THE PRINCIPLES OF EMBRYOLOGY: 487 
Such a view of the processes of embryology brings embryology into harmony 
with comparative anatomy and phylogeny, for it makes the central nervous 
system and not the alimentary canal the most important factor in the develop- 
ment of the host. 
The growth of the individual, whether arthropod or vertebrate, spreads from 
the position of the central nervous system, regardless of whether that position 
is a ventral or dorsal one with respect to the yolk-mass. Where the pabulum 
is, there is the definite gut, the lining walls of which are called in the embryo, 
hypoblast ; but when the pabulum is no longer there, although a tube is formed 
in the same manner as the alimentary canal of the arthropod, it is now called 
an epiblastic tube, and is known as the neural tube of the vertebrate. 
This is the great fallacy of the germ-layer theory, a fallacy which consists 
of an argument in a vicious circle: thus the alimentary canal is homologous in 
all of the Metazoa, because it is formed of hypoblast, but there is no definition 
of hypoblast, except that it is always that layer which forms the definitive 
alimentary canal. 
When, after the process of segmentation has been completed, a free swimming 
blastula results, unprovided with any store of pabulum in the shape of yolk, 
then the same physiological necessity causes such a form to obtain its nutriment 
from the surrounding medium. The simplest way to do this is by a process 
of invagination, in consequence of which food particles are swept into the 
invaginated part and then absorbed. For this reason in such cases true 
gastrule are formed, as in the case of Amphioxus among the vertebrates, and 
Lucifer among the crustaceans; such a formation does not in the least imply 
that the gut of the arthropod is homologous with that of the vertebrate. The 
resemblance between the two is not a morphological one, but due to the same 
physiological necessity. They are analogous formations, not homologous. 
The muscular tissues are found to be formed in close connection with the 
nervous tissues, and in very many cases are described as formed from epiblast, 
so that there are strong reasons for placing them in a special category of the 
so-called mesoblastic tissues. If they be separated out, then it seems to me, the 
rest of the mesoblast would consist of the free-living cells of the body, which 
are not connected with the central nervous system. In watching, then, the 
formation of mesoblast, defined in this way, we are watching the separation 
out from the master-tissues of the body of the independent skeletal and 
excretory cells, 
