CHAPTER XXIV. 

 DEVELOPMENT OF THE VASCULAR SYSTEM. 



Application of the Fundamental Law of Biogeny. The Two Sides. Heredity 

 of Conservative Organs. Adaptation of Progressive Organs. Ontogem- 

 and Comparative Anatomy complementary of each other. New 

 "Theories of Evolution" of His. The "Envelope Theory" and the 

 "Waste-rag Theory." Main Germ and Supplementary Germ. Forma- 

 tive Yelk and Nutritive Yelk. Phylogenetic Origin of the latter from 

 the Primitive Intestine. Origin of the Vascular System from the 

 Vascular Layer, or Intestinal-fibrous Layer. Phylogenetic Significance 

 of the Ontogenetic Succession of the Organ-systems and Tissues. 

 Deviation from the Original Sequence ; Ontogenetic Heterochronism. 

 Covering Tissue. Connective Tissue. Nerve-muscle Tissue. Vascular 

 Tissue. Relative Age of the Vascular System. First Commencement 

 of the Latter; Coeloma. Dorsal Vessel and Ventral Vessel of Worms. 

 Simple Heart of Ascidia. Atrophy of the Heart in the Amphioxns. 

 Two-chambered Heart of the Cyclostoma. Arterial Arches of the 

 Selachii. Double Auricle in Dipneusta and Amphibia. Double Ven- 

 tricle in Birds and Mammals. Arterial Arches in Birds and Mammals. 

 Germ-history (Ontogeny) of the Human Heart. Parallelism of the 

 Tribal-history (Phylogeny). 



''Morphological comparison of the adult conditions should naturally 

 precede the study of the earliest conditions. Only in this way can the 

 investigation of the history of development proceed in a definite direction ; 

 it is thus provided, as it were, to see each step in the formative process in 

 its true relation with the condition which is finally to be reached. Treat- 

 ment of the histoiy of development without preparatory study is only too 



