go SECTIONAL ADDRESSES 
or another for intrinsic reasons: an event starts inside the cells—quite 
independent of any external influence—just as in the exploding molecule 
of a radio-active substance. In other words, the cell has an individuality 
of its own—which is free from the limitations of statistical laws. ‘The 
field of cell cleavage is full of possibilities for future inquiry, and would 
well repay more intensive study. 
We must, however, now turn to certain wider aspects of experimental 
embryology, which are best observed in the eggs of the lower vertebrates. 
Within this field the progress of the past twenty-five years has been 
spectacular. By grafting fragments of the developing embryo of the newt 
into positions which they do not normally occupy, it is possible to get 
a picture of embryological development which is incomparably more 
satisfying than any hitherto available. We know that there exists in the 
egg a region or regions which are capable of influencing the fate of the 
neighbouring tissues. Each of these so-called ‘ organising centres’ 
determines in some way the process of tissue differentiation: the raw 
material is, as it were, full of potentialities for differentiation, but the 
exact line which will be followed is affected by the organisers. Once the 
process of differentiation has reached a critical stage, the fate of the tissue 
is determined ; before that period, the raw tissue can be built up into 
a variety of different structures. Quite recently it has been shown that 
such organisers are curiously non-specific—an organising centre from 
a chick can induce organ-formation in the undifferentiated tissue of 
a mammal ; and, still more remarkable, the organising centre does not 
appear to lose its activity after death. These facts are admittedly be- 
wildering—but two points seem to emerge quite clearly. Firstly, the 
potentiality of the organism to control its fate is established at a very early 
stage. If we carry back the facts of experimental embryology to their 
beginning, we see that the essential biological difference between the 
egg of the sea-urchin and the egg of the mollusc (Dentalium) is a difference 
in the relative time at which development becomes independent of 
organising centres—in the sea-urchin it is relatively late; in Dentalium 
it occurs before the egg begins to cleave. By accepting the concept of 
an organising centre the facts of embryology thus appear to arrange 
themselves in an orderly manner—and this, after all, is the supreme test 
of any scientific hypothesis. The second great inference to be drawn 
from these facts is the present inadequacy of expressing the facts in 
physico-chemical terms. The only point at which the phenomena 
seem to be susceptible to physical analysis is the apparent activity of an 
organising centre after death. This would suggest that the action of an 
organiser is either mechanical in its nature or is comparable to that of 
a trigger which releases specific lines of development from unorganised 
tissues of high potentiality. By physical methods we can hope to elucidate 
the physical attributes of this trigger action, but I do not think that the 
facts, so far as they are known at present, present a very convincing argu- 
ment in favour of a mechanistic hypothesis. From a broad standpoint, 
the obvious conclusion we must draw from the facts of experimental 
embryology and from the regeneration of lost parts is that the organism 
behaves as a co-ordinated system even in the very earliest stages of its 
development ; and that this co-ordination is of a degree of complexity 
