STUDIES IN THE EXrEKlMENTAE ANALYSIS OF SEX. 289 
them occurs wliicli results in the flooding of the blood by the 
iminnue substances, which are therefore present at a future 
time, either to block the access of the parasite to the tissues, 
•or else to fix on to the parasite and disable it. It is the 
undoubted fact ot‘ this production in excess of a substance 
which is in some manner being linked on to the intruding 
parasite which confers on Ehrlich’s side-chain theory of 
immunity its ready acceptation in principle, however much 
authorities muy differ as to its application and detailed 
working out. But it is exactly in the root principle of 
Eiirlich’s side-chain theory, viz. in the principle of the 
regeneration in excess of a substance which is being linked 
on to a parasite, that we find an extraordinarily close parallel 
in the reaction of the crabs to Sacculiua, and we therefore 
may feel justified in casting onr theory of this reaction into 
terms of the side-chain theory. 
If we were to frame this theory in its simplest possible 
form, we might say that the reaction is brought about by the 
Sacculiua roots seizing on the fatty side-chains of the liver, 
which, in consequence, are regenerated in excess. A¥e know, 
however, that this simple statement does not cover two essen- 
tial facts : first, that the exchange takes place through the 
medium of the blood, and second, that fat is not present as 
such in the blood, but in some soluble form in combination 
with some other material. We innst, therefore, represent the 
Sacculina roots as in some way seizing on the fatty part of 
this combination in the blood, and thus setting free the other 
part to take up more fat from the liver and convey it again to 
the Sacculina roots. 
An illustration of the method by which such a reaction 
might be conceived to take place is given in the subjoined 
diagram. 
We may suppose that the proteid molecules of the blood 
(P;^ and Po) are provided with side-chains (l, l) which act as 
fat-links, having the power of seizing on and combining with 
fat-molecules. Po in the diagram is represented in the act of 
^seizing on such fat-molecules in the liver on the right-hand 
