216 



Basic Structure of Vertebrates 



ir "ALVEOLUS 



a- BLOOD VESSEL 



Fig. 184. A section of the pancreas, showing an island of Langerhans among 

 the alveoli of the gland. (X 200.) (Courtesy, Neal and Rand: "Chordate Anat- 

 omy," Philadelphia, The Blakiston Company.) 



age of glycogen in the liver. Pancreatic islands occur in all classes of 

 vertebrates, usually in connection with the pancreas, though in some 

 bony fishes the two glands are independent. Both arise from the 

 endoderm. 



Male Sex-Glands 



Removal of the testes of an immature animal prevents the devel- 

 opment of the secondary sexual characteristics of the male. It is evi- 

 dently due to the action of testicular hormones that the young human 

 male begins to acquire, at puberty, some of the adult male character- 

 istics such as beard, deeper voice, and broadened shoulders. 



Female Sex-Glands 



In the ovary, as in the testis, may be distinguished germ-cells and 

 nongerminal "interstitial cells." That the interstitial cells have an 

 endocrinal function is suggested by the fact that they increase during 

 pregnancy. 



The mammalian ovary is known to produce an important hor- 

 mone, estrone, a fatty substance having the chemical formula Cig- 

 H22O2. Estrone induces puberty and menstruation. Another ovarian 

 hormone is progesterone whose action is, in some respects, opposite 

 to that of estrone which stimulates sexual activity, while progesterone 



