EMBRYONIC DEVELOPMENT 153

an earlier or later stage of its development. Thus there is good
reason to believe that fundamental as well as trivial processes of
morphogenesis depend upon genes for their possibility.

The two main evolutionary changes which made possible the
development of the highly organized multicellular organisms from
their unicellular ancestors were, therefore, the following:

(1) The tendency of the swarm of cells3 descended from their
common ancestor, the egg, to remain together in organic continuity
instead of separating as independent individuals. Thus it became
possible for the cells or larger aggregates of the developing embryo
to provide morphogenetic stimuli for each other, both by emission
of evocator substances and through the more intimate relationship
existing between the constituent members of a morphogenetic field.
Thus also are provided the mechanical supports and barriers which
play a part in determining the direction of movement of embryonic
parts and the shapes of completed organs. In this mechanical
action must also be included the stresses and strains which, as
Weiss (1934) has shown, probably play an important part in
guiding growing nerves to their destinations.

(2) A great elaboration of the gene complexes upon which
depend the inherited potentialities of cells and embryonic parts
to provide and respond to morphogenetic stimuli.

Within the present century a great amount of knowledge has
been obtained about morphogenetic processes, and about genes4
and their mode of action. Here it is only possible to touch in a
general way on both these subjects.

The general conclusion to be drawn from genetic researches
is that the genes provide the necessary conditions under which
the cytoplasm makes specific responses to morphogenetic stimuli.
There is no doubt that morphogenetic processes are cytoplasmic
processes. The movements of embryonic cells and parts are
brought about by the cytoplasm; histological differentiation is

3. I am not forgetting that local differentiations of structure and function are not
necessarily dependent upon multiplicity of cells in the organism. This Is demonstrated by
ampler Protozoa, while Lillic has shown^ that _;i small amount of organ formation
lace in a metazoan etftf (Chaetopterus) in. which cell division has been suppressed
1 means. Nevertheless, full morphogenesis in the higher organisms is always, in
dent upon a multiplicity of differentiated cells. To avoid any misunderstanding, I
that throughout this book I am speaking of cells as the elementary agents of

the more

c.in take

by aiiifici

fact, dcpei

may repe;

the origin m, partly because they are the smallest units known to be capable of prolonged

independent life, and partly to avoid continual repetition of the categories of living agents.

Many, if not all, cells arc nexus of living agents, icpresentcd physiologically by sub-fields in

rhe total cell field; on the other hand, cell aggregates which carry a common field are

agents in the sense we have defined, namely, acting causally as units or other agents,

including their own sub-agents.

4. Further consideration will be given to the genes in chapter vr.