316 STRUCTURE OF THE VERTEBRATES 



of development, they begin functioning. These glands develop 

 from ectoderm, endoderm, or mesoderm; and in one case from 

 two germ layers. In all cases the glandular, secreting, portion of 

 the organ is supported by mesodermal connective tissues. The 

 position, structure and function of the glands is equally diverse. 

 Most of the endocrine organs have been mentioned previously 

 in connection with the structural system with which they 

 develop or are associated; but they are reviewed at this point 

 as they may be considered together as a functional system of 

 structures, although there may be no anatomical connection 

 between them. 



1. Pineal. The pineal gland arises as the epiphysis of the 

 diencephalon, and is connected with the parietal eye in develop- 

 ment. In its original condition in the fish, and in the mammals, 

 it is solely glandular as far as its histological picture is con- 

 cerned. Its function is not known. 



2. Pituitary. As the stomodeal inpocketing meets the hypoph- 

 ysis of the diencephalon, the former spreads over the evagina- 

 tion from the brain and gives rise to the pituitary gland. The 

 gland is divided into three portions, the anterior and posterior 

 lobes being most important. The anterior part secretes a hor- 

 mone (or hormones) which, among other effects, influences the 

 growth of bone. An over secretion causes large heavy bones to 

 be formed. If this occurs before the epiphyses are ossified, 

 gigantism usually results, the degree depending upon the amount 

 of secretion. The results differ if the over function occurs later 

 in life. Those animals with a low secretion of the pituitary tend 

 to be small boned; and, due to some correlation with the mecha- 

 nism of carbohydrate metabolism, usually have an accumulation 

 of fat. 



Recent research has shown that the pituitary secretion, either 

 independently or through some other gland, has a direct influence 

 upon general differentiation and the development of sex. The 

 metabolic influences are too complex to be considered here. 



3. Thyroid. The thyroid gland is considered as homologous 

 with the endostyle of Amphioxus. The development of the 

 thyroid of the cyclostome from the endostyle of the larva has 

 been described. In other vertebrates the gland develops as a 

 median ventral outpocketing of the pharynx about the level of 



