We must not lose sight of the fact, in the midst of these various and diverging observations about the fertilization of the ova of one species by sperms of another species, that there is such a thing as "parthenogenesis," or virgin-birth. In some of the insects and lower forms of animals the egg-cell habitually and regularly develops and gives rise to a new individual without being fertilized at all. And in other cases by special treatment, such as rubbing with a brush, or in the case of marine animals by addition of certain salts to the water in which the eggs are floating–or, again, in the case of the eggs of the common frog by gently scratching them with a needle–the eggs which usually and regularly require to be penetrated by and fused with a spermatozoon or sperm-filament before they will develop, proceed to develop into complete new individuals without the action upon them of any spermatozoon. In such marine animals as the sea-urchins or sea-eggs it has been found that the eggs deposited in pure sea-water, though they would die and decompose if left there alone, can be made to develop and proceed on their growth by the addition to the sea-water of the sperm filaments of a star-fish (the feather star or comatula). The spermatozoa or sperm-filaments do not, however, in this case fuse with the egg-cells. They mechanically pierce the egg-coat, but contribute no substance to the embryo into which the egg develops. They have merely served, like the scratch of a needle on the frog's egg and the brushing of insects' eggs, to start the egg on its growth, to "stimulate" it and set changes going. It appears thus that the fertilizing sperm-filaments of organisms generally have two separate and very important influences upon the egg-cells with which they fuse. The one is to stimulate the egg and start the changes of embryonic growth; the other is to contribute some living material from the male parent to the new individual arising from the growth and shaping of the egg-cell. The first influence can be exercised without the second, as is seen in the case of the eggs of some sea-urchins stimulated to growth by the spermatozoa of some star-fishes. It happens that these marine animals are convenient for study and experiment because their eggs are small and transparent and that they and the spermatozoa are freely passed into the sea-water at the breeding season, in which the fertilization of the eggs takes place.