CHAPTER IX. 



THE VERTEBRATE NATURE OF MAN. 



Relation of Comparative Anatomy to Classification. — The Family-relation* 

 ship of the Types of the Animal Kingdom. — Different Significance 

 a«d Unequal Value of the Seven Animal Types. — The Gastrcea Theory, 

 and the Phylogenetic Classification of the Animal Kingdom. — De- 

 scent of the Gastrsea from the Protozoa. — Descent of Plant-animala 

 and Worms from the Gastraea. — Descent of the Four Higher Classes of 

 Animals from Worms. — The Vertebrate Nature of Man. — Essential and 

 Unessential Parts of the Vertebral Organism. — The Amphioxus, or 

 Lancelet, and the Ideal Primitive Vertebrate in Longitudinal and 

 Transverse Sections. — The Notochord. — The Dorsal Half and the Ven- 

 tral Half.— The Spinal Canal.— The Fleshy Covering of the Body.— 

 The Leather-skin (corium). — The Outer-skin (epidermis).— Body- 

 cavity (cceloma) . — The Intestinal Tube. — The Gill-openings. — The 

 Lymph-vessels. — The Blood-vessels. — The Primitive Kidneys and 

 Organs of Eeproduction. — The Products of the Four Secondary Germ, 

 layers. 



" Know thyself ! This is the source of all wisdom, said the great thinkers 

 of the past, and the sentence was written in golden letters on the temple of 

 the gods. To know himself, Linnaeus declared to be the essential indis- 

 putable distinction of man above all other creatures. I know, indeed, in 

 study nothing more worthy of free and thoughtful man than the study of 

 himself. For if we look for the purpose of our existence, we cannot possibly 

 find it outside ourselves. We are here for our own sake."— Karl Ernst 

 Baer (1824). 



A DIFFICULT task now lies immediately before us in this 

 history of our individual development ; we must trace the 

 complex human body with all its various parts, — organs, 



