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.

These discoveries were confirmed and extended for the chick[188] by the
embryologist Huschke, a pupil of Oken. Like Rathke, he found only three
indubitable gill-slits, but he noticed that the body-wall in front of
the first gill-slit was really composed of two arches, which were on the
whole similar to the gill-arches. The hinder of these two seemed to him
to be a horn of the hyoid, the front one, which was bent at an angle, to
be the rudiment of the upper and lower jaws (p. 401). Between these two
arches he found an opening, just as between two gill-arches a gill-slit.
This opening led into the mouth-cavity, and according to Huschke it
became the external ear-passage. He discovered also three pairs of
aortic arches in close relation with the gill-arches, so close indeed,
that he did not hesitate to call them gill-arteries, and to recognise
their resemblance with the aortic arches of fish. He traced, in part at
least, the metamorphosis which these aortic arches undergo. This part of
his discovery he developed in fuller detail in a pap