200 Stimulus S Mechanism as Factors in Organisation 
I cannot but think that this double aspect of the whole matter, 
the specific nature of the mechanism on the one hand, and the 
modes of excitation of it on the other have often been too little 
insisted on, and some at least of the confusion with which the 
subject of physiological morphology has become invested is to be 
attributed to this source. By applying this hypothesis of the com¬ 
bined result of the activating stimulus upon an actuated mechanism 
as a working hypothesis in endeavouring to gain a more compre¬ 
hensive insight into the nature of the manifestation of vital activity 
I believe we shall be able to reduce to a common standard manv 
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things that seem at first sight to be only remotely related. 
Take the case of the egg and the sperm. Each of these contains 
in itself the latent possibilities of a mature organism—but there is 
no evidence to shew that there is in them any microcosmic image of 
the adult ; rather we are confronted with a substance out of which 
the mature form is gradually evolved in the sense of being built up 
out of the raw materials present in the germ. And this building up, 
that we call ontogenesis, is very clearly the result of a stimulus 
acting on a material characterised by considerable complexity of 
chemical composition. It is most significant that the more recent 
researches on fertilisation have clearly proved, not only that 
ontogeny is initiated by a stimulus, but that this stimulus need not 
necessarily be given by the act of fertilisation. Like the optic 
nerve, the structure of the egg is such that it responds 
to all agencies that can set its chemical machinery agog, 
in one way—by segmentation and development. As long as it 
appeared that fertilisation was a sine qua non in the process, there 
was an element of mystery that seemed to baffle enquiry and elude 
comparison with simpler processes, but now that the various 
methods by which parthenogenetic development can be induced, 
whether by the action of certain salts, as in Loeb’s experiments on 
echinoderm eggs, or by the disturbances produced on raising 
the temperature, as in Marsilea, or in any of the other ways known 
to us, it has become clearly apparent that anything capable of 
setting the machinery in motion may suffice to bring about 
development. The stimuli that continue to produce the further 
changes are equally clearly related to the internal decompositions 
going on in the mass of the young embryo, and again the influence 
of the mechanism becomes prominent in considering the effect of 
early artificial vivisection of its body. When the blastomeres are 
separated at their first formation, development in each is modified, 
