THE FROG AND VERTEBRATES IN GENERAL 207 



tissue. Studied microscopically the thyroid tissue is found to 

 consist essentially of vesicular structures, termed follicles. The 

 walls of the follicles are composed of a single layer of epithelial 

 cells enclosing a central cavity. The latter contains a secreted, 

 transparent material, known as the colloid substance, in which 

 there is the hormone, thyroxine. It is rich in iodin, and is ap- 

 parently formed as a secretion by the epithelial cells which con- 

 stitute the walls of the follicles. 



The blood, which passes through the thyroid glands, picks up 

 thyroxine which is important in the life of the organism. It has 

 been definitely shown in the Frog that this substance has a specific 

 action upon the process of metamorphosis. If there is a lack of it, 

 metamorphosis will be inhibited. On the other hand, by supplying 

 this hormone to young tadpoles, metamorphosis can be brought 

 about at a very early stage. In Man a deficiency of thyroxine 

 results in serious pathological conditions, such as cretinism, 

 myxedema, and goiter. Its fundamental action in the tissues 

 appears to result in an increase in the oxidative processes. (W. f. 

 110.) 



Pituitary Body. This endocrine gland is a composite structure 

 found on the ventral surface of the mid-brain. It consists partly 

 of nerve tissue from the infundibulum, originally derived from 

 the fore-brain, and partly of an ectodermal ingrowth, the hypophy- 

 sis, from the dorsal wall of the anterior end of the alimentary 

 canal. In Man an enlargement of the pituitary results in an in- 

 crease in the size of various tissues, particularly bone tissue of the 

 limbs and face — a disease known as acromegaly. In the Frog 

 there is experimental evidence that the hormone of the pituitary 

 body has a regulatory effect upon the thyroid gland. (W. fs. 140, 

 C; 142.) 



Adrenal Bodies. The pair of adrenals in the Frog has been 

 previously noted, one lying on the ventral surface of each of the 

 kidneys. In the higher Vertebrates they are small rounded bodies 

 situated near the kidneys. The hormone secreted by these glands 

 is known as adrenin, and among other things it acts on certain 

 nerve endings which, in turn, bring about a contraction of the 

 muscles of the walls of the blood vessels. Thus it may be used in 

 stopping a hemorrhage or to cause an increase in the blood pres- 

 sure. (W. f. 132.) 



Combination Glands. It is worthy of note that a number of 



