ONTOGENESIS 



225 



human embryo (twenty-five days) and gives it a striking 

 resemblance to the embryos of lower animals. 



The heart makes its appearance as a simple straight 

 tube similar to that of the invertebrates, becomes 

 separated into an anterior ventricle and posterior auricle 

 like that of the fishes. Later it consists of a curved 

 organ with two incompletely separated auricles and one 

 ventricle, not unlike that of the bactrachians. Much 



FIG. 98. Diagrams illustrating arrangement of primitive heart and aortic 

 arches in the human embryo. By comparing these with the diagrams showing 

 the increasing complexity of the heart in phylogenetic development (Chapter 

 VII) it will be seen that in the development of the human heart the autogeny 

 repeats the phylogeny. (Modified from Allen Thomson.) 



later it differentiates into the four-chambered viscus of 

 the higher vertebrates. 



Not only does the development of the heart thus 

 conform to its phylogeny, but the development of the 

 whole circulatory system coincides as well. The heart 

 and great vessels first appear, the small vessels and 

 capillaries later. Further, the arrangement of the 

 arteries at first conforms with fair accuracy to that 

 15 



