From such a beginning arose in orderly evolution, on the one hand, all the neuro-muscular and neuro-epithelial structures of the body—the so-called master-tissues; on the other, the germ-cells, the blood-corpuscles, lymph-corpuscles plasma and excretory cells, connective tissue cells, cartilage and bone-cells, etc., all of them independent of the central nervous system, all traceable to a modification of the original germ-cells.
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 development 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 gastrulæ 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.
CHAPTER XV
FINAL REMARKS
Problems requiring investigation—
Giant nerve-cells and giant-fibres; their comparison in fishes and in arthropods; blood- and lymph-corpuscles; nature of the skin; origin of system of unstriped muscles; origin of the sympathetic nervous system; biological test of relationship.