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.
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