III.
Let us now recapitulate the ancestral chain of man, as it is set forth in the accompanying diagram (p. [55]), which represents our present knowledge of our descent. For simplicity's sake the many side-issues or branches which lead to groups not in the main line of our descent have been left out, or have been indicated merely. Many of the stages are of course hypothetical, arrived at by the study of comparative anatomy and ontogeny; but an example for each of them has been taken from those living or fossil creatures which seem to be their nearest representatives.
1. The most remote ancestors of all living organisms were living beings of the simplest imaginable kind, organisms without organs, like the still existing Monera. Each consisted of a simple granule of protoplasm, a structureless mass of albuminous matter or plasson, like the recent Chromaceæ and Bacteriæ. The morphological value of these beings is not yet that of a cell, but that of a cytode, or cell without a nucleus. Cytoplasm and nucleus were still undifferentiated.
I assume that the first Monera owe their existence to spontaneous creation out of so-called anorganic combinations, consisting of carbon, hydrogen, oxygen, and nitrogen. An explanation of this hypothesis I have given in my 'Generelle Morphologie.'
The Monera probably arose early in the Laurentian period. The oldest are the Phytomonera, with vegetable metabolism. They possessed the power (characteristic of plants) of forming albumin by synthesis from carbon, water, and ammonia. From some of these plasma-forming Monera arose the plasmophagous Zoomonera with animal metabolism, living directly upon the produce of their plasmodomous or plasma-forming sisters. This is the first instance of the great principle of division of labour.
2. The second stage is that of the simple and single cell, a bit of protoplasm with a nucleus. Such unicellular organisms are still very common. The Amœbæ are their simplest representatives. The morphological value of such beings is the same as that of the egg of any animal. The naked egg cells of the sponges creep about in an amœboid fashion, scarcely distinguishable from Amœba. The same remark applies to the egg-cell of man himself in its early stages before it is enclosed in a membrane. The first unicellular organisms arose from Monera through differentiation of the inner nucleus from the outer protoplasm.
3. Repeated division of the unicellular organism produces the Synamœbium, or community of Amœbæ, provided the divisional products, or new generations of the original cell, do not scatter, but remain together. The existence of such a Cœnobium, a number of equal and only loosely-connected cells, as a separate stage in the ancestral history of animals, is made highly probable by the fact that the eggs of all animals undergo after fertilization such a process of repeated self-division, or 'cleavage,' until the single egg cell is transformed into a heap of cells closely packed together, not unlike a mulberry (morula)—hence morula stage in ontogeny.
4. The morula of most animals further changes into a Blastula, a hollow ball filled with fluid, the wall being formed by a single layer of cells, the blastoderm or germinal layer. This modification is brought about by the action of the cells—they conveying nourishing fluid into the interior of the whole cell colony and thereby being themselves forced towards the surface. The Blastula of most Invertebrata, and even that of Amphioxus, is possessed of fine ciliæ, or hair-like processes, the vibrating motion of which causes the whole organism to rotate and advance in the water. Living representatives of such Blastæads, namely, globular gelatinous colonies of cells enclosing a cavity, are Volvox and Magosphæra.
5. The Blastula of most animals assumes a new larval form called Gastrula, in which the essential characteristics are that a portion of the blastoderm by invagination converts the Blastula into a cup with double walls, enclosing a new cavity, the primitive gut. This invagination or bulging-in obliterates the original inner cavity of the Blastula. The outer layer of the Gastrula is the ectoderm, the inner the endoderm; both pass into each other at the blastoporus, or opening of the gut cavity. The Gastrula is a stage in the embryonic development of the various great groups of animals, and some such primitive form as ancestral to all Metazoa is thus indicated. This hypothetical Gastræa is still very essentially represented by the lower Cœlenterates—e.g., Olynthus, Hydra.
6. The sixth stage—that of the Platodes, or flat-worms—is very hypothetical. They are bilateral gastræads, with a flattened oblong body, furnished with ciliæ, with a primitive nervous system, simple sensory and reproductive organs, but still without appendages, body cavity, vent, and blood-vessels. The nearest living representatives of such creatures are the acœlous Turbellarians—e.g., Convoluta, a free-swimming, ciliated creature.