The Principles of Biology, Volume 2 (of 2)

by Herbert Spencer

§ 288. The early establishment of this primary contrast of tissues answering to this primary contrast of conditions, is no less conspicuous in aggregates of the second order. The feebly-integrated units of a Sponge, with individualities so little merged in that of the whole they form that most of them still retain their separate activities, nevertheless show us, in the unlikeness that arises between the outermost layer and the contained mass, the effect of converse with unlike conditions. This outermost layer is composed of units somewhat flattened and united into a continuous membrane—a kind of rudimentary skin.

Secondary aggregates in which the lives of the units are more subordinate to the life of the whole, carry this distinction further. The leading physiological trait of every cœlenterate animal is the divisibility of its substance into endoderm and ectoderm—the part next the food and the part next the environment. Fig. [147] ([§ 201]), representing a portion of the body-wall of a Hydra seen in section, gives some idea of this fundamental differentiation. The creature consists of a simple sac, the cavity of which is in communication with the surrounding water; and hence the unlikeness between the outer and inner layers has not become great. The essential contrast is that between the differentiated parts of what was originally the same part—a uniform membrane composed of juxtaposed cells.

For here, indeed, we are shown unmistakably how the primary contrast of structures follows upon the primary contrast of conditions. The ordinary form from which low types of the Metazoa set out, is a hollow sphere formed of cells packed side by side—a blastula, as it is called: all these cells being similarly exposed to the environment. The blastula presently changes into what is called a gastrula—a form resulting from the introversion of one of the sides of the blastula. If there be taken a small ball of vulcanized india-rubber, say an inch or more in diameter, and having a hole in it through which the air may escape, and if one side of it be thrust inwards so as to produce a cup, and if the wide opening of the cup be supposed to contract, thus becoming a narrow opening, there will result something like the gastrula form. Manifestly that part of the original layer which has become internal is differently conditioned from the rest which remains external: the one continuing to hold converse with the forces of the environment, while the other begins to hold converse with the nutritive matters taken into the sac-formed chamber—the archenteron or primitive stomach. Interesting evidence of the primitive externality of the digestive cavity is yielded by the fact that whereas the blastula consisted of ciliated cells, and whereas the ciliation persists throughout life on the outer layer, or parts of it, in sundry low types—even in some Chætopods—it persists also on the alimentary tract of sundry low types: not only in the Hydra but commonly in Nemertines, in some Platyhelminthes, and even in some leeches.

Besides being enabled thus to understand how an aggregate of Amœba-form units, originally consisting of a single layer, may pass into an aggregate consisting of a double layer; we may also understand under what influences the transition takes place. If the habit which some of the primary aggregates have, of wrapping themselves round masses of nutriment, is followed by a secondary aggregate, there will naturally arise just that re-differentiation which the Hydra shows us.

§ 289. This account of the primary differentiation carries us only half-way towards a true conception of the distinction between outer and inner tissues. Though, using words in their current senses, this introverted part of the primitive layer has become internal in contrast with the remainder, which continues external, yet this introverted part has not become internal in the strict physiological sense. For it remains subject to the actions of those environing matters which are taken in as food: such environing matters, when they happen to be moving prey, acting upon it much as they might act upon the exterior. So that this introverted part has a quasi-externality. It has not the same absolute internality as have those parts which never come in contact with products of the outer world. Here we must briefly recognize the distinction between these parts and the parts thus far considered.

Reverting to our symbol, the india-rubber ball, it will be seen that the introversion may be so complete that the cavity is obliterated, with the result that the internal surfaces of the outer and inner layers come in contact. This is the state reached in the simplest cœlenterate animal, the Hydra: there being in it nothing more than a thin structureless lamella between the ectoderm and endoderm, as shown in Fig. [147]. This lamella represents all that there is of strictly internal tissues. But the introversion, instead of bringing the inner surfaces of the ball into contact, may be so far incomplete as to leave a space, and in various creatures and embryos of others, symbolized by this arrangement, this space becomes occupied by a tissue formed from one or other or both of the two primary tissues—the mesoblast or mesoderm. This intermediate layer, sometimes, as in the Medusa, growing into a mass of jelly serving as a fulcrum for the creature’s contractions, or, as in the Sponge, giving a passive basis to the active tissues, becomes in higher animals the layer out of which the structures that support the body and move it about, as well as those that distribute prepared nutriment, are developed. From it arise the bones, the muscles, and the vascular system—the masses of differentiated tissue which are truly internal and occupy what is called the body-cavity or peri-visceral space.

In the higher types of animals this space comes to be partially occupied by a structure that may be described as a cavity within a cavity—the cœlom. Most zoologists regard this as arising by a re-introversion of the archenteron or primary alimentary sac. It is easily to be perceived that after the introversion which produces this digestive cavity, the wall of the cavity may be again introverted in such way as to intrude into the peri-visceral space. The cœlom thus formed is subsequently shut off. Becoming included among the more truly internal structures, and in part giving origin to certain lining membranes, it has for its chief function the formation of organs for the excretion and emission of nitrogenous waste and of the generative products: some portions of it retaining, as a consequence, indirect connexions with the environment and characters usually accompanying such connexions.

Here we are not concerned with further details: the aim being simply to indicate the way in which out of the original layer, wholly external, there arise, by primary and secondary introversions, and the formation of intermediate membranes and spaces, the chief contrasts between outer and inner tissues, and how there simultaneously go on the differentiations accompanying different conditions.

§ 289a. Another all-important differentiation between outer tissues and inner tissues has now to be set forth—that by which the nervous system becomes established and distinguished. Strangely enough, like the one above described, it is sequent upon an introversion: the nervous system is primarily a skin-structure and develops by the infolding of this skin-structure.

In creatures possessing the earliest rudiments of nerves these exist in certain superficial cells. Each has a small tubular orifice from which projects a minute hair, and each has on its under side processes running into the tissue below, and serving, as it seems, to conduct impressions from the projecting hair when it is disturbed by contacts with foreign bodies. A plexus of fibres bringing the inner processes of such cells into communication arises, and forms something like a nervous layer capable of propagating impulses in all directions. At a subsequent stage some of the superficial cells, ceasing to be themselves the recipients of external stimuli, sink inwards and become ganglion-cells connected with the nervous plexus—agents, as we must suppose, for the reception, multiplication, and diffusion of the impulses received from the outer cells.