EMBRYONIC DEVELOPMENT 171

join up again with its end organ. This outgrowth is not a mere
passive growth. The end of the axon is not merely pushed
forward by the lengthening of the fibre. It advances by pseudopodia
thrust out from the advancing tip, and may have to reach its
destination by threading its way through obstacles, such as scar
tissue developed in its path.

(6) One of the characteristics of morphogenetic fields is that
if a portion of the carrier of the field is removed, the remainder
will re-organize itself into a similar field of smaller size.9 Out of
this will differentiate the proper organs, smaller but correctly
proportioned to the whole. Examples of this are the re-organization
of a portion of a planarian worm into a complete worm of properly
proportioned parts, and the development of a fragment of the optic
cup into a complete miniature cup. Very many other examples
could be quoted, including the familiar one of the development of
a whole embryo out of one of the first two blastomeres. The
experiments of Lashley described in chapter ir show that a portion
of the brain field of the rat will also re-organize itself into a similar
field of smaller size. His experiments showed that every part of
the cortex is involved in the learning process, but learning can
take place after removal of any part of it so long as not less than
about one fifth of it remains. Moreover, rats which had learnt
the maze with an intact brain remembered it after the removal of
a large part of the cortex. This indicates a field organization in
the cortex accompanying the process of learning, which involves
the whole cortex or such parts of it as are present. After removal
of a part, the remainder reorganizes itself into a functionally
similar field of smaller size. The fact that a certain minimum
amount of cortex is necessary for learning the maze, or for reten-
tion, of what has already been learnt, is paralleled by the fact that
structural reorganization of fragments of hydroids or planarian
worms does not take place if they are below a minimum size. This
cannot be due solely to lack of material, since these fragments have
enormously more material than the eggs of these animals, from
which complete organisms develop.

(7) Both in embryonic agents and in the completed organism
instincts appear and disappear by the process of maturation. The
disappearance with age of the sucking instinct of young mammals,
and the development of the sexual instinct later, can be compared

9, See Wciat), 1926, 1928, for many generalizations about morphogenetic fields.