It seems that von Baer
arrived at his types (from the study of adult structure) independently
of Cuvier, though the priority of publication rests with Cuvier.[174]

Now it is clear that the development of the individual, which is
essentially an _Ausbildung_, a differentiation, is directly comparable
with the grade-differentiation of forms within the type. And just as the
type rules all its varied modifications, so does the development of the
individual take place always within the bounds imposed by type. This is
von Baer's chief contribution to the theory of embryonic
relationships--the law that "the type of organisation determines the
manner of development" (p. xxii.). Development is not merely from the
general to the special--there are at least four distinct "general"
types, from which the special is developed. The type is fixed in the
very earliest stages of development--the embryo of a Vertebrate is from
the very beginning a Vertebrate (p. 220), and it shows at no time any
agreement in total organisation with any Invertebrate. The types are
independent of one another; differentiation and development follow a
different course in each of them. Not but what some analogies can be
found between the very earliest stages of embryos of different type.
Thus vertebrate and annulate embryos agree in certain points at the time
of the formation of the primitive streak. And in the earliest stage of
all, the egg-stage, there is probably agreement between all the types.
In eggs with yolk, whether vertebrate or annulate, there is always a
separation into an animal and a plastic layer. It seems, too, as if a
hollow sphere were a constant stage in the development of all animals
(pp. 224, 258). Apart from these analogies, development takes an
entirely independent course in each of the four main types, and no
embryo of one of the higher types repeats in its development the
peculiar organisation of any adult of the lower types.

If we consider now development within the type, which is the only
legitimate thing to do, we arrive at certain laws governing the relation
of embryos to one another. For instance, at a certain stage vertebrate
embryos are uncommonly alike. Von Baer had two in spirit which he was
unable to assign to their class among amniotes; they might have been
lizard, bird, or mammal, he could not say definitely which.[175] Generally
the farther back we go in the development of Vertebrates the more alike
we find the embryos.