464 COMPARATIVE ANATOMY 



in failure of sexual development and atrophy of the sex glands. Another 

 effect is reduced secretion by the suprarenal and thyroid glands, so that 

 it is quite possible that the effect of the pituitary upon sexual development 

 is indirectly through the effect upon suprarenals and thyroid. 



While the histological structure of the posterior lobe does not suggest 

 a glandular function, treatment with extract of the posterior lobe causes 

 a marked rise in blood pressure. Two kinds of substances which act as 

 endocrines have been obtained from the posterior lobe of the pituitary. 

 One of them, vasopressin, increases blood pressure; the other, oxytocin, 

 stimulates the contraction of the muscles of the uterus and is now used 

 following childbirth to bring the uterus back to normal size. Posterior 

 lobe extract is found to reduce obesity and certain types of obesity are 

 now ascribed to deficiency of this hormone. Increase of posterior lobe 

 secretion causes decreased urination, possibly brought about indirectly 

 through the nervous system. 



The wide-spread and contrasting effects of the secretion of the pitui- 

 tary, which may have as many as eight different hormones, taken together 

 with the experimental evidence of the mutual interaction of endocrinal 

 organs, demonstrates the enormous complexity of the problem of the 

 biology of endocrines. Under the circumstances, we may follow Hoskins 

 in suggesting that "To those who, in the present state of our knowledge, 

 would glibly re-write physiology and psychology in terms of pituitary 

 functions the timorousness of the proverbial angel is commended." 



Development. The pituitary gland has a double origin. The anterior 

 lobe comes from an ectodermal sac, Rathke's pouch, in the roof of the 

 mouth, while the posterior lobe is formed as an outgrowth of the base of 

 the diencephalon. Rathke's pouch makes its appearance in a 3 mm. 

 embryo. The posterior end of this sac comes into contact with the 

 infundibular outgrowth from the base of the brain. As development 

 proceeds, Rathke's pouch loses connexion with the ectoderm and breaks 

 up into vesicles, the cavities of which are remnants of the pouch cavity. 

 In this way, the anlage of the anterior lobe assumes its definitive position 

 anterior to the infundibulum. The ventral end of the infundibulum 

 becomes thickened as the anlage of the posterior lobe. The anterior 

 lobe enlarges and partly surrounds the posterior lobe. The cells which 

 later become differentiated as pars intermedia and pars tuberalis are 

 derived from the anlage of the anterior lobe. By the end of the fourth 

 month, the gland attains its adult form and appearance. Blood vessels 

 and connective tissue capsule are of mesenchymatous origin. 



Evolution. The pituitary gland occurs only in vertebrates. No 

 comparable structure has been identified in the protochordates. The 

 elements of the pituitary make their first appearance in cyclostomes. 

 In myxinoids (Myxine, Bdellostoma) the neural (posterior) lobe is repre- 



