50 THE EVOLUTION OF MAN 



parathyroid, and the thymus glands (Fig. 19). Another im- 

 portant endocrine organ, the pituitary body, or hypophysis, 

 has a double origin in that a portion of it develops as an 

 upgrowth from the ectoderm which lines the extreme anterior 

 end of the alimentary canal, or buccal cavity, and another por- 

 tion, which fuses with the former, develops as a downgrowth 

 from the brain. These glands, through their internal secre- 

 tions, or hormones, influence development in various ways. 

 All of them ultimately lose their connection with the ali- 

 mentary canal. The thyroid gland moving down the ventral 

 side of the neck stops just above the thorax. The thymus, 

 which in the calf is commonly called the sweetbread, descends 

 into the thorax until it lies just above the heart. Early in pre- 

 natal life the glandular structure of the thymus disappears and 

 a lymphoid tissue, such as we find in the tonsils, takes its place. 

 This organ continues to grow until the child is about two years 

 of age and then gradually diminishes in size, disappearing at 

 about twenty years of age. This same remarkable history of 

 the thymus is found in other mammals as well as in man. Little 

 is known definitely with regard to the function of this organ. 



Lower down the alimentary canal, just below the spindle- 

 shaped enlargement which represents the developing stomach, 

 two other organs grow out and, by repeated branching, form 

 the liver and pancreas (Fig. 19) . These always remain in con- 

 nection with the alimentary canal by ducts which carry their 

 secretions. The pancreas develops as two outgrowths which 

 fuse. Usually only one of the ducts persists. Occasionally 

 it is found in man that both persist and the pancreas pours its 

 secretion into the intestine by two ducts instead of one. The 

 explanation of such cases undoubtedly lies in the double origin. 



The Vascular System. The heart differentiates from a por- 

 tion of the mesoderm lying underneath the pharynx in the head 

 region of the embryo. Two straight tubes are first formed 

 (Fig. 20). These quickly fuse for part of their length form- 



