[59] Loeb, J., and Bancroft, F. W., _Jour. Exper. Zoöl._, 1912, xii., 381. CHAPTER IV SPECIFICITY IN FERTILIZATION 1. We have become acquainted with two characteristics of living matter: the specificity due to the specific proteins characteristic for each genus and possibly species and the synthesis of living matter from the split products of their main constituents instead of from a supersaturated solution of their own substance, as is the case in crystals. We are about to discuss in this and the next chapter a third characteristic, namely, the phenomenon of fertilization. While this is not found in all organisms it is found in an overwhelming majority and especially the higher organisms, and of all the mysteries of animated nature that of fertilization and sex seems to be the most captivating, to judge from the space it occupies in folklore, theology, and “literature.” Bacteria, when furnished the proper nutritive medium, will synthetize the specific material of their own body, will grow and divide, and this process will be repeated indefinitely as long as the food lasts and the temperature and other outside conditions are normal. It is purely due to limitation of food that bacteria or certain species of them do not cover the whole planet. But, as every layman knows, the majority of organisms grow only to a certain size, then die, and the propagation takes place through sex cells or gametes: a female cell--the egg--containing a large bulk of protoplasm (the future embryo) and reserve material; and the male cell which in the case of the spermatozoön contains only nuclear material and no cytoplasmic material except that contained in the tail which in some and possibly many species does not enter the egg. The male element--the spermatozoön--enters the female gamete--the egg--and this starts the development. In the case of most animals the egg cannot develop unless the spermatozoön enters. The question arises: How does the spermatozoön activate the egg? And also how does it happen that the spermatozoön enters the egg? We will first consider the latter question. These problems can be answered best from experiments on forms in which the egg and the sperm are fertilized in sea water. Many marine animals, from fishes down to lower forms, shed their eggs and sperm into the sea water where the fertilization of the egg takes place, outside the body of the female.