Further comparative work by Rathke is found in the two volumes of _Abhandlungen_ and in a book, _Zur Morphologie, Reisebemerkungen aus Taurien_ (1837), which contains embryological studies of many different types, including a study of the uniform plan of arthropod limbs. Later on Rathke devoted himself more to vertebrate embryology, producing among other works his classical papers on the development of the adder (1839), of the tortoise (1848), and of the crocodile (1866). He laid the foundations of all subsequent knowledge of the development of the blood-vascular system in a series of papers of various dates from 1838 to 1856. The diagrams in his paper on the aortic arches of reptiles (1856) were for long copied in every text-book. Rathke was a foremost worker in another important line of embryological work, the study of the development of the skeleton and particularly of the skull. We shall discuss the history of the embryological study of the skull in some detail below; meantime, we note the two other important lines of research which characterise this period. One is the intensive study of the development of the human embryo, a study pursued by, among others, Pockels, Seiler, Breschet, Velpeau, Bischoff, Weber, Mueller, and Wharton Jones.[194] The other important line--the early development of the Mammalia--was worked chiefly by Valentin,[195] Coste,[196] and, above all, by Bischoff, whose series of papers[197] was justly recognised as classical. What interests us chiefly in the work of this embryological period is, of course, the relation of embryology to comparative anatomy and to pure morphology. The embryologists were not slow to see that their work threw much light upon questions of homology, and upon the problem of the unity of plan. Von Baer, we have seen, recognised this clearly in 1828; Rathke, in one of his most brilliant papers, the _Anatomisch-philosophische Untersuchungen ueber den Kiemenapparat und das Zungenbein_ (Riga and Dorpat, 1832), used the facts of development with great effect to show the homology of the gill-arches and hyoid throughout the vertebrate series; Johannes Mueller made great use of embryology in his classical _Vergleichende Anatomie der Myxinoiden_ (i. Theil, 1836), and, according to his pupil Reichert, firmly held the opinion that embryology was the final court of appeal in disputed points of comparative anatomy;[198] Reichert himself in a book of 1838 (_Vergleichende Entwickelungsgeschichte des Kopfes der nackten Amphibien_) discussed the two different methods of arriving at the "Type"--the anatomical method of comparing adults, and the embryological method of comparing embryogenies. Of the embryological method, he says, "Its aim is to distinguish during the formation of the organism the originally given, the essence of the type, and to classify and interpret what is added or altered in the further course of development. Embryologists watch the gradual building up of the organism from its