egg is not 

 implanted in the uterine wall, the corpus luteum degenerates, and its 

 hormone is no longer produced. The thickening of the uterus conse- 

 quently disappears, and the pituitary is relieved of its inhibition. The 

 latter gland therefore begins to produce its follicle-stimulating hormone, 

 and the cycle is started all over again. 



Why the corpus luteum persists if the egg is implanted is not entirely 

 clear, but its hormone is essential to the continued development of the 

 embryo, and the pituitary gland is in some way responsible for its 

 persistence. Some have supposed that a hormone from the placenta 

 guides the pituitary in this particular function, but this is not established. 

 Increase in the size of the mammary glands during pregnancy, with 

 their secretion of milk at birth, is also caused by a hormone of the pitui- 

 tary, but the persistent corpus luteum seems to be the mentor of the 

 pituitary in this control. 



The cycle in other vertebrate animals is likewise controlled by hor- 

 mones, but, since their young are developed outside the mother's body 

 and are not nourished with milk after birth, much of the complexity of 

 the reproductive rhythm is wanting in them. In the amphibia, the repro- 

 ductive cycle is an annual one. Eggs ripen during the winter and are 

 laid in early spring. During the summer the ovaries are small flabby 

 organs, in which the oocytes gradually increase in size into the fall, 

 but normally none is liberated until the next spring. If, however, an 

 extract of the anterior lobe of the pituitary gland is injected into one of 

 these animals in the fall, eggs are released from the ovary in three or four 

 days. 



References 



Carlson, A. J., and V. Johnson. The Machinery of the Body. The University of 

 Chicago Press. (Pp. 360-533.) 



Corner, G. W. The Hormones in Human Reproduction. Princeton University 

 Press. 



Mitchell, P. H. A Textbook of General Physiology. 2d Ed. McGraw-Hill Book 

 Company, Inc. (Chap. IV, reflexes; Chap. V, correlating action of nervous sys- 

 tem; Chap. VI, receptors.) 



Rogers, C. G. A Textbook of Comparative Physiology. McGraw-Hill Book Com- 

 pany, Inc. ' (Includes many invertebrates: Chap. XXVIII, nervous system; 

 Chap. XXV, hormones.) 



Sherrington, C. S. The Integrative Action of the Nervous System. Charles 

 Scribner's Sons. (Chap. I, simple reflexes.) 



