The first stage in the existence of any organism, from the simplest form of plant to man, consists of a single cell (in animals called the ovum or egg), which differs in no essential points of structure from an ordinary cell.
There is moreover a class of organisms in which the individuals never go beyond this stage, but pass their entire lifetime in the state of a simple unicellular organism. The individuals composing this group (Protista), though insignificant in point of size, may vie with the higher plants and animals in number, species, and variety of form, so that they might well be placed in a kingdom by themselves (as has been proposed), apart from the vegetable and animal kingdoms.
Fig. 6. Unicellular organism. Small amoeba. (Cadiat).
The group of these organisms which most resembles animals, is called Protozoa, and is divided from other animal forms by the manner of development of the ovum of the latter, which divides into cells that subsequently become differentiated into tissue. This group is called the Metazoa.
Fig. 7. Stages in the division of the egg cell (ovum), showing the production of a multiple mass by division. (Gegenbauer).
In the Protozoa the ovum never divides, the animal always remaining a single cell. On the contrary, the ovum of the Metazoa changes its characters during its development. At first possessing a stage common to both divisions, viz., a single cell, it soon passes through rapid stages of cell proliferation, and is converted into a multiple mass, the mulberry stage or Morula.
The cells forming this Morula stage approach the periphery of the mass, where they arrange themselves in two layers, and form a cavity in the centre. This is known as the Gastrula stage. Following, then, this cell multiplication, we find a qualitative differentiation of the cells, by which certain groups of cells assume special peculiarities, fitting them for some specific duty.
Thus we arrive at the production of special textures and organs such as are met with in the higher animals, and which are necessary for the efficient discharge of the various functions carried on during their lives. The division of the original mass of indifferent cells into two layers of special cells is the first step toward tissue differentiation, and in some animals is the only one arrived at in their life history, throughout which they remain a simple sac made up of an external layer, Ectoderm, and an internal layer, Endoderm.
The groups of cells forming the outer and inner layers of this stage of development, not only form the primitive tissues, but also represent the first appearance of organs or parts with a specific function. The external or ectodermic layer is the supporting, protecting, motor and respiratory organ, while the inner or endo-dermic layer is devoted to a primitive form of digestion, preparing the food for assimilation, and generally presiding over the nutrition of the body.
Although this sac-like (Gastrula) stage is supposed to have formed a step in the life history of nearly all animals, yet it forms a less striking part in the development of the individuals as we ascend the scale, and in the higher animals no such stage has been recognized. In the Vertebrates, the germ cells derived from the ovum are from an early period divided into three distinct layers, as those which correspond to the Ectoderm and Endoderm' of the lower organisms form between them a third layer or Mesoblast.
From these germinal layers all the organs and tissues of the body are subsequently evolved. In embryological language the three primitive layers are called Epi-, Meso-, and Hypo-blast.
Thus it can be seen that, as we can compare the primitive unicellular state of the lowest animals with the first egg-cell stage of existence of the highest animals, so we can compare all the steps of tissue and organ differentiation as we trace them in the embryo of a mammal, with the steps of elaboration in organic and tex-tural parts that we find in ascending the scale of animal life.
Fig. 8. Diagram showing the first differentiation of the organism into an-external and internal layer, (a) Mouth, (6) alimentary cavity, (d) ectoderm, (c) endoderm. (Gegenbauer).
The history, then, of the development of any mammal from a single cell or egg to the complex adult individual, is analogous with the more protracted history of the evolution of the animal kingdom from the Protista upward.
It is impossible to separate the differentiation of tissues and organs, or to say which is of older date in the history of animal evolution. Even in unicellular animals, where we have no trace of tissue difference (Paramaecium, Vorticella), there being only one cell, we have a distinct foreshadowing of organ and functional differentiation (vide Chapter III (Chemical Basis Of The Body)). And in creatures made of many parts, the same cells have several duties to perform. But when an aggregation of specialized cell units exists, it may be said to be a tissue. If these cells have no very special characteristic, then the tissue may be called primitive or embryonic. But, as has just been stated, the aggregation of embryonic cells - in the higher forms of life - have special characters from the very first, which mark them off from one another as destined for different functions.
Fig. 9. Transverse section of blastoderm of chick. A. Epiblast. B. Mesoblast. C. Hypoblast. pr. Primitive groove.