THE CASE OF BONELLIA The marine worm, Bonellia viridis, displays a remarkable de gree of sex-dimorphism. The female has a body about the size of a plum. The male is a microscopic pigmy whose internal organs, save those concerned with reproduction, are degenerate and who lives as a parasite within the body of the female. The fertilised eggs hatch out as free-swimming larvae. If a larva, settles down upon the sea bottom, it becomes, with few excep tions, and after a short neuter period, a female; but if it settles upon the proboscis of a female it becomes a male. Baltzer took larvae at various periods after they had settled upon a female but before they had become completely male and forced them to lead an independent life, and as a result he obtained inter sexes, the degree of intersexuality varying with the length of time the larva had been upon the female's proboscis. It has been shown that the larvae absorb material from the proboscis and that this is responsible for the arrest of growth and the direction of the sexual differentiation. The arrested pigmy male passes from the proboscis of the female into her mouth and then emerges and ensconces himself in her reproductive duct.
An ingenious hypothesis has been advanced to cover the ob served facts concerning the case of Bonellia. Goldschmidt (1923) supposes that in all the individuals there is at first an excess of male-differentiating substance, but that the production of female-differentiating substance after a time overtakes this.
Further, he supposes that the secretion of the proboscis of the female of Bonellia has the effect of accelerating the processes of differentiation as opposed to the processes of growth, antedating, as it were, the period during development when sexual differenti ation occurs. When differentiation is rapid, the sex-organisation matures under the influence of the male-differentiating substance; when it is not accelerated, under that of the female-differentiating substance. The mode of sex-differentiation is determined by a varying physiological state in connection with varying environ ment and secretions from other individuals.
Baltzer (1925) agrees in principle with this physiological inter pretation of the case of Bonellia but holds that the cause of the intersexuality cannot be an acceleration of the rate of develop ment but is rather a retardation. He points out that in experi mental cultures of Bonellia there first appear normal females, then females with sperm, and lastly intersexes and males; and that the male organisation, compared with that of the female, is to be regarded as a lower grade of development, being character ised by the absence of various organs.