BLOOD SYSTEM. 357 



of mammals, and even of man. (1) In the earliest stages 

 of egg development — i. e., before the organs are formed 

 — there is, of course, no circulating fluid of any kind, 

 no blood or blood system. This corresponds remotely 

 to the lowest cell-aggregate animals in which there is 

 yet no circulation. (2) The next thing observed is the 

 liberation of nucleated tissue-cells along certain lines and 

 the oscillatory movement of the liberated cells along 

 these lines. This is the beginning of blood vessels and 

 of a blood which has colorless nucleated cells like the blood 

 of invertebrates. A heart is added later, by the develop- 

 ment of a part of the vascular system. (3) The nucleated 

 corpuscles become reddened, and we have now nucleated 

 red corpuscles like the blood of the lower vertebrates. 

 (4) The small non-nucleated round disks begin to appear 

 among the others, which gradually disappear, and we 

 have true mammalian blood. 



It would seem now that in logical order we ought to 

 take up the circulatory system. But the course of the 

 blood is so wholly determined by its distribution through 

 the respiratory organs that it would be impossible to 

 understand that course, especially its comparative 

 morphology, without a previous knowledge of the 

 organs. These, therefore, must be first taken up. On 

 the other hand, the function of respiration is wholly a 

 katabolic process and the most important of all these 

 processes, and must be treated along with these in the 

 fourth chapter. Therefore, our plan will be to take up, 

 first, the morphology of the respiratory organs in vertebrates, 

 then the morphology of the circulatory system, also in 

 vertebrates; then the morphology of circulation and res- 

 piration together in the invertebrates; and, finally, the 

 physiology or function of respiration — which is the same 

 in all animals. This, of course, will be treated with the 



