MULTICELLULAR ANIMALS 207 



do with the secretion of other hormones. One group of these, found 

 in the fluid of the graafian follicle surrounding the maturing 

 ovum, has an influence on the mating impulse of the female and 

 causes histological changes in the wall of the uterus in preparation 

 for the reception of the fertilized egg; another, secreted by the fol- 

 licular cells after the ovum has been shed and fertilized activates the 

 mammary glands prior to and during lactation and has other func- 

 tions concerned with the development and birth of the young. 



General Considerations. Certain general characters of the 

 hormones of vertebrates are worth further discussion. In the first 

 place, some of these endocrine glands represent rather extreme mod- 

 ifications of some simple structures which occur in the other chor- 

 dates and lower vertebrates. For example, the thyroid is found to be 

 homologous with a small gland in the floor of the pharynx of a 

 primitive chordate, Amphioxus. In Amphioxus this gland secretes 

 a slimy ribbon of material which, floating free in the pharyngeal 

 cavity, entraps particles of food from the water and carries them 

 down the digestive canal. In Man this gland originates from the 

 endoderm in the same relative position. In the embryo it has a 

 tubular connection with the floor of the pharynx but later this 

 connection is closed and the adult gland has no outlet except 

 through the absorption of its products by the blood. The wide 

 adaptability and remarkably eflicient characters of the higher ani- 

 mals have been profoundly influenced by the development of the 

 endocrine function of these glands, controlling, as they do, the type 

 of growth and development, of physical and psychical characters, 

 and the maintenance of life in such a complicated body system. 



Secondly, the hormone from one vertebrate when placed in the 

 body of another type has the same action as the hormone produced 

 by that animal's own gland. For example, if a bit of the thyroid of 

 an ox is planted in a frog tadpole from which the native thyroid has 

 been removed, it causes the tadpole to metamorphose into an adult 

 frog in a typical fashion. Advantage is taken of this interchangeable 



