“The reason why _Sacculina_ causes the assumption of the adult female state in _Inachus_ is found in the facts: (1) that the roots of _Sacculina_ elaborate a yolk-substance from the blood of _Inachus_ of a similar nature to that which is elaborated in the ovaries of an adult _Inachus_; (2) that in order to elaborate this yolk-substance the roots take up from the blood of _Inachus_ the female sexual formation substance, which is the necessary material for forming the yolk; (3) that the female sexual formative substance being absorbed by the _Sacculina_ roots is regenerated in excess; (4) that the presence of the female formative substance continually circulating in large quantities in the body-fluids of the infected crabs causes the production of adult female secondary sexual characters, and, when the parasite dies, of yolk-containing eggs.”
In brief, the evidence consists in showing that in the parasite a yolk-substance appears, which Smith says comes from the blood of the crab that produces it under the influence of the parasite. Incidentally, as it were, this is said to be the same yolk-substance (but no sufficient evidence that it is the same is given) that the egg stores up inside itself, and it is _assumed_ that it is a formative substance that causes the cell that gets it (or contains it or secretes it--details are wanting) to become an egg-cell. It is the excess of this substance produced by the male crab, while still a male, under the influence of the parasite, that affects the abdomen and its appendages in such a way that they assume the female condition. There are too many assumptions in the argument, some of which are scarcely of a kind that our knowledge of development, incomplete as it is, can allow us to accept without more direct evidence in their support, to make this view very plausible. Until better evidence is forthcoming, I fail to be convinced by Smith’s interpretation of his facts.
Into Smith’s and Robson’s interesting observations on the blood of crabs, described in Smith’s later paper (part 7, 1911), it is not necessary to enter here, since the evidence taken as a whole offers little further in support of his view than had been already assumed. The argument on page 263 should not, however, pass unchallenged. Smith says: