Page:Popular Science Monthly Volume 85.djvu/29

Rh that the same sort of response follows when a frog's egg is pricked by a needle, thus showing that in this case the egg does not distinguish between the prick of the needle and that of the spermatozoon. The spermatozoon is usually a locomotor cell and it responds differently to certain stimuli, just as many bacteria and protozoa do; spermatozoa are strongly stimulated by weak alkalies and alcohol, they gather in certain chemical substances and not in others, they collect in great numbers around fertilizable egg cells, etc.

The movements of fertilized egg cells, cleavage cells, and early embryonic cells are usually limited to flowing movements within the

 a, b, c. Spirilla  a, condition immediately after adding crystals; b and c, later stages in the reaction.

x y z, repulsion of Spirilla by distilled water. The upper drop consists of sea water containing Spirilla, the lower drop of distilled water. At x these have just been united by a narrow neck; at y and z, the bacteria have retreated before the distilled water. (From Jennings, after Massart.)

individual cells. These movements, which are of a complicated nature, are of the greatest significance in the differentiation of the egg into the embryo; they are caused chiefly by internal stimuli and by non-localized external ones. Modifications of the external stimuli often lead to