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, 2 comparison which exists only for the larval form, and not 
for theadult. 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 outi: 
onsider 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 cesophagus 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 
