CHAPTER XII. 



THE GERM-MEMBRANES AND THB 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 exactly 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 Differeiices appear. — At first, the 

 Human Embryo resembles those of all other Mammals ; later, it 

 resembles only those of the Higher Mammals. — The Ap| endages and 

 Membranes of the Human Embryo. — The Yelk-sac. — The Allantois and 

 the Placenta. — The Amnion. — The Heart, the First Bluod-vessclrf, 

 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. germinafiva) : 

 Yelk-arteries and Yelk-veins. — Second Embryonic Circulation of the 

 Blood, in the Allantois: Navel-arteries and Navel-veins. — Divioions of 

 Human Germ-history. 



** Is man a peculiar organism? Does he originate in a wholly different 



way from a dog, bird, frog, or fish.? and does he thereby justify those who 



assert that he has no place in nature, and no real relationship with the 



lower world of animal life ? Or does he develop from a similar embryo, 



and undergo the same slow and gradual progressive modifications ? The 



answer is not for an instant doubtful, and has not been doubti'ul for the last 



thirty years. The mode of man's origin and the earlier stages of his 

 2G 



