harmonious relations existing between the animal and vegetable world.  I would only attempt to show that in the plan of Creation the maintenance of types has been secured through a variety of means, but under such limitations, that, within a narrow range of individual differences, all representatives of one kind of animals agree with one another, whether derived from eggs, or produced by natural division, or by budding; and that the constancy of these normal processes of reproduction, as well as the uniformity of their results, precludes the idea that the specific differences among animals have been produced by the very means that secure their permanence of type.  The statement itself implies a contradiction, for it tells us that the same influences prevent and produce change in the condition of the Animal Kingdom.  Facts are all against it; there is not a fact known to science by which any single being, in the natural process of reproduction and multiplication, has diverged from the course natural to its kind, or in which a single kind has been transformed into any other.  But this once established, and setting aside the idea that Embryology is to explain to us the origin as well as the maintenance of life, it yet has most important lessons for us, and the field it covers is constantly enlarging as the study is pursued.  The first and most important result of the science of Embryology was one for which the scientific world was wholly unprepared.  Down to our own century, nothing could have been farther from the conception of anatomists and physiologists than the fact now generally admitted, that all animals, without exception, arise from eggs.  Though Linnaeus had already expressed this great truth in the sentence so often quoted,—­“Omne vivum ex ovo,”—­yet he was not himself aware of the significance of his own statement, for the existence of the Mammalian egg was not then dreamed of.  Since then the discoveries of von Baer and others have shown not only that the egg is common to all living beings without exception, from the lowest Radiate to the highest Vertebrate, but that its structure is at first identical in all, composed of the same primitive elements and undergoing exactly the same process of growth up to the time when it assumes the special character peculiar to its kind.  This is unquestionably one of the most comprehensive generalizations of modern times.

In common parlance, we understand by an egg something of the nature of a hen’s egg, a mass of yolk surrounded with white and inclosed in a shell.  But to the naturalist, the envelopes of the egg, which vary greatly in different animals, are mere accessories, while the true egg, or, as it is called, the ovarian egg, with which the life of every living being begins, is a minute sphere, uniform in appearance throughout the Animal Kingdom, though its intimate structure is hardly to be reached even with the highest powers of the microscope.  Some account of the earlier stages of growth in the egg may not be uninteresting to my readers.  I will take the egg of the Turtle as an illustration, since that has been the subject of my own especial study; but, as I do not intend to carry my remarks beyond the period during which the history of all vertebrate eggs is the same, they may be considered of more general application.