THE PRINCIPLES OF EMBRYOLOGY 457 



First, I will take the teachings of vestigial organs and the arrange- 

 ment of organs found in the vertebrate embryo. Here it is impossible 

 to say that my theory is contrary to the teaching of embryology, for 

 as the previous chapters have shown again and again, the argument 

 is based very largely upon the facts of embryology. In the first 

 place, the comparison which I have chiefly made is a comparison 

 between the larval form of a very low vertebrate and the arthropod 

 group, a comparison which exists only for the larval form, and not 

 for the adult. The whole theory, then, is based upon a developmental 

 stage of the vertebrate, and not upon the anatomy of the adult. 



Throughout the whole history it seems to me perfectly marvellous 

 how completely the law of recapitulation is vindicated by my theory 

 of the origin of the vertebrate. The theory asserts that the clue 

 to the origin of vertebrates is to be found in the tubular nature 

 of the central nervous system of the vertebrate ; in that the verte- 

 brate central nervous system is in reality formed of two things : (1) 

 a central nervous system of the arthropod type, and (2) an epithelial 

 tube in the position of the alimentary canal of the arthropod. 



Is it possible for embryology to recapitulate such a phylogenetic 

 history more clearly than is here the case ? In order to avoid all 

 possibility of our mistaking the clue, the nerve-tube in the embryo 

 always opens into the anus at its posterior end, while in the larval 

 Amphioxus it is actually still open to the exterior at the anterior end. 

 The separateness of the tube from the nervous system at its first 

 origin is shown especially well in the frog, where, as Assheton has 

 pointed out, owing to the pigment in the cells of the external layer 

 of epithelium, a pigmented tube is formed, on the outside of which 

 the nervous tissue is lying, and step by step the gradual inter- 

 mingling of the nerve- cells and the pigmented lining cells can be 

 followed out. 



Consider the shape of the nerve-tube when first formed in the 

 vertebrate. At the cephalic end a simple bulged-out tube with two 

 simple anterior diverticula, which passes into a narrow straight spinal 

 tube; from this large cephalic bulging a narrow diverticulum, the 

 infundibulum, passes to the ventral surface of the forming brain. 

 This tube is the embryological expression of the simple dilated cephalic 

 stomach, with its ventral tesophagus and two anterior diverticula, 

 which opens into the straight intestine of the arthropod. Nay, more, 

 by its very shape, and the invariable presence of two anterior 



