D.—ZOOLOGY. 113 
The fate of the cellular material with which the differentiator deals 
depends not on their pre-determined nature but on the changes they 
undergo in passing to their final place in the organism, and to the company 
they keep when they get there. So far as their fate is concerned they 
“may say with Hamlet ‘the readiness is all.’ In the hands of the three 
co-ordinate gradients that radiate from the ‘ differentiator,’ it matters 
“nothing whether the cells they hand on to build the back or the side are 
those naturally presumed to fit the part. Cells that would under normal 
circumstances form skin cells on the outer surface, and that lie outside 
_the reach of the differentiator, will if grafted into it become kidney-tissue, 
“muscle-tissue, or gut-tissue. And the converse is true. Tissue of the 
differentiator itself, presumably destined to become kidney or muscle, may 
“be grafted into the wound left in the skin by the previous excision, and there 
it will become skin. So the surface tissue that would become brain if left 
alone will, if grafted into the differentiator, become intricately involved, 
and after travelling inwards and forwards find itself transformed into the 
likeness of those with which it is now a companion in function. With 
“increasing zest we may repeat Huxley’s great metaphor concerning the 
cells of the early embryo: ‘ They are no more the producers of the vital 
phenomena than the shells scattered along the sea-beach aretheinstruments 
é by which the gravitative force of the moon acts upon the ocean. Like 
these, the cells mark only where the vital tides have been and how they 
have acted.’ 
___ The events that I have briefly described constitute the prelude to two 
other phases through which the life of a multicellular animal passes. We 
may call them collectively the indeterminate, the determinate, and the 
in tegrated phases. During the first, the three waves of chemical activation 
ppssort the cellular material along the axis of the body and next determine 
tevocably its fate as organs of the individual. This period begins in the 
frog with the closing of the blastopore and of the neural groove. From 
‘ow onwards the evolution of the organs proceeds from determined begin- 
nings impressed upon the constituent cells by their relation to each other 
and to the gradients. Remove the rudimentary organ from its normal 
position—the heart, the kidney, and the brain—and it will complete or at 
least continue its evolution even in the solitude of a moist chamber. But 
under normal circumstances this phase of organic determination leads 
Msensibly to that condition of full and inter-related activity that we may 
call integration. The muscles may be able to develop apart from the 
hervous system, but without organic contact with that controlling system 
they cannot function. The kidney may exhibit characteristic complexities 
of origin and evolution without the aid of humoral or hormonic influence, 
but it cannot function apart from these. The primary factors of life—the 
metabolic gradients—are supplemented by new structural factors and new 
chemical factors, and together constitute personality. 
_ Meanwhile, the inevitable price, senescence, is paid for advance. The 
Stream of animal life, unlike its prototype, sedimenting most elaborately 
where it runs most strongly, is running down. Stability of construction 
brings the penalty of diminished dynamic activity, and the advent of 
| puberty marks for many animals the shadow of the fell sergeant. But 
life has still its reserves, or at least one means of continuing the life-cycle 
‘in its descendant, if not in its undivided personality. In those lower 
1924 I 
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