ORIGIN OF THE ENDOCRINE GLANDS 877 



b. Posterior Lobe 



The posterior lobe (lobus nervosus, pars neuralis) is derived from the distal 

 part of the infundibulum. 



c. Pars Intermedia 



The pars intermedia or intermediate lobe is associated closely with the 

 posterior lobe but has the same embryonic origin as the pars distalis and pars 

 tuberalis of the anterior lobe. 



In Petromyzon fluviatilis, the hypophysis is a flat, tube -like organ attached 

 to the infundibular evagination of the floor of the diencephalon. The anterior 

 lobe is represented by the hypophyseal duct which ends blindly below the 

 infundibulum. From this duct are proliferated the cells of the intermediate 

 lobe (fig. 365B). The pituitary gland shows great similarity, in all higher 

 vertebrates, being composed of three main parts, viz., pars anterior, pars 

 intermedia, and pars posterior (fig. 365C-E). However, in the chicken, whale, 

 manatee, and armadillo, the intermediate lobe is missing (Selye, '48). 



The pars anterior and the pars intermedia of the pituitary gland develop 

 from Rathke's pouch as evaginations of the middorsal area of the stomodaeal 

 pocket, although in the frog Rathke's pouch develops precociously from the 

 so-called neural ectoderm above the stomodaeal invagination (fig. 365F-I). 

 Rathke's pouch gradually comes into contact with the ventrally directed in- 

 fundibular evagination from the diencephalon. The distal part of the infundib- 

 ular evagination forms the pars neuralis, while Rathke's pouch differentiates 

 into the pars distalis, pars intermedia, and pars tuberalis. 



3. Thyroid Gland 



The thyroid gland (fig. 366B) was described first in 1656 by Thomas 

 Wharton, the English anatomist, who called it the thyroid gland because of 

 its association with the thyroid or shield-shaped cartilage of the larynx. 



After about 50 years of work by many observers on the thyroid gland and 

 its activities, the crystalline form of the secretory principle of the thyroid 

 gland was isolated by Kendall in 1919, and he called it thyroxine. This com- 

 pound contained 65 per cent of iodine by weight and its empirical formula 

 was subsequently determined as Ci-,HnO,Nl4. 



One of the thyroid's functions is to govern carbohydrate metabolism, and, 

 in general, the gland .controls the basal metabolism of the animal together 

 with growth processes. In man and the cat, the thyroid gland is in the form 

 of two lateral lobes, located on the ventro-lateral aspect of the thyroid cartilage 

 of the larynx, the two lobes being joined by an isthmus. In birds, there are 

 two glands, both being located within the thoracic cavity; in fishes, including 

 the Cyclostomes, the thyroid is an unpaired structure and is to be found 

 generally between and near the posterior ends of the lower jaws. The gland, 

 therefore, is a constant feature of all vertebrates. 



