Soon one side of the blastula is invaginated or pushed in, as one would push in one side of a hollow india-rubber ball. The result of this invagination (called gastrulation technically) is the formation of a sort of cup. This is the gastrula ([Fig. 10], 6) phase of man’s existence. It is a higher and fundamentally different phase of existence than either the morula or the blastula. It corresponds, not to a Protozoan, but to the higher Metazoan. It possesses the fundamental anatomical qualities of a low Cœlenterate (Polyp).
The mode of gastrulation is different in man from that just described, and varies in different animals, but the essential point common to all is the formation of a double cellular-membrane; the outer membrane being called epiblast or ectoderm, and the inner one hypoblast or endoderm, the enclosed cavity being the primitive digestive cavity. These two layers are the primary germinal layers. A third layer is subsequently formed between them, by the agency of one or both of them, and is called the mesoblast. From these three simple membranes, which are composed exclusively of cells, are formed all the complex tissues and organs of the adult man. The epiblast develops into the nervous system (brain, spinal cord, and nerves), and the cuticle, hair, etc. The hypoblast develops into the cellular parts of the digestive canal, the liver, lungs, etc. The mesoblast develops into the muscles, bones, ligaments, blood-vessels, etc. To trace the details of this evolution of a human being from the microscopic oösperm is a fascinating and instructive study, but is beyond the limits and purpose of this little book. It will well repay further careful study by the reader.
A careful study of the life-histories of the few unicellular animals and plants mentioned in the preceding pages will help to let one realize the wonderful powers of nucleated pieces of protoplasm (cells). If isolated pieces of protoplasm can accomplish so much, one will not be astonished to learn that many diverse cells associated together in intimate correlations, as occurs in the higher animals, may accomplish results that are profoundly interesting and marvelous. As far as anatomy and physiology alone can reveal, it is the result of millions of cells acting together that makes possible the existence of such a living, sentient, thinking creature as man or the highly intelligent elephant or any other multicellular animal. It is due to the mysterious powers of protoplasm that one little microscopic cell, like the fertilized ovum of a woman, is able to hold all the heritages of the race, and gradually unfold them as it builds up the body into myriads of diverse cells intimately associated together.
All of the wonderful results of Embryology are accomplished through cell-multiplications, cell-differentiations, cell-associations, invagination and evaginations of cell-groups (tissues or organs), and unequal growth of parts (cells or groups of cells).
Fig. 10.—Segmentation of the fertilized ovum and Gastrulation: 4, morula; 5, section through blastula showing hollow sphere; 6, gastrula showing outer layer of cells (epiblast) and inner layer (hypoblast); the 6 is at the mouth of the cavity (enteron) of the gastrula.
In concluding this brief but, we hope, useful study of a few selected cells, we may say that an eminent English physiologist has made the statement that a student who has not looked through the microscope and observed the circulation of the blood in the web of a frog’s foot is not fit to study medicine. However beautiful, fascinating and instructive the sight of this circulation may be, we are tempted to make the assertion that the student who has not looked through the microscope at some of the superficial ooze from the bottom of any slow-running stream, in summer, and observed the structure and actions of that wonderful little unicellular animal, the Amœba proteus, is still less prepared to study medicine. In this little speck of protoplasm the problems of life are reduced to their simplest forms, for all higher plants and animals may be regarded as groups of more or less modified amœbæ peculiarly associated together. In our studies of the amœba we will be forcibly reminded of a very clever trick which is practiced in India and is called the mango-trick. In this trick a seed is put into the ground and covered up, and after divers incantations a full-blown mango-bush appears within five minutes. We have never met any one who knew how this thing was done, nor have we ever seen a person who believed it to be anything else than a conjuring trick. So it is with the amœba, a beautiful and fascinating trickster of nature. We understand some of its activities, interesting and exceedingly instructive, but there are many others beyond our ken. We see the commencement and ending of many of its chemical activities, but there are numerous other intermediary processes that take place in the hidden recesses of the protoplasm, and concerning which we know nothing. It may fall to the lot of some reader of this little book, as a patient and keen observer, to unravel some of these mango-like tricks of the amœba or other unicellular creature.