MAMMALIA 291 



derived respectively from its anterior and posterior lobes. 

 It appears to maintain blood pressure, promote growth, 

 and stimulate the secretion of the kidney. If in excess 

 the secretion gives rise to the disease acromegaly and to 

 increased growth ; if it atrophies, to reduced growth and to 

 failure to mature. 



The adrenal bodies derived from the mesoderm and the 

 sympathetic system, and situated as prominent yellow bodies 

 anterior to the kidneys, secrete adrenalin, which has the effect 

 of bringing the body to attention in a case of fright by a quick 

 change in blood pressure. 



The interstitial tissue of the ovary and testis, with maturity 

 of these organs, gives rise to secretions which develop secondary 

 sex characters, and in certain cases, at all events in aquatic 

 groups of animals, produce migrations. It has been shown 

 also that the fertilised ovum in the uterus gives a special 

 structure to the follicle it occupied in the ovary, and this in 

 turn conveys an intimation to the mammary glands. 



The integration produced by the blood conveying messages 

 from one region to another is well preserved in mammals. It 

 is a system which is doubtless even more ancient than the 

 blood, and probably inherited from the circulation of the sap 

 of the mesenchyme spaces. 



The spleen lies in the mesentery, close to the stomach, as a 

 large red organ. It is a product of the mesoderm, and acts as 

 a blood reservoir where important changes in the corpuscles 

 take place. In all vertebrates, it may be regarded as pro- 

 viding a reservoir of blood for the liver. The marrow of the 

 bones likewise is concerned with maintaining a supply of red 

 corpuscles and leucocytes. The latter also are derived from 

 lymphoid tissue. 



Nervous System. The neural canal of the embryo, after the 

 closure of the neuropore anteriorly and the neurenteric canal 

 posteriorly, is a closed vesicle. It is expanded in front to 

 form the brain, and the three primary vesicles the fore-, 

 mid-, and hind-brain are defined. The hindbrain posteriorly 

 graduates into the condition of the spinal cord. The flexure 

 of the body at its anterior end bends the brain ventrally, and 

 the mesencephalic flexure still further rotates the front end of 



