ere is also a longitudinal axis of the body and a
bilateral symmetry in the arrangement of parts; the head and tail are
marked, and the right and left sides are distinct. But the prominent
tendency in this type is the development of the dorsal and ventral region;
here above and below prevail over right and left. It is the back and the
lower side that have the preponderance over any other part of the
structure in Articulates. The body is divided from end to end by a
succession of transverse constrictions, forming movable rings; but the
character of the animal, its striking features, are always above or below,
and especially developed on the back. Any collection of Insects or
Crustacea is an evidence of this; being always instinctively arranged in
such a manner as to show the predominant features, they uniformly exhibit
the back of the animal. The profile view of an Articulate has no
significance; whereas in a Mollusk, on the contrary, the profile view is
the most illustrative of the structural character. In the highest
division, the _Vertebrates_, so characteristically called by Baer the
_Doubly Symmetrical_ type, a solid column runs through the body with an
arch above and an arch below, thus forming a double internal cavity. In
this type, the head is the prominent feature; it is, as it were, the
loaded end of the longitudinal axis, so charged with vitality as to form
an intelligent brain, and rising in man to such predominance as to command
and control the whole organism. The structure is arranged above and below
this axis, the upper cavity containing all the sensitive organs, and the
lower cavity containing all those by which life is maintained.

While Cuvier and his followers traced these four distinct plans, as shown
in the adult animal, Baer opened to us a new field of investigation in the
embryology of the four types, showing that for each there was a special
mode of growth in the egg. Looking at them from this point of view, we
shall see that these four types, with their four modes of growth, seem to
fill out completely the plan or outline of the animal kingdom, and leave
no reason to expect any further development or any other plan of animal
life within these limits. The eggs of all animals are spheres, such as I
have described them; but in the Radiate the whole periphery is transformed
into the germ, so that it becomes, by the liquefying of the yolk, a hollow
sphere. In the Mollusks, the germ lies above the yolk,