Each of these component parts bears witness to a succession of past periods. The yelk with its germ was first formed, escaping naked, or clothed only with its own excessively delicate membrane, from its ovisac into the oviduct. Through the course of this tube it now slowly descended, receiving successive investments as it proceeded. The albumen was deposited layer upon layer from the mucous membrane of the upper part of the oviduct; the first depositions condensing into the _chalaza_. By and by it came down to a region of the oviduct where a tenacious secretion was poured out, which, investing the albumen, soon hardened into a substance resembling thin parchment, and formed the _membrana putaminis_; two successive layers of this were deposited, between which a bubble of gas, chiefly composed of oxygen generated in the interval, was inclosed. Then it descended still farther, to a part where the lining membrane of the duct was endowed with the power of secreting calcareous matter, which, as above stated, was deposited in a thin layer of polygonal atoms. And now, having received all its components, and having arrived at the orifice of the duct, the egg was laid. Here, then, there is abundant evidence of successive processes, which must have preceded the existence of this complete and perfect egg. But there is yet one more evidence which I have reserved to the last, because it is peculiarly distinct and palpable, even to the senses. The _chalaza_, we see, is twisted at each pole of the yelk-globe, until it resembles a piece of twine: what is the meaning of this? It was, as I observed, deposited as a loosely enveloping membrane in the upper part of the oviduct; the yelk-globe, however, was progressively descending; and, as it descended, _it continually revolved upon its axis_; by means of which rotation the investing membrane was gathered at each pole into a spirally twisted cord, stretching from the yelk to the ends of the _membrana putaminis_. Thus it presents us with an unmistakeable record of what took place in the earlier periods of the descent. We saw distinct traces of the past in the structure of a feather. But the feathers have already begun to develop before the young bird leaves the egg. And the structure of the egg carries us back to the oviduct of the parent-fowl. At what stage of existence, then, could a bird, by possibility, have been created, which did not present distinct records of prochronic development? If we come to the MAMMALIA, the impossibility of finding such a stage becomes only more and more obvious. For it is a law in physiology, that the higher the grade of organization assigned to any being, the more it is assisted in infancy by the parent.