Page:Popular Science Monthly Volume 76.djvu/321

Rh in the amount or quality of heat, light, food, gravity, soil, manure and atmosphere may be easily verified.

Among the low metazoa the influence of temperature, light and food is not so striking as among plants, still the changes may be called remarkable. For instance, the body-length of Echinus larvæ may be made, by raising the temperature, to increase about 25 per cent, of the average growth attained under a low temperature, while the arm-lengths in certain forms are at the same time increased from 200 to 300 per cent. If the number of larvæ of Echinus and Strongylocentrotus, which are growing together, be increased from under 1,500 per liter to over 3,000 per liter the mean length of the anal arms and the oral anus may be made to diminish from the figures 121.2 and 118.4 to 56.6 and 68.5, respectively. No modifications like this have been induced among the higher vertebrates.

In the embryology of placental mammals the dorsal or ventral surface may be facing either up or down, right or left; all forces work out their destiny in disregard to the force of gravity. This force of gravity so important in moulding plants has been proved to be also very influential in the development of hydroids. Pieces of Antennularia antennia produce new stems that grow upward, and stolons that turn downward. Even if the piece is inverted the root arises from the lower end and the stem from the upper. Driesch observed in a species of Sertularia that whenever he altered the position of the piece the new growth changed its position so that the new part turned away from the center of the earth.

The most remarkable of all experiments on low metazoa are the regeneration experiments. Since the exposed cells are subjected to a very considerable alteration of their normal environment, regeneration experiments may be considered as coming under the head of modification experiments. The power of regeneration possessed by worms and hydroids is so well known that only a passing reference is needed here. It is significant that among worms the tail will regenerate more than the head and that the reproductive organs, if removed, never regenerate at all, and the worm remains "incapable of reproducing itself." This last fact is interesting as supporting the view that germ-cells are never reproduced from somatic cells of any kind, much less from