The facts to which I would first direct the reader's attention, though ignored by many of the professed instructors of the public mind, are easy of demonstration and are universally agreed to by men of science; while their significance is so great, that whoso has duly pondered over them will, I think, find little to startle him in the other revelations of Biology. I refer to those facts which have been made known by the study of Development. It is a truth of very wide, if not of universal, application, that every living creature commences its existence under a form different from, and simpler than, that which it eventually attains. The oak is a more complex thing than the little rudimentary plant contained in the acorn; the caterpillar is more complex than the egg; the butterfly than the caterpillar; and each of these beings, in passing from its rudimentary to its perfect condition, runs through a series of changes, the sum of which is called its Development. In the higher animals these changes are extremely complicated; but, within the last half-century, the labours of such men as Von Baer, Rathke, Reichert, Bischof, and Remak have almost completely unravelled them, so that the successive stages of development which are exhibited by a Dog, for example, are now as well known to the embryologist as are the steps of the metamorphosis of the silkworm moth to the school-boy. It will be useful to consider with attention the nature and the order of the stages of canine development, as an example of the process in the higher animals generally. The Dog, like all animals, save the very lowest (and further inquiries may not improbably remove the apparent exception), commences its existence as an egg: as a body which is, in every sense, as much an egg as that of a hen, but is devoid of that accumulation of nutritive matter which confers upon the bird's egg its exceptional size and domestic utility; and wants the shell, which would not only be useless to an animal incubated within the body of its parent, but would cut it off from access to the source of that nutriment which the young creature requires, but which the minute egg of the mammal does not contain within itself. The Dog's egg is, in fact, a little spheroidal bag (Fig. 12), formed of a delicate transparent membrane called the _vitelline membrane_, and about 1/130 to 1/120th an inch in diameter. It contains a mass of viscid nutritive matter--the "_yelk_"--within which is inclosed a second much more delicate spheroidal bag, called the "_germinal vesicle_" (_a_). In this, lastly, lies a more solid rounded body, termed the "_germinal spot_" (_b_). [Illustration: FIG. 12.--A. Egg of the Dog, with the vitelline membrane burst, so as to give exit to the yelk, the germinal vesicle (_a_), and its included spot (_b_). B. C. D. E. F. Successive changes of the yelk indicated in the text. After Bischoff.]