234 GEOFFREY SMITH. 



seen, the yolk-containing ova may be formed in the gonad 

 of either sex. 



Meantime the continued production and circulation in the 

 blood of the infected Inachus, whether male or female, of 

 this yolk material, or rather of the substances from which the 

 yolk is built up, is accompanied by the production of the 

 secondary sexual characters proper to the adult female. 

 These yolk-forming substances, or substance, are therefore 

 identical with the " sexual formative substance," whose 

 existence we deduced in Part 2 of these studies. We may 

 summarise the above argument as follows: The Sacculina 

 roots require for their nourishment a substance in the blood 

 of the crab which they can work up into yolk material. 

 This substance is provided for them in the female sexual 

 formative substance, which is circulating in small quantities 

 in normal male crabs as well as, in greater quantities, in 

 female crabs. But the Sacculina roots must have the power, 

 not only of abstracting this material from the crab's blood, 

 but also of forcing the crab to go on forming this substance 

 in excess. This may seem to be a great assumption; but it 

 is exactly here that a very close parallel can be drawn 

 between the phenomenon we are dealing with and the 

 general processes of immunity to parasites and organic 

 poisons. Immunity has been interpreted, especially by 

 Ehrlich, to mean that when a poison acts upon an organism it 

 combines with and anchors certain organic molecules, which 

 are then regenerated in excess and poured out into the blood- 

 stream as antibody. If we suppose, therefore, that the 

 Sacculina roots anchor the molecules of the female sexual 

 formative substance, and this, from the fact of their forming 

 yolk material, they appear to do, it is in accordance with the 

 facts of immunity to suppose that the molecules of the sexual 

 formative substance, w^herever they are formed, will be 

 regenerated in excess. 



The continued operation of this process, namely, the pro- 

 duction of female sexual formative substance in the blood- 

 stream, and its abstraction by the Sacculina I'oots, would 



