So much for the embryonic process in itself. The application to evolution has been a long and laborious task. Briefly, it was necessary to show that _all_ the multicellular animals passed through these three stages, so that our biogenetic law would enable us to recognise them as reminiscences of ancestral forms. This is the work of Chapters VIII and IX. The difficulty can be realised in this way: As we reach the higher animals the ovum has to take up a large quantity of yelk, on which it may feed in developing. Think of the bird’s “egg.” The effect of this was to flatten the germ (the _morula_ and _blastula_) from the first, and so give, at first sight, a totally different complexion to what it has in the lowest animals. When we pass the reptile and bird stage, the large yelk almost disappears (the germ now being supplied with blood by the mother), but the germ has been permanently altered in shape, and there are now a number of new embryonic processes (membranes, blood-vessel connections, etc.). Thus it was no light task to trace the identity of this process of _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 _Gastræa,_ and in the second volume we shall see a number of living animals of this type (“gastræads”).
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 _Cœlomula._ 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 cœlom), 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 _Cœlomæa._