214 ESSENTIALS OF ZOOLOGY 



communicate with delicate lymph -vessels which amalgamate 

 to form more distinct lymph- vessels. The lymph is restored to 

 the blood at four points, where lymph heart dilatations are 

 formed to pump the lymph into the blood. The anterior pair 

 communicate with the subscapular vein at a point just behind 

 the transverse process of the third vertebra. The posterior 

 pair open into the femoral vein, and the heart in this case 

 may be observed beating in the living frog on each side of 

 the posterior end of the urostyle. 



Endocrine Organs. The spleen is derived from splanchnic 

 mesenchyme, and is a red globular body which lies between 

 the stomach and the large intestines. The thymus is a whitish 

 round body which is situated just behind and above the angle 

 of the lower jaw. It arises from epithelial buds of the dorsal 

 region of the gills, especially from the second gill. The thyroid 

 is double, and the two rounded masses lie on each side near the 

 posterior cornua of the hyoid and in front of the glottis. 

 They originate as a single outgrowth of the floor of the mouth. 

 After it loses its connexion with the floor of the mouth it is 

 resolved into right and left halves. The pituitary originates 

 early in the tadpole as a median ingrowth of the ectoderm 

 below the forebrain. A rod of cells is gradually formed which 

 penetrates between the enteron and the brain and expands 

 into a body behind the infundibulum. As the stomodeum is 

 forming below, the pituitary loses its connexion with the 

 ectoderm and, becoming hollow, is attached to the infundibulum. 

 The adrenals have a double origin as in Elasmobranchs. The 

 peritoneal derivative appears as paired segmental buds, prac- 

 tically coextensive with the pronephros and mesonephros. 

 Sympathetic ganglia furnish the chromophile elements. In the 

 adult frog the adrenals are elongated yellow bodies which 

 extend along the ventral face of the kidneys. The endocrine 

 organs have not greatly modified their functions in the land 

 Craniates. The thymus is the organ of adolescence, and its 

 activity, communicated by its secretion to the blood, postpones 

 maturity. The thyroid, on the other hand, hastens maturity. 

 Both are thus to some extent opposite, and even contending. 

 They are related to the general well-being, and their action may 

 influence growth. The gonads communicate their coming 



