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 

 gastrulas 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. 



