EMBRYOLOGY, that branch of biological science' which is concerned with the develop ment of the organism from the egg. The term is applied to the development of plants as well as animal organisms, but in the present article qnly the latter will be considered. Though every species of metazoan or multicellular animal produces eggs, not every individual arises directly from the egg. Indeed, in some groups asexual reproduction is commoner than sexual. It may occur by fission, or division of the organism into two or several individuals, as in certain flat-worms and annelids, or by gemination, where new individuals bud or sprout out from the older ones, and either separate completely, or remain attached, form ing colonies as in hydroids and bryozoans. However, strictly speaking, embryology applies only to the development of the organism from the zygote or fertilized egg-cell, or in some cases from eggs which develop by partheno fertilization by a male gamete.
Histoncal.— Before the invention of the miscroscope observations on development were of the most superficial sort and the genesis of the organism from the egg was chiefly a prob lem for the philosopher. The relation of the embryo to the two parents was not in any sense comprehended and as late as the middle of the 17th century spontaneous generation was be lieved to occur in some animals, even by so great a physiologist as William Harvey. Dur ing the 17th and 38th centuries the theory of tevolution,D later known as preformation, of which Bonnet, Leibnitz and Haller were among the greatest exponents, was the dominant view. Evolution in this sense denotes mere unfolding, like the flower from the bud, and has no rela tion to evolution in the sense of a theory of descent with modification. In brief, preforma tion is the doctrine that all the structures of the adult body are present in miniature in the germ and that development consists merely in their unfolding and growth. According to this theory nothing arises anew; as a corollary, known as the c'emboitemene or box-within-box theory, the germ must contain in diminishing series the germs of all succeeding generations. Naturally, most of the preformationists believed the germ to be contained in the egg, but after the discovery of the spermatozoa by Hamm in 1677, a new school arose known as the sper mists or animalculists, who adopted the view that these minute motile bodies, so obviously living, contained the germs, the egg serving merely as a nutrient medium in which the minute but fully formed offspring of male origin was en abled to grow. Some of the spermists even published figures showing a miniature human body, the homunculus, enclosed in the sperma tozoon.
An important advance was made in 1759 by C. F. Wolff, who demonstrated, from observations on the developing hen's egg, that bodily parts are not performed but actually arise anew in an orderly sequence, a theory which had been advocated though not proved by Harvey a century earlier and even vaguely stated by Aristotle. This conception, which is termed epigenesis, shortly supplanted the purely speculative preformation theory, but what regu lated this epigenetic differentiation remained a problem and still remains the great problem of embryology, notwithstanding a vast amount of observation and experimental research. During the 19th century great progress was made in morphological or descriptive embryology and if space permitted many important discoveries might be enumerated. The greatest of the early investigators in this field is generally 2.41 mitted to be Karl Ernst von Baer (1792-1876), sometimes called °the father of embryology," who, working mainly on the chick, was the first to give an orderly account of the chief phe nomena of development, including cleavage of the egg, formation of germ-layers and the differentiation of organs. Von Baer also laid the foundations of comparative embryology.
The cell theory, formulated by Schleiden and Schwann in 1838, which has so completely revolutionized biological thought, led only gradually to the recognition of the unicellular character of the gametes, egg and sperrna tozokin, and despite the much earlier germ theory of the spermists it was not until nearly the middle of the 19th century that the sperma tozoa were generally recognized as the agents of fertilization; indeed by many naturalists they were regarded as parasitic isms, accidentally present in the fertilizing fluid.
In 1843 Martin Barry witnessed the penetration of the rabbit's egg by the spermatozoon, but strange to say the unicellular character of the two gametes, a fact of fundamental importance, was not clearly demonstrated until after 1860. As a consequence of the rapid development of comparative embryology during the middle and latter part of the 19th century, together with the newly awakened interest in organic evolu tion, came the recognition of embryology as one of the greatest sources of evidence of phylogenetic relationship, and it is not surpris mg that a generalization known as the are capitulation theory," namely, that the individual in its development repeats in brief its racial history, should have been developed. Though this theory has frequently been forced farther than the facts warrant, it is unquestionably true that embryology has yielded highly important data as to the relationships of classes and smaller groups within the same phylum, thus confirming in many instances evolutionary evi dence from comparative anatomy and palaeon tology. The latter part of the 19th century and the earlier years of the 20th witnessed the de velopment of a school of experimental em bryology, concerned with the physiology and the philosophy of development, with the old problem of what makes the egg develop and what factors regulate the progressive differentia tion of the embryo. In this field of morpho genesis some of the leaders have been Roux, Herbst and Driesch in Europe, and Loeb, Mor gan and Lillie in America. Experimental studies have shown that while organs are not preformed in the egg, still in many cases the egg substance is differentiated into formative zones at, or even before, fertilization, that it exhibits in greater or less degree cgerminal prelocaliza don* of material for future organs, but not the organs themselves. This predeterminism in the egg has been termed In eggs of some animals this is so definite that removal of a portion of the egg will result in the building up of an incomplete embryo, while in other eases a fragment of an egg, or each of the first four or eight cells of the segment ing egg if artificially separated, will Five rise to an entire dwarf embryo; hence it is not pos sible to make categorical statements regarding promorphology in general. It is, however, a very different conception from the old pre formation theory and does not imply a negation of epigenesis. In some types the normal pro morphology, even though very early established, is readily alterable, in other cases it is not As to the general factors of differentiation, the majority of physiologists undoubtedly incline toward a purely mechanistic explanation, or interpretation in terms of chemical and physical laws, but vitalism also has able exponents, notably Hans Driesch. A discovery of peculiar interest in connection with promorphology is the phenomenon known as cpolyembryony,a or the development of two or more embryos from a single zygote. The most familiar example is the production of the so-called uidentical twins' in man and other spedes. These are always of the satne sex. Ordinary or dissimilar twins, of Course, arise from different ova and may or may not be of the same sex, as is the case in ordinary litters of young in mammals. In the nine-branded armadillo a litter contains four young, all of the same sex, and these have been conclusively shown to come from a single egg, and in a related species the polyembryomc litter contains eight or nine. In certain hymenopterous insects (chalcids) a single ovum produces a great number, in some cases hun dreds of individuals. It follows from the method by which sex is determined at fertiliza tion that all embryos thus arising from a single zygote must be of the same sex.