THE STUDY OF ORGANS. 47 



In the embryo tlie red corpuscles of the blood arise 

 in the inside of large branched mesoblast cellsj which 

 are called " blood-islands." It is not quite certain how 

 they arise in the adult : there are several conflicting 

 theories ; one theory is, that they come out of the 

 splenic or some of the lymphatic glands, another, that 

 they are formed in the marrow of bones, and yet 

 another, that they are formed in some way from the 

 white corpuscles. In mammalia, they have no nuclei. 

 The majority of invertebrates have blood with white 

 corpuscles, often of stellate or amoeba-shaped form ; 

 if colouring matter is present, it is usually in the fluid 

 of the blood. The colouring matter is not always 

 red ; in some invertebrates it is green. Some worms, 

 however, have red blood; a significant fact, compared 

 with what is stated elsewhere as to the relation of the 

 vertebrata to this group. The use of the colouring 

 matter of the blood, wherever situated, is as a carrier 

 of oxygen. It consists of a substance which unites 

 readily with oxygen in the respiratory organs, parts 

 with it as readily to the body in the course of the 

 circulation, and then renews the supply. 



The kidneys, or excretory organs, and the re- 

 productive organs, must also be mentioned in 

 our list of the organs of the body. While in verte- 

 brates the excretory organs take the form of a pair 

 of oval masses of minute and extremely convoluted 

 tubules (little tubes), in the higher worms they exist 

 as a series of isolated pairs of tubules situated in 

 successive rings of the body. These are called 



