But the reproduction of ancestral forms in the course of the embryonic development is by no means always clear, or even always present. Many of the embryonic phases do not recall ancestral stages at all. They may have done so originally, but we must remember that the embryonic life itself has been subject to adaptive changes for millions of years. All this is clearly explained by Professor Haeckel. For the moment, I would impress on the reader the vital importance of fixing the distinction from the start. He must thoroughly familiarise himself with the meaning of five terms. _Biogeny_ is the development of life in general (both in the individual and the species), or the sciences describing it. _Ontogeny_ is the development (embryonic and post-embryonic) of the individual (_on_), or the science describing it. _Phylogeny_ is the development of the race or stem (_phulon_), or the science describing it. Roughly, _ontogeny_ may be taken to mean embryology, and _phylogeny_ what we generally call evolution. Further, the embryonic phenomena sometimes reproduce ancestral forms, and they are then called _palingenetic_ (from _palin_ = again): sometimes they do not recall ancestral forms, but are later modifications due to adaptation, and they are then called _cenogenetic_ (from _kenos_ = new or foreign). These terms are now widely used, but the reader of Haeckel must understand them thoroughly.
The first five chapters are an easy account of the history of embryology and evolution. The sixth and seventh give an equally clear account of the sexual elements and the process of conception. But some of the succeeding chapters must deal with embryonic processes so unfamiliar, and pursue them through so wide a range of animals in a brief space, that, in spite of the 200 illustrations, they will offer difficulty to many a reader. As our aim is to secure, not a superficial acquiescence in conclusions, but a fair comprehension of the truths of science, we have retained these chapters. However, I will give a brief and clear outline of the argument, so that the reader with little leisure may realise their value.
When the animal ovum (egg-cell) has been fertilised, it divides and subdivides until we have a cluster of cohering cells, externally not unlike a raspberry or mulberry. This is the _morula_ (= mulberry) stage. The cluster becomes hollow, or filled with fluid in the centre, all the cells rising to the surface. This is the _blastula_ (hollow ball) stage. One half of the cluster then bends or folds in upon the other, as one might do with a thin indiarubber ball, and we get a vase-shaped body with hollow interior (the first stomach, or “primitive gut”), an open mouth (the first or “primitive mouth”), and a wall composed of two layers of cells (two “germinal layers”). This is the _gastrula_ (stomach) stage, and the process of its formation is called _gastrulation_. A glance at the illustration (Fig. 29) will make this perfectly clear.