110 SECTIONAL ADDRESSES. 
development. That family will be one of daughters only. The existence 
and the control of lethal factors is one of the most significant discoveries 
of the underworld. 
It is with the results of one branch of this experimental study that 
I wish to deal. For several years experimental morphology has been 
actively pursued by zoologists in Hurope and America. For the most 
part the egg has been selected as the natural point of departure, and the 
construction of the embryo or the development of the egg and of its parts 
has yielded results of great interest, though the search for a principle 
of organisation has not yet suceeded. To the developing organism it 
would seem to be all one whether it builds with one egg, with two eggs, or 
with a piece of an egg.(1) If there be any preformed or self-determined 
* organisation,’ it may be shattered to bits without prejudicing the appear- 
ance ofanormalembryo. The nuclei of the segmented egg may be shaken 
about as a bag of marbles, yet there will still remain the capacity for 
normal differentiation. It is therefore not surprising that there is as yet 
no unanimity of interpretation. Some investigators seek the explanation 
of development in an innate ‘ organisation,’ thereby postponing by a 
process of infinite regress the attack on the problem. Others assume by 
an unconscious petitio principw the very problem they set out to solve. 
Others take refuge in a metaphysical solution, and lately the problem of 
‘organisation’ has been regarded as an ultimate category that stands 
beside those of matter and energy. (1) 
Experimental zoology is a young science, and it is unlikely that we have 
reached Ultima Thule. Rather than regarding our position as one with 
our backs to the wall, I would ask leave to consider the report of the advance 
under the leadership of Professor Child of Chicago that has entered new 
territory. Instead of attacking the problem of the development of the 
organism from the egg, Child has long been working at the ‘ regulation ’ 
of regeneration and organic development. From his analytical studies 
(2) and (3) he has arrived at certain conclusions that have far-reaching 
consequences. Though based on the behaviour of the lower Invertebrates 
and Vertebrates, these conclusions have already proved of wide application. 
I believe I am right in stating that no more fertilising biological idea has 
been disseminated in the last ten years than Child’s hypothesis of metabolic 
gradients. It has captured the imagination of the younger generation of 
zoologists and is exercising an increasing influence upon them. 
The Individual considered as a Reaction System. 
It is no easy task to express the principles of Child’s theory of the 
organic individual without reference to fundamental questions on which 
differences of opinion prevail, and about which our knowledge is incomplete. 
Perhaps the best way is to give a concrete instance taken from the fresh- 
water Planarians, those highly organised ‘ animated pellicles’ that divide 
by spontaneous fission. This process is initiated externally by a trans- 
verse constriction far down the parent body, but without any morphological 
distinction at this region. The tail-end after separation develops a new 
head, brain, eyes, and other organs. The head-end develops a new tail, 
and the process is eventually repeated. On turning up a stone in a stream 
running through limestone country one can find certain species of Planaria 
actively engaged in multiplication by this method. 
