ABSTRACT
The first sign that a sea-urchin egg has been fertilized is a change in cortical structure which spreads over the egg surface from the point of attachment of the fertilizing spermatozoon (Rothschild & Swann, 1949). The velocity of propagation is not uniform, there being a reduction in rate when the change approaches the equator of the egg relative to the point of attachment of the spermatozoon, and a marked increase when the cortical change has nearly covered the egg surface. The conduction mechanism of the cortical change might be similar to that occurring in stimulated nerve or muscle. Alternatively, it might be due to the diffusion of a substance, directly or indirectly derived from the head of the spermatozoon, found the cortex or through the cytoplasm, a change in cortical structure being effected by the arrival at the cortex of a particular concentration of the diffusing substance. If this hypothetical substance diffuses round the cortex and not through the cytoplasm, we must postulate a sympathetic reaction by ‘affected’ parts of the cortex, or an entirely separate cytoplasmic reaction, also initiated by the spermatozoon, to account for the intracellular changes that precede mitosis. By ‘affected’ parts of the cortex, reference is made to that fraction of the cortex which, owing to the arrival of a particular concentration of the diffusing substance, has changed its structure. On the other hand, if diffusion takes place through the cytoplasm, the one process might account for the two phenomena, cortical and intracellular changes, if cortex and cytoplasm are affected at different rates by the diffusing molecules.
