The first 'podia] step in unraveling the mor phology of the arthropod classes was taken by Savigny (1810), u•ho showed that the mouth parts of insects, crustaceans, etc., were modified limbs, and this gave a clue to the segmental nature of the head of these animals, which is composed of a number of segments, different in each class, as shown by the later researches of embryologists. Savigny also taught that as a rule a segment bears Ina a single pair of ap• pent while Audimin showed that the form of the artlu•opodan body was due to the atrophy of certain segments, or parts of segments, and the correlated hypertrophy of others. Other im portant advances in classification and anatomy were made by the labors of H. Milne-Edwards, the successor of envier, who established the prin ciple of the division of physiological labor, sepa rated the Tunicata from the Mollusea, divided vertebrates into those with and without an al lantois, and confirmed De Blainville's separation of amphibians from reptiles.
During the middle of the nineteenth century the great German anatomist, Willer, made ex tensive investigations on the embryology and metamorphoses of the echinoderms. revolution ized the classification of the fishes, and placed physiology on a new and higher basis. Mean while Vaughan Thompson (1836), by his dis covery of the nauplins larva of the barnacles, hail shown that instead of being worms they were but greatly degraded, crustaceans. In 18-18 leuelsart broke up the Radiata of Cnvier, and separated the Ccelenterata from the eehino derms, regarding them as a distinct branch or type. Wiegman transferred the Rotifera from the Protozoa to Vermes; Siebold (1848) having established the group of Protozoa, showed their one-celled structure, and founded the type of Arthropoda while Rudolphi, Leuckart, and Sie bold pointed out the worm-like nature of the flat worms, now referred to a separate phylum.
The animal nature of sponges was not proved until after the middle of the nineteenth century, through the researches of Lieberkfihn, Carter, Clark, and finally Haeckel, who, however, placed them with the coelenterates, while their true po sition as a distinct phylum was assigned them by Hyatt. Through the investigations of Morse the Braehiopoda were removed from the Mollusea, as also were the Alolluseoidea, and regarded as more nearly allied to the Annelida. Louis Agas siz (q.v.), one of the foremost of his time (1307-73), greatly advanced our knowledge of the coelenterates. among his discoveries being that of the hydroid nature of Millepora. He worked on the embryology of the fishes and rep tiles. his great work on fossil fishes bringing order out of a chaotic mass of unclassified material. He also indicated the chief faunal divisions of North America. From the middle
of the nineteenth century there has been pub lished an uninterrupted series of monographic anatomical and embryological works on single animals in the line of those of ROaumur, Lyonnet, Strauss-Durekheim, and "Herold, among which should be indicated those of the numerous stu dents of Johannes 'Muller and Leuekart; the works of Agassiz and of Huxley and his pupils; and of Lacaze-Duthiers and his successors.
Among the most remarkable and fruitful ad vances in embryology is Kowalcvsky's discovery (1866) of the presence of the notochord and hol low nervous system in the embryo and larva of aseidians, which resulted in the removal of the Tunicata from the Molluseoida, and afterwards the %vomits, to the neighborhood of the verte brates; and to the establishment of the phylum of the Chuniota, as Nye] I as the breaking down of the distinction between vertebrates and inverte brates; these, with the later researches of Scm per, indicating the origin of verteln•ates from some primitive annelid worm. The fact that the embryos of all animals above the Protozoa through a gastrula stage led llnecla•I to suppose, that his hypothetical gastren was the ancestral form of all the ninny-celled animals.
The two founders of modern histology were lialliker and Leydig. The former, besides his early discovery of the mode of development of sperm cells, and the proof he afforded of the ex istence of animal cells destitute of a cell•wall, gave a great impulse to the general histology of vertebrates, and the latter to that of the inver tebrates, especially the sense-organs. Their pu pils and successors, by the invention of refined methods and the use of reagents, especially by the invention of the microtome, have pushed fruitful investigations in embryological and mor phological directions, greatly extending our knowledge of reproduction and cell-division, and laying the foundation for future special research in comparative physiology, especially of the brain, and in psychology, bearing especially on reflex, instinctive, and mental actions.
The studies of the earlier histologists and em bryologists led to the foundation of cytology, or cell-science, and to a realization of the complex nature of protoplasm; work in which Fleming, Strasburger, E. Van Beneden, the two Hertwigs, Roux, Boverl Wilson, and others took a leading part. On the results of their labors are based the theory elaborated by Weismann that the par ticles of the chromatin of the unclear substance (chromosomes) are the bearers of heredity. A new department of cytology is that of the me chanics of development (q.v.), which is in the line of mechanical or dynamic evolution, taught by Lamarek• H. Spencer, Ryder, Cope, Hyatt, Roux, and others.