i292 
GEOFFEEY SMITH. 
alter the composition of this medium, and thus lead to the 
-stimulation of the development of the female secondary 
sexual characters by means of the excess of fat-links and 
fat-link -f fat combinations. 
It is also not difficult to explain on this theory why it 
it happens that the infected male crabs, on recovery, may 
regenerate an ovary instead of a testis, because the fat-links 
and fat Avhich are present in the blood are the specific food- 
material of the ovary, and hence the indifferent germ-cells 
which remain at the end of infection are supplied with the 
specific female food-material and naturally grow into ova. 
In fact, by showing that the substances which stimulate the 
development of the secondary sexual characters are identical 
with the specific food-materials or food-carriers of the repro- 
ductive gland, we not only gain a rational explanation of the 
-effect of Sacculina on its hosts, but we can put our finger on 
the common formative substances which lie at the back of 
sexual differentiation, both primary and secondary. 
In the special form in which our hypothesis was presented 
we supposed that the result of the Sacculina^s or ovary^s 
activity was to load the blood with two principal substances, 
the free fat-links and the fat-link + fat combinations. 
Either or both of these substances may be concerned in 
-Stimulating the development of the secondary sexual charac- 
ters. There may even be a further series of complicated 
reactions which are initiated by the presence of the unsatisfied 
fat-links in large numbers. To determine whicli of these 
alternatives is correct must be the task of the future, and it 
is not unlikely that we may find a means of testing and 
extending the hypothesis. The criticism has been urged 
against statements of this hypothesis which I have previous!}^ 
put forward, that the mere presence of fat in the blood could 
not be the direct cause of the development of the secondary 
.sexual characters. It will be recognised from the fuller 
explanation of the mode of action of the Sacculina roots 
which has now been given, that the mere presence of fat in 
the blood is not claimed as the cause, but as an accompani- 
