er of 1828,[189] in
which he gave some excellent figures.

Shortly after Huschke's first paper, von Baer published his views and
observations on this subject in a short memoir in Meckel's _Archiv_.[190]
In this paper he confirmed Rathke's discovery, and described the slits
and arches in the dog and the chick. Both Rathke and he found gill-slits
in the human embryo about this time (p. 557). There were generally
present, he found, four gill-slits, and, as Rathke had suggested, the
first gill-arch became the lower jaw. Von Baer also confirmed Rathke's
discovery of the operculum, assigning it, however, to the second
gill-arch. He refused to accept Huschke's derivation of the auditory
meatus from the first gill-slit. Von Baer saw what had escaped Rathke
and Huschke, that there were, not three nor four, but as many as five
aortic arches.

In his view of the metamorphosis of the aortic arches in the chick the
first two pairs disappeared completely, the third pair gave rise to the
arteries of the head and the fore-limbs, the right side of the fourth
arch became the aorta, the left half of the fourth and the right half of
the fifth arch became the pulmonary arteries, while the left half of the
fifth arch disappeared. This schema, which for the last three arches was
the same as Huschke's, von Baer upheld for the chick even in the second
volume of his _Entwickelungsgeschichte_ (p. 116); he rectified it,
however, for mammals in the same volume (p. 212), deriving both
pulmonary arteries from the fifth arch, and the aorta from the fourth
left. He fully recognised the great analogy of the embryonic arrangement
of gill-arches and gill-arteries in Tetrapoda with their arrangement in
fish (i., pp. 53, 73).

Huschke, in a paper of 1832,[191] chiefly devoted to the development of
the eye, figured and described the developing upper and lower jaws, and
maintained against von Baer that the first slit turns into the auditory
meatus and the Eustachian tube.

These were the first papers of the embryological period. Before going on
to discuss the principles which guided embryological research during the
next ten or twenty years it is convenient to note what were the main
lines of work characterising the period.

The typical figure of the period is Rathke, who produced a great deal of
first-class embryological work. He was, even more than von Baer, a
comparative embryologist, and there were few groups of animals that he
did not study. His fi