Chap. 15 CHEMICAL REGULATION ENDOCRINE GLANDS 257 



cal substances (steroids); the other, of complex substances (proteins). They 

 occur in the blood in remarkably minute quantities, and as drugs are extraordi- 

 narily powerful. The hormone of the thyroid gland is so potent that one grain 

 of it in circulating human -blood will raise the rate of metabolism in an adult 

 by about one-third. Sparsity of only one of the hormones of the anterior pitui- 

 tary in a child can make it a dwarf, and an oversupply of the same hormone 

 can create a giant. In certain animals and human tribes these conditions have 

 become hereditary; among dogs, the Great Danes are giants and toy Pom- 

 eranians are well-formed dwarfs. Much of individuality originates in hor- 

 mones. They also aid and regulate embryonic development and growth. 



Distribution of Hormone Production among Animals 



Hormones take part in the control of essential activities in the lives of many 

 invertebrates and of all vertebrates. 



Invertebrates. Hormones have been clearly demonstrated in arthropods, 

 especially crustaceans and insects. In each eyestalk of crayfishes and shrimps 

 there is a minute endocrine gland, the sinus gland (Fig. 15.2). Results of 

 experiments indicate that these glands secrete at least five hormones: three 

 that regulate the pigment in the chromatophores (pigment cells) of the skin, 

 one that stimulates the movement of pigment grains into a location in the cell 

 characteristic of them when the eye is adapted to full light, and one that delays 

 molting until a particular time. 



The hormonal control of molting in insects has been definitely established. 





y:^ 



Fig. 15.2. Left, Part of the head of a shrimp (Palaemonetes exilpes), showing 

 the position of the sinus glands in the eye stalks. They produce hormones which 

 influence the movement of pigment in the cells of the retina when the eyes change 

 from a dark to a light adapted state. Right, A diagram illustrating the dispersion 

 and concentration of pigment in cells (chromatophores) when the skin changes 

 from dark to pale color and the reverse; full color effect in a pigment cell occurs 

 when pigment granules occupy the numerous branches of the cell; the least pos- 

 sible display of color in the same cell when the granules are crowded into the 

 center. One cell may contain several kinds of pigment and the granules of each 

 kind may be dispersed and concentrated independently. (Left, courtesy, Turner: 

 General Endocrinology. Philadelphia, W, B. Saunders Co., 1948.) 



