CHAPTER XIV 



THE PRINCIPLES OF EMBRYOLOGY 



The law of recapitulation. — Vindication of this law by the theory advanced 

 in this book. — The germ-layer theory. — Its present position. — A physio- 

 logical not a morphological conception. — New fundamental law required. — ■ 

 Composition of adult body. — Neuro-epithelial syncytium and free-living 

 cells. — Meaning of the blastula. — Derivation of the Metazoa from the Pro- 

 tozoa. Importance of the central nervous system for Ontogeny as well as 

 for Phylogeny. — Derivation of free-living cells from germ-cells. — Meaning 

 of coelom. — Formation of neural canal. — Gastrula of Amphioxus and of 

 Lucifer. — Summary. 



In a discussion upon this theory of mine, which took place at 

 Cambridge on November 25 and December 2, 1895, it was said that 

 such a theory was absolutely and definitely put out of court, because 

 it contravened the principles of embryology, was opposed, therefore, 

 to our surest guide in such matters ; and the law was laid down with 

 great assurance that no claim for genetic relationship between two 

 groups of animals can be allowed which is based upon topographical 

 and structural coincidences revealed by the study of the anatomy of 

 two adult animals, however numerous and striking they may be, if 

 there are fundamental differences in the embryology of the members 

 of these two groups. 



According to my theory the old gut of the arthropod still exists in 

 the vertebrate as the tubular lining of the central nervous system, 

 and the vertebrate has formed a new gut. According to the principles 

 of embryology as held up to the present, in all animals above the 

 Protozoa, the different structures of the body arise from three definite 

 embryonic layers, the epiblast, mesoblast, and hypoblast, and in all 

 cases the gut arises from the hypoblastic layer. In the vertebrate 

 the gut also arises from the hypoblast, while the neural canal is 

 epiblastic. My theory, then, makes the impossible assertion that 

 what was hypoblast in the arthropod has become epiblast in the 

 vertebrate, and what was epiblast in the arthropod has become 

 hypoblast in the vertebrate. Such a conception is supposed to be so 



