488 The Polar Organization of Animals. [ May, 
layer. The former is the seat of the nutritive, the latter of the 
sensory function. Thus very early in life the animal possesses a 
digestive and a sensitive layer of cells. It is an animate stomach 
with a sensitive outer skin. There is no chemical differentiation 
of tissue. The duty performed by each layer of cells is a conse- 
quence of its position. The hydra, for instance, may be turned 
inside out, and the functions of the two layers become reversed 
without injury to the animal. In the higher animals, however, 
these two layers grow unlike in character and incapable of replac- 
ing each other. And yet in the highest animal there may be no 
fundamental distinction. Each layer gains special organs, which 
would not subserve the purpose of the other, but the character of 
- their protoplasm may remain unchanged. 
Between these two layers arises a third, the mesoderm, whose 
origin is yet somewhat unsettled, though there is no reason to 
doubt that it springs from one or both of the original layers. An 
examination of this mesodermal layer, from the point of view here 
` taken, leads to certain interesting conclusions. For, in the highest 
animals, we find it to consist of several distinct tissues, which we > 
may generalize into three. One of these is the nervous and 
its related muscular tissue. A second is the vascular and its 
related lymphatic tissue. The third is the connective or support 
~ ing tissue, with its various forms of fiber, cartilage, bone, &c. The 
~ mesoderm, then, seems to be a direct outgrowth of both the endo- 
dermal and ectodermal layers. The external layer pushes inw 
its channels of sensory inflow, which permeate every region 
` the body, each line of inflow terminating in a muscle, or motor 
organ. The internal layer pushes outward its channels of ai: 
` tive outflow, which permeate every region of the body, and as the 
` nerves may be said to deposit their conveyed force in the muscle, 
so the vessels deposit their conveyed nutriment in the lymphatic 
assimilative spaces. This is really about all we find in the bodys 
complicated as it may appear. If we consider its basic ha | 
istics, we are brought back to the two original layers, OF even | 
the external and internal regions of the rhizopod. 445 t net 
` grows in bulk the external layer extends itself inward im å ae 
plex mass of nerve conductors, nerve cells and muscles. a i 
manner the internal layer extends itself outward, in an Cen 
complex mass of blood channels and lymph vessels. ge 
therefore, we have but the two primary layers, with their 
