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. — Ontogeny 

 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. — 

 J)eviation 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 ; Coaloma. — Dorsal Vessel and Ventral Vessel of Worms. 

 — Simple Heart of Ascidia. — Atrophy of the Heart in the Amphioxus. —  

 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 history of development without preparatory study is only too 



