gastrulation in all the animals. It has been done, however; and with
this introduction the reader will be able to follow the proof. The
conclusion is important. If all animals pass through the curious
gastrula stage, it must be because they all had a common ancestor of
that nature. To this conjectural ancestor (it lived before the period
of fossilisation begins) Haeckel gives the name of the Gastraea, and
in the second volume we shall see a number of living animals of this
type ("gastraeads").

The line of argument is the same in the next chapter. After laborious
and careful research (though this stage is not generally admitted in
the same sense as the previous one), a fourth common stage was
discovered, and given the name of the Coelomula. The blastula had one
layer of cells, the blastoderm (derma = skin): the gastrula two
layers, the ectoderm ("outer skin") and entoderm ("inner skin"). Now a
third layer (mesoderm = middle skin) is formed, by the growth inwards
of two pouches or folds of the skin. The pouches blend together, and
form a single cavity (the body cavity, or coelom), and its two walls
are two fresh "germinal layers." Again, the identity of the process
has to be proved in all the higher classes of animals, and when this
is done we have another ancestral stage, the Coelomaea.

The remaining task is to build up the complex frame of the higher
animals--always showing the identity of the process (on which the
evolutionary argument depends) in enormously different conditions of
embryonic life--out of the four "germinal layers." Chapter 1.9
prepares us for the work by giving us a very clear account of the
essential structure of the back-boned (vertebrate) animal, and the
probable common ancestor of all the vertebrates (a small fish of the
lancelet type). Chapters 1.11 to 1.14 then carry out the construction