It was largely a German science, this new embryology, and its leaders were all personally acquainted. Pander, von Baer and Rathke were on friendly terms with one another; von Baer dedicated his master-work to Pander; Rathke dedicated the second volume of his _Abhandlungen_ to von Baer. Interest in the new science was, however, not confined to Germany. In Italy, Rusconi commenced in 1817 his pioneer researches on the development of the Amphibia with a _Descrizione anatomica degli organi della circolazione delle larve delle Salamandre aquatiche_ (Pavia), in which he traced the metamorphoses of the aortic arches. This was followed in 1822 by his _Amours des Salamandres aquatiques_ (Milan), and in 1826 by his memoir _Du developpement de la grenouille_ (Milan). In this last paper he described how the dark upper hemisphere of the frog's egg grows down over the lower white hemisphere and leaves free only the yolk plug; he observed the segmentation cavity and the archenteron, but thought that the former became the alimentary canal; he observed and interpreted rightly the formation of the medullary folds. The circular blastopore in the frog in later years often went by the name of the anus of Rusconi. In France Dutrochet[183] investigated the foetal membranes in various vertebrate classes; Prevost and Dumas studied the very earliest stages of development in birds, mammals and amphibia (_Ann. Sci. nat._, ii., iii., 1824, xii., 1827). A little later came Duges' studies of the osteology and myology of developing amphibia (1834),[184] and Coste's careful researches into the early developmental history of mammals.[185] [Illustration: FIG. 8.--Gill-slits of the Pig Embryo. (After Rathke.)] It was in 1825 that Heinrich Rathke (1793-1860), published his famous discovery of gill-slits in the embryo of a mammal,[186] a discovery which aroused considerable interest, and greatly stimulated embryological research. He describes how in a young embryo of a pig he saw four slits in the region of the neck, going right through into the oesophagus. They were separated by partitions which he called _Kiemenbogen_ (gill-arches), and immediately in front of the first gill-slit lay the developing lower jaw. He compared these gill-slits with those of a dogfish. We reproduce his drawing of the pig-embryo (_Isis_, Pl. IV., fig. 1). Later in the same year Rathke discovered gill-slits in the chick,[187] in this case finding only three. He described growing out from in front of the first slit a structure which he compared to the operculum or gill-cover of a fish.