94 EVOLUTION AND GENETICS 



system. The function of conductivity is naturally retained in some 

 degree by all cells, but it too is highly developed in the specialized 

 ectoderm of complex nervous systems. Contractility, the means 

 of accomplishing immediate response to environmental stimuli, 

 likewise becomes an ectodermal function in the Coelenterata, but 

 with the development of the third germ layer, this function shifts 

 almost completely to the third layer, the mesoderm. 



Inasmuch as the body form in the Coelenterates almost com- 

 pletely removes the endoderm from environmental contacts and 

 protects it by the ectoderm from all necessity for direct response 

 for protection or the securing of food, it is not to be expected that 

 this layer would retain the same qualities as the ectoderm. The 

 additional fact that it lines a cavity of sufficient size to contain 

 particles of food too large for ingestion by single cells, suggests its 

 logical association with digestion, and we find that the initial steps 

 of metabolism are henceforth functions of endodermal tissue. 

 The development of large and complex glands such as the liver and 

 pancreas, involving quantities of mesodermal tissue, still involves 

 the endoderm as the source of the epithelium — the glandular 

 portion — of these organs. 



The Endocrine Glands. One evidence of similarity in the func- 

 tions of similar organs is sufficiently striking to be worthy of com- 

 ment, inasmuch as valuable therapeutic results have been ob- 

 tained on this basis. The several ductless or endocrine glands of 

 vertebrates produce secretions known as hormones which exert 

 specific correlating influences on the body through their power to 

 activate or inhibit the development and functions of various 

 parts. These glands include the pituitary body, thyroid, thymus, 

 gonads and various other parts. Because of their importance in 

 the human body they have been made the subject of extensive 

 study in lower animals, and have been found to exert the same 

 influence upon individuals of the species producing them and 

 upon others, even of different classes. 



The secretion of the thyroid, whose effect is evident through 

 modification in the various types of goiter and in the congenital 

 insufficiency of cretinism, is now extracted from domestic animals 

 for therapeutic use. Its effect on the normal course of physical 

 development is exerted not only on man, a member of the same 

 class, the mammals, but also on tadpoles and larval salamanders. 

 Tadpoles to which the extract (thyroxin) is administered undergo 



