The type-characters are first to appear, then the
class characters, then the characters distinguishing the lesser
classificatory groups. "From a more general type the special gradually
emerges" (p. 221). The chick is first a Vertebrate, then a
land-vertebrate, then a bird, then a land-bird, then a gallinaceous
bird, and finally _Gallus domesticus_. Development within the type is a
progress from the general to the special, a real evolution. The more
divergent two adults are, the farther back we must go in their
development to find an agreement between their embryos. We can sum up
the case in the following laws:--

"(1) _That the general characters of the big group to which the embryo
belongs appear in development earlier than the special characters._ In
agreement with this is the fact that the vesicular form is the most
general form of all; for what is common in a greater degree to all
animals than the opposition of an internal and an external surface?

"(2) _The less general structural relations are formed after the more
general, and so on until the most special appear._

"(3) _The embryo of any given form, instead of passing through the state
of other definite forms, on the contrary separates itself from them._

"(4) _Fundamentally the embryo of a higher animal form never resembles
the adult of another animal form, but only its embryo_" (p. 224).

These laws relating to development within the limits of type are
destructive of even a limited application of the theory of parallelism,
for not even within the limits of the type is there a real scale which
the higher forms must mount; each embryo develops for itself, and
diverges sooner or later from the embryos of other species, the
divergence coming earlier the greater the difference between the adult
forms. It is only because the lower less-differentiated adult forms
happen to be little divergent from the generalised or embryonic type,
that they show a certain similarity with the embryos of the higher more
differentiated members of the group. Such similarity, however, is due to
no necessary law governing the development of the higher animals; it is,
on the contrary, merely a consequence of the organisation of these lower
animals (p. 224).

Von Baer goes on to show what are the distinguishing embryological
characters of the types and classes, working out a dichotomous schema of
development, which each embryo must follow, branching off early or late
to its terminal p