Blond and Blood Vessels. 219 



address, and leave you to judge for yourselves whether this 

 method of viewing the subject gives a wider and truer view 

 of physiological truths than the older plan or not. 



We all recognize the fact that any individual can be but 

 indifferently understood apart from his antecedents ; hence 

 the importance we attach to biographical sketches of those 

 persons that interest us. It is really an acknowledgment 

 of the influence of the environment on the organism, both 

 during its own life-time and that of its ancestors. 



Why, then, is not the consideration of every function of 

 the body preceded by an account of the development of the 

 structures involved, as well as by ordinary anatomical or 

 histological details ? 



No advanced morphologist hopes to clear up the relations 

 of any animal group without taking its embryology into 

 consideration. Up to the present, this method has been 

 almost wholly ignored by physiologists. Allow me to sug- 

 gest in this connection a few considerations which seem to 

 put the student in the possession of a clew to otherwise 

 very obscure relations. 



All are agreed that the blood-cells, whatever their later 

 history, arise in the embryonic mesoblast at the same 

 time as the heart and blood-vessels themselves. To 

 consider, therefore, the heart, blood-vessels, and blood 

 wholly separately, or without a perception of their unitj T , is 

 a mistake that has practical as well as theoretical conse- 

 quences. When we bear this i*elation in mind, it is possible 

 to understand that there may be cases in which the whole 

 vascular system, including the contained blood, may be 

 imperfectly developed, and with ail the consequences of 

 recurrent anaemia. There can be no doubt that any crop 

 of blood-cells must bear relations to the preceding one, and 

 if the original ancestors are defective, their descendants are 

 likely to be similarly weak, apart from any unfavorable 

 circumstances in the environment. 



Until recently, the functions of the white corpuscles, if 

 considered at all in works on physiology, were dismissed 

 in a very few lines. When we remember that the leuco- 



