i 7 8 
Journal of Agricultural Research 
Vol. XXVI, No. 4 
produce another type of monster—the partially double-headed kind— 
eight of which have been found in other stocks and which the experiments 
of Stockard {8, 9) indicate are determined by arrests in development at a 
moment preceding the arrests which determine the cyclopean condition. 
There is no apparent correlation between the distribution of otocephali 
and other types of monsters among the experiments. 
It would seem likely that genetic factors with an effect on general 
metabolism are very common, but at first thought it may seem as if 
such factors could play only a minor part in determining the type of de¬ 
velopment. But this is not a necessary conclusion. Study of the num¬ 
erous orderly, nicely adjusted anatomical changes brought about in 
otocephali by a factor of this kind suggests the possibility that the whole 
course of development may be controlled by such factors. The funda¬ 
mental properties of cells are much the same throughout the organic 
world, however diverse the structural patterns of the developed organisms. 
The Mendelian unit factors must be self-perpetuating entities within the 
cells. It may be possible that the action of Mendelian factors is merely 
to depress or accelerate metabolism. Their specificity lies in the un¬ 
locking of their activity at a particular moment when the cell has reached 
a particular condition. The pattern of development is then controlled 
by the particular succession of inhibitions and accelerations to which the 
various cells are subjected, as the result ultimately of the reaction of 
genetic factors with environmental ones. Just as the whole range of 
thought can be expressed by particular successions of dots and dashes in 
the Morse code, so the whole range of developmental patterns, from the 
one-celled alga to the sunflower, from the amoeba to man, may be the 
result of different sequences of inhibitions and accelerations. Develop¬ 
ment in a given cell lineage under this view is a chain reaction in which 
each gene reacts only in the presence of certain conditions, in part 
environmental relative to the cell lineage in question, and in part the 
result of the action of genes previously called into action. 
This of course is taking an extreme view. It is not necessary to sup¬ 
pose that the action of all genes on cell metabolism is equally general. 
In addition to factors with a general influence released by a specific set 
of conditions, we may have genes which catalyze only a particular 
reaction and which can come into action only when the reacting substances 
are present. 
On the discovery of units of heredity it was natural to compare them 
with the living units which had entered into previous speculations. The 
gemmules of Darwin, the biophores of Weismann, and all of their kind 
were essentially more or less sublimated representatives of the various 
morphological features of the adult organism. The developmental process 
was conceived of as a sorting out of these elements. This view, as Child 
has justly insisted (j), really explains nothing in development and is 
vitalistic in its implications. It was, nevertheless, adopted by many 
geneticists who began to look for determiners for the various parts of the 
body. Discoveries in genetics, however, while demonstrating ever more 
securely that the heredity of an organism is composed of constant units, 
have continually led away from this naive conception of their nature. 
In the case of otocephaly, specific unit factors have not been demon¬ 
strated, but their presence is probably indicated by the evidences of seg¬ 
regation in early generations and the sudden jumps in percentage of 
monsters at points in particular inbred lines. There is at any rate an 
