1883. ] The Polar Organization of Animals. 487 
trols the whole development of the animal kingdom, and is as 
strongly displayed in the man as in the protozoan. Both alike 
possess a distinct nutritive and sensory polarity, each of these 
functions having its separate pole. 
This polarity is, in fact, particularly displayed in the fully pro- 
gressed animal, as we shall hope to show. In its generalized con- 
dition, in the rhizopod, it consists of a differentiation of function in 
the two regions of the body. The external layer of the rhizopo- 
dal body alone comes into contact with and feels the impressions 
of outer force. Thus such sensitiveness as is possessed must be 
confined to this layer. The impressions received, however, seem 
to extend'inwardly, and to result in the production of a motor 
function. The internal layer, on the contrary, is distinctly nutri- 
tive in function. The food is, in various methods, brought into 
contact with it, digested and assimilated. And as the sensory 
impressions received by the external layer are transmitted and 
produce motion throughout the entire body, so the nutriment 
received by the internal layer is diffused throughout and assimila- 
ted by the entire body. 
At the very outset of life, therefore, a separation of its two active: 
functions takes place as a necessary result of the opposed rela- 
tions of its substance. The external layers of protoplasm become 
„Opposite in function to the internal. Contact, irritation, motor in- 
tigation, have their seat in the surface, and proceed inward. Nu- 
trition has its seat in the interior, and proceeds outward. The 
development of the one tends to the production of special chan- 
- nels of sensory inflow, and special regions of motor contraction. 
The development of the other tends to the production of special 
m tive regions and special channels for the conveyance of nu- 
triment, Thus the sensory and motor functions of the body 
originate in the surface and spread inward. Its vascular functions 
originate in the interior and spread outward. These separate 
pactions are distinct only as regards their points of origin and 
their characteristics, for throughout the whole intermediate region 
they inosculate and interweave with each other. | 
Separation of functional regions, thus indicated in the Pro- 
; is markedly displayed throughout the whole line of devel- 
pment of the Metazoa. In all the Metazoa an early step of 
embryonal differentiation is the formation of two distinct layers of 
cells, one bounding an internal cavity, the other forming a surface 
