52 GENERAL BIOLOGY 



direct connection with the digestive system, but may conveniently be 

 dealt with here. They are bright yellow, finger-like bodies grouped in 

 front of the testes or ovaries as the sex may be. They develop from 

 the anterior portion of the genital ridges whose posterior portions alone 

 give rise to the sexual organs. In the autumn they are of great size and 

 loaded with fat-cells, a certain amount of lymphatic tissue being also 

 present. In the spring they are much reduced. It is probable that they 

 also perform other functions at all seasons of the year, but on this point 

 we have no precise knowledge." 



GLANDS 



Glands may be conveniently classed into two groups : 

 Exocrine glands, that is, glands whose products are used externally or 

 on substances entering the body, and which generally leave by way of 

 ducts. 



Endocrine glands, that is, glands whose products act on the body 

 itself, not on substances brought into it. This type of gland generally 

 has no duct ; or if so, as in the case of ovaries and pancreas, the 

 "endocrine" portion of the secretion is absorbed by the blood vessels 

 and does not leave the gland by way of a duct. The term "ductless 

 glands" has been used to designate these glands, but has been found 

 inappropriate. The products of the endocrine glands are known fre- 

 quently as "internal secretions," and are composed of active agents simi- 

 lar to enzymes. The name of "hormone" (excitant) has been given to 

 these agents, which differ from enzymes primarily in that their activity 

 is not destroyed by boiling. 



It has been found, however, that the action of the endocrines may 

 be both stimulatory and inhibitory, as, for instance, in certain experi- 

 ments on tadpoles which were fed with thymus and thyroid gland. 

 Gudernatsch (1912-14) found that the thyroid food stimulated develop- 

 ment and inhibited growth, while thymus stimulated growth and in- 

 hibited development. Thyroid-fed tadpoles matured in four weeks, as 

 contrasted with the normal period of twelve weeks, but were dwarfs and 

 pigmies in size ; while thymus-fed tadpoles were gigantic in size, but 

 after sixteen weeks showed no indication of transformation, in fact, had 

 not yet developed their hind legs. From certain other experiments on 

 the sexual glands it has been similarly concluded that the internal secre- 

 tion from the sex glands (specifically that of the interstitial cells) acts 

 both as a stimulant to the body so that it will develop the characteristics 

 pertaining to its proper sex, and as an inhibitor in suppressing those of 

 the other sex. The excitant has been named hormone, the inhibitor 

 chalone. 



The products of the various endocrines are regulatory in nature, and 

 control or affect such processes of the body as growth, puberty, sec- 

 ondary sexual characters, blood pressure, metabolism, distribution and 



