ABSTRACT

DEVELOPMENT in sexually reproducing organisms is usually considered to begin at the time when the sperm unites with the egg. But actually these two types of cells are among the most complex formed in the animal body, and themselves undergo very important processes of development before they are ready to perform their characteristic functions. The undifferentiated cells which will eventually give rise to them are collectively known as gametocytes, and separately as oocytes if they will form ova, or spermatocytes if they will form sperm. The fully differentiated cells are known as gametes. The male type are called sperm, or spermatozoa, both terms being correct and with exactly the same meaning. The female type are referred to as eggs or ova, but these two words do not mean quite the same thing. The word ovum refers strictly to the gamete-cell; and this often makes up only a part of the body known as the egg, which may include a number of membranes, layers of jelly, shell, etc., which strictly speaking lie outside the ovum, and are no part of it. Thus in the hen's egg, the ovum is only that part conventionally known as the 'yolk'.