With regard to the first point, we saw that among the Protozoa conjugation occurs for the most part between two individual cells which are alike in size and (to all appearance) alike in constitution; and this conjugation leads to reproduction. But when among the higher forms sex begins to show, the conjugating cells—sperm-cell and germ-cell—are generally unlike in size, and often in the higher animals extremely unlike—as in the human _spermatozoon_ and _ovum_, of which the latter is a thousand times the volume of the former;[10] and this has sometimes led, as remarked before, to an exaggerated view of the preponderant importance of one sex. But the curious fact seems to be that when the spermatozoon of the human or higher animal penetrates the ovum, there is a preliminary period before its nucleus actually combines with the nucleus of the ovum, during which the nucleus rapidly absorbs nourishment from the surrounding protoplasm, and _grows_—grows till it becomes of _exactly the same size_ as the nucleus of the ovum. The situation then is that there are two nuclei of the same size and both charged with chromatin of the same general character, in close proximity, and waiting to fuse with each other. The product of that fusion is a new being; and as far as can at present apparently be observed, the parts played by the two sexes in the process are quite equal. There may be _difference_ of function but there is no inequality. “Both male and female cells,” says Professor Rolleston,[11] “prepare themselves for conjugation long before it takes place, and neither of them can be said to be a more active agent in fertilization than the other. Not ‘fertilization’ but ‘fusion’ is the keyword of the process. The mystical conception, as old as Plato, of the male and female as representing respectively the two halves of a complete being, turns out to be no poetic metaphor. As regards the essential features of reproduction, it is a literal fact.”