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 colom.—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 
