STUDIES IN THE EXPERIMENTAL ANALYSIS OF SEX. 263 
characters could not be applied here, since at the time that 
the alterations in the secondary sexual characters take 
place no ovary is present to give rise to the required 
stimulus. It was suggested, therefore, that in some way the 
stimulus must reside in the roots of the Sacculina, and it 
was observed that these roots were in process of elaborating 
from the blood of the host a pigmented yolk material, 
similar in every respect to the yolk which is stored in the 
ovary of a normal female crab at maturity. This clue 
formed the basis of the idea that the Sacculina roots, by 
continually abstracting the yolk material from the blood of 
the host, stimulated its constant production in excess, and 
that the presence of this material circulating in the body- 
fluids was the stimulus for the development of the female 
characters. An analogy was therefore drawn between this 
reaction and the reaction of warm-blooded animals to 
bacterial infections, and the phenomenon of parasitic castra- 
tion was seen to approach the category of an immunity 
phenomenon. Mr. Robson and I have now attempted to 
follow more closely these internal changes in the fat-meta- 
bolism of the host, and a further step has been gained. We 
have shown that the blood of normal crabs does undergo 
periodic changes according to the sex of the animal, and 
that the blood of the female, at the time of the ripening of 
the eggs, is actually flooded by an excess of fatty material, 
which can be estimated quantitatively. Robson has found 
that in Inachus, where the parasitic affection is most 
complete, the blood of the infected individuals similarly is 
charged with fat material more constantly than is the case 
under normal conditions, while both in Carcihus and 
Inachus the infected individuals show a constant and 
excessive formation of fat in the liver. 
It may therefore fairly be claimed that, though many 
points in our interpretation of the phenomenon may be lack- 
ing, yet on the whole the attempt to reduce the observed 
changes to an alteration in the condition of fat-metabolism 
has met with success, and that whereas formerly the 
