CONTENTS. 



PAGl 



tion of the Skull from the Head-plates. Gill-openings and Gill- 

 arches. Sense-organs. Limbs. The Two Front Limbs and the 

 Two Hind Limbs 328 



CHAPTER XII. 



THE GERM-MEMBRANES AND THE FIRST CIRCULATION OF 

 THE BLOOD. 



The Mammalian Organization of Man. Man has the same Bodily 

 Structure as all other Mammals, and his Embryo develops in 

 extfctly the same way. In its Later Stages the Human Embryo is 

 not essentially different from those of the Higher Mammals, and in 

 its Earlier Stages not even from those of all Higher Vertebrates. 

 The Law of the Ontogenetic Connection of Systematically Related 

 Forms. Application of this Law to Man. Form and Size of the 

 Human Embryo in the First Four Weeks. The Human Embryo in 

 the First Month of its Development is formed exactly like that of 

 any other Mammal. In the Second Month the First Noticeable 

 Differences appear. At first, the Human Embryo resembles those 

 of all other Mammals ; later, it resembles only those of the Higher 

 Mammals. The Appendages and Membranes of the Human 

 Embryo. The Yelk-sac. The Allantois and the Placenta. The 

 Amnion. The Heart, the First Blood-vessels, and the First Blood, 

 arise from the Intestinal-fibrous Layer. The Heart separates 

 itself from the Wall of the Anterior Intestine. The First 

 Circulation of the Blood in the Germ-area (a. germinativa) : Yelk- 

 arteries and Yelk-veins. Second Embryonic Circulation of the 

 Blood, in the Allantois: Navel-arteries and Navel-veins. Divisions 

 of Human Germ-history 31 



CHAPTER XIIL 



THE STRUCTURE OF THE BODY OF THE AMPHIOXUS AND 

 OF THE ASC1D1AN. 



Causal Significance of the Fundamental Law of Biogeny. Influence 

 of Shortened and Vitiated Heredity. Kenogenetic Modification of 

 Palingenesis. The Method of Phylogeny based on the Method of 

 Geology. Hypothetic Completion of the Connected Evolutionary 

 Series by Apposition of the Actual Fragments. Phylogenetic 

 Hypotheses are Reliable and Justified. Importance of the Amphi- 



