Tissues and Embryos. Biology

Nor did the biologist have to depend on the somewhat alien world and work of the chemist to become aware of the basic unity of life. The developing excellence of the microscope eventually made this point visible to the eye.

At first, the microscope made too much visible to the eye, or, rather, to the imagination. Some of the early microscopists, fascinated by the glimpse into the infinitesimal, insisted on making out details beyond the power of their poor instruments to offer them. Thus, they painstakingly drew pictures of microscopic human figures ("homunculi") within the spermatozoa of the semen they studied.

They imagined, too, there might be no end to smallness. If an egg or sperm already contained a tiny figure, that tiny figure might contain within it a still tinier one that was someday to be its offspring and so on indefinitely. Some even tried to calculate how many homunculi within homunculi within homunculi might have existed in Eve in the first place; and wondered whether the human race might not come to an end when those nested generations were exhausted. This was the doctrine of "preformation" and was clearly an antievolutionary view since, according to it, all possible members of a species already existed in the first member of that species and there was no reason to suppose that there would be a change of species any-where along the line.

The first major attack on this point of view came from a German physiologist, Caspar Friedrich Wolff (1733-94). In a book published in 1759, when he was only 26, he described his observations of the development of growing plants. He noted that the tip of a growing-plant shoot consisted of undifferentiated and generalized structures. As the tip grew, it specialized, however, and one bit eventually developed into a flower while another bit (completely indistinguishable at first) developed into a leaf. Later, he extended his observations to animals such as the embryonic chick. Undifferentiated tissue, he showed, gave rise to the different abdominal organs through gradual specialization. This was the doctrine of "epigenesis," an expression first used by William Harvey in a book on the birth of animals, published in 1651.

From this viewpoint, all creatures, however different in appearance, developed out of simple blobs of living matter and were alike in their origins. Living things did not develop out of a tiny, but already specialized, organ or organism.

Even fully developed organisms were not as different as they might appear to be, when studied properly. A French physician, Marie Francois Xavier Bichat (1771-1802), working without a microscope (!) was able to show, in the last years of his short life, that various organs consisted of several components of different appearance. These components he named "tissues" and thus founded the science of histology, the study of tissues. It turned out there were not very many different tissues (some important varieties in animals are epithelial, connective, muscle and nerve tissues) and that different organs of different species were built up out of these few varieties. Particular tissues did not differ from species to species as radically as the whole organisms did.

And one can go still further than that, too. As was explained earlier in the book (see page 30), Hooke, in the mid-seventeenth century, had observed that cork was divided up into tiny rectangular chambers which he called cells. These were empty, but then cork was a dead tissue. Later investigators, studying living, or recently living, tissues under the microscope came to realize that these, too, were built up out of tiny, walled-off units.

In living tissue, the units are not empty, but are filled with a gelatinous fluid. This fluid was eventually to receive a name thanks to a Czech physiologist, Johannes Evangelista Purkinje (1787-1869). In 1839, he referred to living embryonic material within an egg as "protoplasm," from Greek words meaning "first formed." The German botanist, Hugo von Mohl (1805-72), adopted the term the next year but applied it to the material within tissues generally. Although the partitioned units of living tissue were not empty, Hooke's word "cell" continued to be applied to them.

Cells were more and more commonly found and a number of biologists speculated that they might exist universally within living tissue. This belief crystallized in 1838, when a German botanist, Matthias Jakob Schleiden (1804-81), maintained that all plants were built up of cells and that it was the cell that was the unit of life; a little living thing out of which entire organisms were built.

In the next year, a German physiologist, Theodor Schwann (1810-82), extended and amplified this idea. He pointed out that all animals, as well as all plants, were built up out of cells; that each cell was surrounded by a membrane separating it from the rest of the world; and that Bichat's tissues were built up of cells of a particular variety. Usually, then, Schleiden and Schwann share the credit for the "cell theory" though many others also contributed, and with them begins the science of cytology (the study of cells).

The assumption that the cell was the unit of life would be particularly impressive if it could be shown that a cell was capable of independent life, that, to be living, it was not necessary for it to be combined into conglomerates of billions and trillions. That some cells actually were capable of independent life was shown by a German zoologist, Karl Theodor Ernst von Siebold (1804-85).

In 1845, Siebold published a book on comparative anatomy which dealt in detail with protozoa, the little animals first detected by Van Leeuwenhoek (see page 30). Siebold made it quite clear that protozoa had to be considered as consisting of single cells. Each protozoon was surrounded by a single membrane and possessed within itself all the essential faculties of life. It ingested food, digested it, assimilated it, and discarded wastes. It sensed its environment and responded accordingly. It grew, and it reproduced by dividing in two. To be sure, the protozoa were generally larger and more complex than the cells making up a multicellular organism such as man; but then the protozoan cell had to be, for it retained all necessary abilities that made independent life possible, whereas individual cells of a multicellular creature could afford to discard much of this.

Even multicellular organisms could be used to demonstrate the importance of individual cells. The Russian biologist, Karl Ernst von Baer (1792-1876), had, in 1827, discovered the mammalian egg within the Graafian follicle (see page 28) and then went on to study the manner in which the egg developed into an independently living creature.

Over the course of the next decade, he published a large two-volume textbook on the subject, thus founding the science of embryology (the study of the embryo, or developing egg). He revived Wolff's theory of epigenesist (which had been largely ignored in its own time) in more detailed and better-substantiated form, showing that the developing egg forms several layers of tissue, each of which is undifferentiated to begin with, but out of each of which various specialized organs developed. These original layers he called "germ layers" ("germ" being a general term for any small object containing the seed of life).

The number of such germ layers was finally fixed at three, and in 1845, the German physician, Robert Remak (1815-65), gave them the names by which they are still known. These are "ectoderm" (from Greek words meaning "outer skin"), "mesoderm" ("middle skin"), and "endoderm" ("inner skin").

The Swiss physiologist, Rudolf Albert von Kolliker (1817-1905), pointed out, in the 1840s, that the egg and sperm were individual cells. (Later, the German zoologist, Karl Gegenbaur (1826-1903), went on to demonstrate that even the large eggs of birds were single cells.) The fusion of sperm and egg formed a "fertilized ovum" which, Kolliker showed, was still a single cell. (This fusion, or "fertilization," initiated the development of the embryo. Although biologists were already assuming, by mid-nineteenth century, that the process took place, and though a number of observations supporting this assumption were made over the preceding decades, it was not actually described in detail until 1879, when the Swiss zoologist, Hermann Fol, witnessed the fertilization of a starfish egg by a sperm.)

By 1861, Kolliker had published a textbook on embryology- in which Baer's work was reinterpreted in terms of the cell theory. Every multicellular organism began as a single cell, the fertilized ovum. As the fertilized ovum divided and redivided, the resulting cells were not very different to begin with. Slowly, however, they specialized in different directions until all the complexly interrelated structures of the adult form were produced. It was epigenesis reduced to cellular terms.

The concept of the unity of life was greatly strengthened. One could scarcely differentiate between the fertilized ovum of a man, a giraffe, and a mackerel and, as the embryo developed, differences were produced only gradually. Small structures in the embryos, scarcely distinguishable at first, might develop into a wing in one case, an arm in another, a paw in a third, and a flipper in still a fourth. Baer felt, quite strongly, that relationships among animals could more properly be deduced by comparing embryos than by comparing adult structures, so that he is also the founder of comparative embryology.

The change from species to species, viewed through the process of cellular development, seemed a matter of detail only, and to be well within the capacity of some evolutionary process to bring about. Baer was able to show, for instance, that the early vertebrate embryo possessed a "notochord" temporarily. This is a stiff rod running the length of the back and there are very primitive fishlike creatures that possess such a structure throughout life. These primitive creatures were first studied and described in the 1860s by the Russian zoologist, Alexander Kowalewski (1840-1901).

In vertebrates, the notochord is quickly replaced by a spinal cord of jointed vertebrae. Nevertheless, even the

temporary appearance of the notochord seems to show a relationship to the animals described by Kowalewski. It is for this reason that the vertebrates and these few invertebrates are lumped together in the phylum, Chordata. Moreover, it is even attractive to suppose that the notochord, appearing so briefly in the vertebrate embryo (even in the human embryo) , is an indication that all the vertebrates are descended from some primitive creature with a notochord.

From several different fields then—from comparative anatomy, from paleontology, from biochemistry, from histology, cytology, and embryology—all signs at first whispered, then, as mid-nineteenth century approached, shouted that some sort of evolutionary view was a necessity. Some satisfactory mechanism for evolution simply had to be presented.