168 EMBRYOLOGY tary canal and air passages and of the cellular parts of the internal glands. These researches of Remak appear in some measure to reconcile the views of Von Baer with those of other embryologists, as to the constitution of the blastoderm and the relation of its several layers to the fundamental systems and organs of the embryo. Recent observation, though modifying them in some respects, has not led to any im portant invalidation of their general results; and we may therefore in the meantime regard them as forming the principal basis or starting-point of modern embryological inquiries, although much still remains to be ascertained as to the source of the mesoblast and its relation to the two primitive layers of the blastoderm. More especially important in a comparative embryological view is the for mation of the coelom or somato-visceral cavity, as connected with the gradual appearance in the animal series of the lymphatic and blood vascular cavities. But while the researches of Remak and others had thus in the commencement of the sixth decade of our century brought the history of the general phenomena of develop ment or embryogeny into a consistent and systematic form, especially as known in the higher vertebrates, much still remained to be done in the more minute investigation of the origin of the ovum and its germ, and the intimate nature of the process of fecundation, as well as in regard to the histological and morphological changes in which the organogenic processes consist. The progress of discovery in these departments has been greatly promoted by the very great improvements which have been introduced into the methods of investigation, the successful prosecution of which has had an equally favourable influence on the whole range of minute anatomy and histology, viz. (1), the hardening, clearing, and tinting processes of preparation; (2), the method of fine section of the parts to be observed; and (3), the permanent preservation of specimens in the moist or dried state. The first of these methods may be said to have had its origin in the introduction of the use of chromic acid as a hardening agent by Hannover of Copenhagen in 1840; and the works on practical histology since published bear ample testimony to the prodigious advance in refinement in the adaptation of this and other methods of hardening and dis tinctive coloration of the tissues, which have in recent times rendered the minuter investigation of the tissues compara tively elegant and exact, and indeed now almost exhaustive. The second method, or that of sections, as applied to embryological research, obviously suggested by the diagrams of Pander and Von Baer, seems first to have been practi cally applied by Allen Thomson in 1831, though without the assistance of finer modern appliances, in the ascertain ment of the earliest double condition of the aorta in the bird s embryo. It was soon carried to a much greater extent by Reichert, and later by Remak, and it is now universally pursued as a principal means of embryological investigation. To show the extent to which the successful combination of the above-mentioned metho.ls is now carried by the use of the most approved chemical reagents and the best sectional instruments, it may be stated that as many as several hundreds of perfectly clear sections may be made through the body of an embryo of only half an inch in length, and that similarly thin sections may be made in any de sired direction through the smallest as well as larger ova, and that, notwithstanding the extreme delicacy of some of the parts and the inequality of their density, every one of the sections may be made to present a distinct and true view both of the microscopic histological characters and of the larger morphological relations of the parts observed. Accordingly, during the time which has elapsed since the publication of Remak s work, the number of contributions to different parts of our subject, by the history of original observations made mainly by the way of sections, has been immense, and it goes on increasing to the present time. Among the more important of these, as influencing the general progress of embryological science, the following may be mentioned. First, in connection with the development of Fishes, the researches of Lereboullet "On the Pike and the Perch " (Annal. dcs Sciences Nat., 1862,) ; those of Joseph Oellacher " On the Trout " (Zeitsch. fur Wissensch. Zool., 1872); those of His also " Oa Osseous Fishes," appearing in 1875, and the important and elaborate researches of F. M. Balfour " On the Elasmobranch Fishes," in 1874 and following years (Journ. of Anat. and Physiol. and Quart. Journal of Microscopic, Anatomy); the prize memoir of Max Sclmlze On the Development of tJie River Lamprey, Haarlem, 1856 ; and the researches of Kowalewsky " On the Development of the Amphioxus" (in the Mem. of the St Petersburg Acad., torn xi., 1867), are deserving of notice. Second, in regard to Amphibia, after the memoirs of Rusconi, Reichert, Remak, and C. Vogt of earlier date, the most important recent contributions are those of V. Bambecke " On the develop ment of Pelobates fuscus" (Mem. de I Acad. de. Bclgique, vol xxxiv., 1868), and the very beautiful work of C. Gb tte On the De* velopment of the Toad, Bombinator igncus (Leipsic, 1874, folio). Third, in regard to Reptilia, not much has been done since Rathke s work On the Development of the Turtle was published in 1848. But there may be mentioned as valuable contributions to this department, the Account of the Development of the Crocodile, by Rathke himself in 1866, and the "Embryology of the Turtle," by H . J. Clark, in Agassiz s Contribidions to the Natural History of the. United States (vol. ii. 1857). Fourth, in the class of Birds, the most notable work which has appeared in recent times on the earlier phenomena of their develop ment is that of His, entitled Researches on the First Foundation of the Body in Vertebrate Animals (Leipsic, 1868), in which a careful revision of the subject is undertaken from original observations, and a clearer distinction established between the axial or central and the lateral parts of the blastoderm. Under this head come also the researches of Dursyupon the primitive trace of the chick (Lahr, 1866), F. M. Balfour s paper on the same subject (1873), and the important observations of Peremeschko on the formation of the layers of the blastoderm, especially the middle one ( Vienna Acad., 1868), Affanasieff on the first circulation in the fowl s embryo (in 1866), E. Klein on the development of blood vessels and blood corpuscles from the middle layer (1868), along with which may also be quoted the observations of Waldeyer, Oellacher, Strieker, Gotte, Balfour, and Kolliker, as tending to throw light on the origin of the blastoderm ic layers. Fifth, in regard to Mammalia, the most recent observations after those of Bischoff on the process of development in this class, are those of Hensen, in Zeitsch. fur Anat. und Entwickclungsgesch. , vol. i., 1875-6 ; the observations cf Kolliker in the new edition of his systematic work, 1876 ; those of Reichert, in his Account of the Development of the Guineapig, Berlin, 1862, and his Description of an Early Human Product, &c., Berlin, 1873 ; also in the papers of E. A. Schafer, from Physiol. Laborat. Univ. Coll. London, and Proceedings Roy. Soc., 1876. On the structure and morphology of the ovum may be quoted the article " Ovum " in the Cyclopaedia of Anatomy and Physiology, by Allen Thomson (1852-56); the contributions of Gegenbaur, 1861 and 1864, and of Cramer, 1868; and the very able "Memoire Couronne" of Edward van Beneden, Recherches sur la composition et la signification de I Ocuf, Brussels, 1870. With respect to the process of segmentation of the ovum and earliest steps in the formation of the germ, the most interesting re searches have recently been communicated by Auerbach, Butschli, Strasburger, Edw. van Beneden, Oscar Hertwig, and others, which are still in progress, and will be referred to in the article GENE RATION. Several systematic works or text-books on embryology have ap peared since it assumed the form and dimensions of a special branch of science. The first of these, by Valentin, referring to the development of man, mammals, and birds, was published in 1835. The next was that of Bischoff, published in 1842, as one of the volumes of the encyclopaedic system of anatomy nauied after Soemmering. The third work of this kind was that of Kolliker, in the form of lectures, published in 1861, and giving a very full account of the development of the ovum and embryo in man and the higher animals. Of this work a second edition is now in pro gress, the first part having appeared in 1876. To this excellent work, as the production of one who has contributed a very large amount of original observations on embryology and the whole range of minute anatomy, the reader may be referred for the fullest and most accurate systematic information on the subject. In comparative
embryology we have the interesting short treatise of Rathke.