CHAPTER XII. 



THE GEKM-MEMBRANES AND THB FIRST CIRCULATION 

 OP 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 

 Sacond 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. 



"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 doubtful for the last 

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

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