SYSTEMATIC SURVEY OF THE HUMAN RACE. 185 



THIRD MAIN PERIOD IN TRIBAL HISTORY. 



The Invertebrate Intestinal Animal Ancestors of Man. 



The ancestors of man have the form-value of individuals of the third 

 order, of inarticulate individuals. The body encloses an intestinal cavity 

 with a mouth, and consists at first of two primary germ-layers, afterwards 

 of four secondary germ-layers. 



Fifth Stage : Gastraea Series (Figs. 174-179, p. 65). 



Tbe ancestors of man have the form-value and structure of a Gastrula. 

 The body consists merely of a simple primitive intestine, the wall of which 

 is formed of the two primary germ-layers. 



Sixth Stage : Chordonium Series (Figs. 184-188, p. 80-90). 



The ancestors of man are worms : at first, primitive worms, allied to the 

 Turbellaria; afterwards worms of higher rank, Scolecida; finally, notochord- 

 animals with the organization of the ascidian larvae. The body is composed 

 of four secondary germ-layers. 



FOURTH MAIN PERIOD IN TRIBAL HISTORY. 



The Vertebrate Ancestors of Man. 



The ancestors of man are vertebrates, and their form-value is, therefore, 

 that of an articulated individual, or a chain of metamera. The skin-sensory 

 layer is specialized into the horn-plate, medullary tube, and primitive 

 kidneys. The skin-fibrous layer has divided into the leather-plate, primitive 

 vertebrae (muscular plate and skeleton-plate), and the notochord. From 

 the intestinal-fibrous layer originates the heart with the main blood-vessels 

 and the fleshy intestinal wall. From the intestinal-glandular layer, the 

 epithelium of the intestinal tube is formed. The formation of metamera is 

 constant. 



Seventh Stage : Acrania Series (Fig. 189 ; PI. XL Fig. 15). 



The ancestors of man are skull-less vertebrates, like the extant Amphi- 

 oxus. The body already forms a chain of metamera, several primitive 

 vertebrae having separated off. The head is not yet entirely distinct from 

 the trunk. The medullary tube has not separated into brain-bladders. The 

 heart is very simple, without chambers. The skull is still wanting ; as are 

 also the jaws and limbs. 



