In the beginning...animal fertilization and sea urchin development.

What I most wished to discover [in my study] was the role that spermatozoids play in fertilization. In order to determine this, I put a droplet of red liquid, and at a small distance, a similar droplet of white liquid in a little trough on the viewing slide of the microscope; then, after covering all of this with a thin strip of glass, I added a drop of sea water. I was then able to watch the spermatozoids advance progressively towards the eggs. Some of [the eggs] were soon encircled by a compact mass of moving corpuscles; others, farther away, only found themselves in contact with a very small number [of sperm]; in both cases, I saw the signs of fertilization. The first apparent effect of this union is the almost immediate appearance of a perfectly transparent envelope that encircles the yolk at a certain distance, which is manifested by the appearance of a circular line. I saw this envelope manifest when in contact with a very small number of spermatozoids (three or four, sometimes even one only).