It seems that in most cases the spermatozoa swim around at random and that their union with the eggs is assured only by their enormous number; only in a few cases in plants have there been discovered special stimuli of a chemical nature, which attract the spermia to the egg. But we cannot enter here more fully into the physiology of fertilisation, and shall only remark that its real significance is by no means clear.[6] [6] The older theories, attributing to fertilisation (or to “conjugation,” *i.e.* its equivalent in Protozoa), some sort of “renovation” or “rejuvenescence” of the race, have been almost completely given up. (See Calkins, *Arch. für Entwickelungsmechanik*, xv. 1902). R. Hertwig recently has advocated the view, that abnormal relations between the amounts of nuclear and of protoplasmatic material are rectified in some way by those processes. Teleologically, sexual reproduction has been considered as a means of variability (Weismann), but also as a means of preserving the type! THE FIRST DEVELOPMENT PROCESS OF ECHINUS Turning now definitively to the special kind of organism, chosen of our type, the common sea-urchin, we properly begin with a few words about the absolute size of its eggs and spermatozoa. All of you are familiar with the eggs of birds and possibly of frogs; these are abnormally large eggs, on account of the very high amount of reserve material they contain. The almost spherical egg of our Echinus only measures about a tenth of a millimetre in diameter; and the head of the spermatozoon has a volume which is only the four-hundred-thousandth part of the volume of the egg! The egg is about on the extreme limit of what can be seen without optical instruments; it is visible as a small white point. But the number of eggs produced by a single female is enormous and may amount to hundreds of thousands; this is one of the properties which render the eggs of Echinus so very suitable for experimental research; you can obtain them whenever and in any quantity you like; and, moreover, they happen to be very clear and transparent, even in later stages, and to bear all kinds of operations well. The spermia enters the egg, and it does so in the open water--another of the experimental advantages of our type. Only one spermia enters the egg in normal cases, and only its head goes in, the tail is left outside. The moment that the head has penetrated the protoplasm of the egg a thin membrane is formed by the latter. This membrane is very soft at first, becoming much stronger later on; it is very important for all experimental work, that by shaking the egg in the first minutes of its existence the membrane can easily be destroyed without any damage to the egg itself.