112 SECTIONAL ADDRESSES. 
self-determined changes appear always to lead in animals to the formation 
of a central nervous system if they go far enough, the conclusion is reached 
that the nervous system is the final expression, both in arrangement and in 
mode of action, of the system of metabolic gradients. 
A corollary of great importance can be deduced from the case of the 
Planarian. The degree of individuality of the daughter is a measure of the 
loss of control of the head-end, a not unfamiliar phenomenon. As this 
occurs, the daughter becomes more and more physiologically isolated and 
her metabolic processes proceed at a faster rate. Hence physiological 
isolation is a fundamental factor in asexual reproduction. 
The Development of the Frog Egg as a System of Gradients. 
In the light of this conception of the individual being as a reaction- 
system, we may now take the unfertilised ovarian egg, say of the frog, as 
a primary individual. It possesses an axial gradient. One pole is the 
region of highest metabolic rate determined by therelations of the egg to the 
maternal tissues and the other external agencies. There is evidence that, 
from this apical pole, chemical change proceeds in waves of decreasing order 
of intensity through the protoplasm towards the opposite or basal pole. 
Though there may as yet be no visible structural change in the colloidal 
medium, yet the factors that produce the first visible change are there. 
Differentiation on this view is the expression of chemical change along the 
gradient. The cell or ovum is in fact a creature ‘ with a kind of a heid 
upon it—man could say nae mair.’ 
The changes that ensue during the maturation of this egg or primary 
individual are too involved, and too familiar, to zoologists for me to 
enumerate. The little sphere, still without visible differentiation, becomes 
a stratified power-station. The apical pole remains chemically active, the 
basal pole accumulates stores of potential food and energy. The whole 
globular microcosm becomes enclosed in a non-permeable membrane, and 
is shut off as a closed system from the outer world. If only one of its 
extruded polar bodies returned ; if only something could break this too, 
too solid envelope; if only some messenger from the outer world, some 
Orpheus could visit the cold Eurydice, then development might begin. 
And it so happens. In the natural sequence, Orpheus—the spermatozoon 
—is the winged key that unlocks the imprisoned one. He casts a shadow 
—the grey crescent—that heralds the advent of the new gradient, the one 
that takes sides, and that prophetically unseams the germ from the nave 
to the chaps, that separates the right side from the left. As if to justify 
the use of emotional language, the germ at that moment of release takes 
an explosive breath as though the crisis were over. It will never take a 
deeper one. The process of development is begun. 
The first trace of the embryo is the apical region or brain, formed as 
part of that region of greatest metabolic activity known as the dorsal lip 
of the blastopore, or the ‘ differentiator.’ (4) This region provides the 
three co-ordinate lines or ‘ metabolic gradients’ along which the main 
features of structure are elaborated—the primary gradient along which 
the central nervous system forms; the secondary gradient for the axial 
organs; and the transverse gradient along which the lateral organs are 
developed. 
