Thus the four explanations do not satisfactorily explain, and the subject of the method of evolution is not yet decided. Neither teleology with the initiatory psychic energy working towards definite ends, nor Bergson’s vital impulse or original profound cosmic force, nor Lamarck’s appetency or use and disuse, nor Darwin’s natural selection, furnish an unobjectionable satisfactory explanation of organic evolution. Still the world’s thinkers and scientists have accepted organic evolution as a fact which may be proven by embryonic development, in conformity with Haeckel’s biogenetic axiom that ontogeny is only a short recapitulation of phylogeny.
The ovum or even the zygote (i. e., the impregnated ovum) is a single-celled organism and resembles the animals of the first or lowest type in the animal kingdom. The protozoa are nothing else than single-celled animals. Some of them have even a lower structure than the common cell. The Monera, e. g., has neither nucleus nor membrane. The manifestations of life are recognizable only by the possession of the faculties of the assimilation of food and of propagation by segmentation and division.
CUT I.
a, cell; b, morula; c, blastula; d, gastrula; ec, ectoblast; en, endoblast.
Like the protozoon, the ovum, immediately after its impregnation, begins to undergo a certain division, by a series of successive segmentations, into 2, then 4, 8, 16, 32, 64, etc., cells. By continuous cell segmentations a great mass, the morula or mulberry, is produced. The structure of the morula corresponds with that of the coelenterata, or the animals of the second type. To this type belong the animals with gemmiparous reproduction, or multiplication by means of buds. The divided animals remain together and form colonies, as e. g. in sponges or corals.
The next event in the formation of the embryo is the blastula. The solid spherical mass of cells becomes hollow like a rubber ball. In the subsequent stage the blastula becomes flat at one pole. By degrees a depression is formed at this point, which becomes deeper step by step, until the inner layer reaches the outer layer, representing half a sphere of two layers, like a collapsed rubber ball. In the farther growth the edges approach the middle line till they finally meet and fuse together. The oval body, called the gastrula, thus consists of two layers, the primitive germinal membranes, the ectoderm and entoderm. The gastrula resembles in its structure that of the worms, or the third type of the animal kingdom.
CUT II.
Diagram, showing the development of the organs.
ek, ektoderm; mp, medulary plate; ms, mesoderm just forming from the ed, entoderm; c, coelom or body cavity; nt, nerve-tube or spinal cord; ac, abdominal cavity; cd, chorda dorsalis; it, intestinal cavity.