Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/173

Rh E M B E M B 1G3 near the right bank of the Durance, 25 miles east of Gap. It has woollen and linen manufactures. Its principal build ings are the cathedral, said to have been founded in the time of Charlemagne, a handsome Gothic structure, surmounted with a lofty tower; the archiepiscopal palace ; the aacient college of the Jesuits, now converted into a prison ; and the ancient convent of the Capuchins. Embrun was an import ant military station in the time of the Romans. It was the seat of a bishop in the time of Constantine, and from the 9th century till the Revolution it ranked as an arch bishopric. It has been sacked successively by the Vandals, the Huns, the Lombards, the Saxons, and the Saracens; and in the reign of Louis XIV. it was bombarded and taken by the duke of Savoy. The population in 1872 was 3075. EMBRYOLOGY is a branch of biological inquiry com prising the history of the young of man and animals, and it may be also of plants. The term is derived from the Greek e/.i(3pvov, signifying a growing part or thing, and has been somewhat vaguely applied to the product of genera tion of any plant or animal which is in process of formation. Among the higher animals, and especially in the human species, the Latin word foetus has sometimes been employed in the same signification as embryo, but it is more generally held to denote a more advanced stage of formation, while the term embryo is applied to the earlier condition of the product of conception before it has assumed the character istic form and structure of the parent. In all animals, with the exception of the Protozoa, the new being, deriving its origin from a definite organized structure termed the ovum or egg, passes during the pro gress of its formation and growth from a simpler to a more complex form and organic structure by a series of con secutive changes which come under the general denomina tion of development. The consideration of these changes, which is mainly an anatomical subject, being partly mor phological as affecting the larger and more obvious organic form, and partly histological as belonging to the minute or textural structure, constitutes by far the greater part of tho science of embryology, but the latter word may also include the history of all other living phenomena mani fested by the young animal in the progress of its growth to maturity. The formative process through which the embryo passes is necessarily of very different degrees of complexity, ac cording to the more simple or complex organization of the adult animal to which it belongs. But it presents throughout the whole range of animals certain general features of similarity dependent on the fundamental re semblance of the organized elements from which all animals derive their origin. A minute mass of protoplasm constitutes not only the simplest, but also the invariable, form presented by the germinal part of the ovum or egg, and in all animals, ex cept the Protozoa, in which the nature of the germ is still doubtful, it takes at first the form of an organized cell, or it is a definite spherical and nucleated mass of protoplasm. It is therefore a germ-cell. In all ova the first stage of the formative process, follow ing upon fecundation of the germ, consists in the multipli cation of the egg or germ-cell by a process of the nature of fissiparous division, so that when this division has pro ceeded some length, it results in the production of a mass or congeries of organized cells descended from that which formed the primitive germ, and containing in combination the molecular elements of the materials contributed by the male and female parents to the formation of the fertilized germ. This is the mulberry stage, or morula, of Haeckel. In a more advanced stage among the higher animals, the cells of this mass assume more or less of a laminar arrange ment, constituting the blastoderm or germinal membrane of Pander and succeeding authors ; and in the first and lowest forms of this structure two layers are distinguished, corre sponding to the outer and inner cellular laminae of which the earliest form of the embryo consists in the higher, and the whole of the body in the lower, forms of animals. These layers are the ectoderm and endoderm of the embryo- logist and comparative anatomist (Huxley and Allman). In the lowest animals little if any further differentiation of the germinal structures ensues ; but in animals higher in the scale there arises a third or intermediate layer, the mesoderm, which takes an important part along with the other two layers in the formation of the animal organism. The cellular blastoderm, therefore, is already the embryo of the lowest animals; while in the higher that term could scarcely with propriety be applied to the product of development in the egg until some of the characteristic lineaments, however rudimentary, of the new animal are apparent. But in the whole of this process of embryonic develop ment, whether it be of the simplest or of the most complex kind, it is to be observed that it is solely by the multipli cation and differentiation of cells which have descended more or less directly from the original germ-cell that the organizing process ia effected. It follows from this that the processes of organic growth or embryonic development pre sent a textural or histological uniformity to a remarkable degree throughout the whole zoological series. There is also a very striking similarity in the morphological phenomena of development within large groups of animals. Our know ledge, indeed, of the mode of formation of the young in all the varied forms of animal organization is still too limited to admit of our affirming that a uniform and progressive morphological type pervades the whole animal kingdom; but already many ascertained facts point strongly to such a conclusion, and the more our knowledge of the process of development in individual animals (ontogeny] advances, the greater resemblance do we recognize in the formative processes; so that it becomes more and more probable that the morphological development of any of the higher animals includes, or as it were repeats within certain limits, the various steps of the process which belong to the inferior grades of the animal kingdom. Hence we are led to the further conclusion that there is an essential correspondence between the individual development or ontogeny of the higher animals and the progressive advance of the organi zation in the whole animal series. If, further, we adopt the Darwinian view of the evolution of auimal life and organization by descent of one species of animals from others preceding it, we shall see that the embryological history of any animal is at the same time the history of its relation to other animals and of its phylogenetic development or gradual derivation as a species from more simple progenitors in the lapse of time. It is obvious, therefore, that we must look to the future pro gress of embryology as well as of paleontology for a large portion of the facts upon which the confirmation of the modern theory of evolution will rest. From what has been said it will be apparent that it would be impossible, within the limits of one article, to trace even in the briefest possible manner the phenomena of embryological development in all different animals. But special descriptions, so far as required, will find their ap propriate places under the divisions of animals to which they respectively belong ; and as there are some considera tions relating to embryology which require to be stated besides the history of development, it has been deemed advisable to bring the more important facts of develop ment of the embryo into connection with those relating to reproduction in general under the heading GENERA TION, to which article, therefore the reader is referred.