THE NATURAL HISTORY OF MAN 51 



ing a single tube which is bifurcated at each end. At this stage 

 of development, the human heart resembles that found in the 

 adult of the lowest vertebrates. Later the single tube of the 

 developing heart partially subdivides into two cavities, the 

 auricle and the ventricle, and it now resembles the heart of 

 the adult of the next higher vertebrates, the fish. The auricle 

 is then subdivided into two cavities, and the human heart of 

 three cavities resembles the fully developed heart of the next 

 higher vertebrate, the amphibian. Later the ventricle is sub- 

 divided and the 'human heart contains four cavities, which is 

 characteristic of the adult heart of the highest vertebrates. 

 Thus it is evident that the human heart in its development 

 passes through stages representing the different adult stages 

 of the various ascending vertebrate classes. This again is an 

 illustration of the so-called Law of Recapitulation, which holds 

 in essence that man in his individual development repeats the 

 evolutionary history of the race, or that "ontogeny repeats 

 phylogeny." 



The blood vessels, consisting of the arteries, capillaries, and 

 veins, are tubes which differentiate from the mesoderm cells 

 in all parts of the body and become connected with the heart. 

 The blood cells, which are present in countless numbers in the 

 blood, likewise are derived from mesoderm cells, and we find 

 that the red blood cells of the human embryo when first formed 

 are large and nucleated. In this stage they resemble those 

 of the fishes and amphibians; later their structure is similar to 

 those of the reptiles. Finally, before birth, they become in 

 man, as in all mammals, non-nucleated and biconcave. 



The complete natural history of the human red blood cor- 

 puscles has never been learned. It is known that they are con- 

 stantly formed in the adult from the cells of the red marrow 

 in the ends of the long bones and that they are nucleated at 

 first but lose their nuclei before they enter the circulation. 

 Although it is believed that the corpuscles are constantly being 



