ull. 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
foundations, and distinguish the fundament, the primordial form, the
type, from the individual developments; they reach thus, following
Nature in a certain measure, the essential structure of the organism,
and demonstrate the laws that manifest themselves during embryogeny" (p.
vi.). The embryologists, influenced in this greatly by von Baer,
gradually felt their way to