The Riddle of the Universe at the close of the nineteenth century

by Ernst Haeckel

VII. This phylogenetic conclusion, based on the comparison of ontogenetic facts, is confirmed by the circumstance that there are several of these gastræades still in existence (gastraemaria, cyemaria, physemaria, etc.), and also some ancient forms of other animal groups whose organization is very little higher (the olynthus of the sponges, the hydra, or common fresh-water polyp, of the cnidaria, the convoluta and other cryptocæla, or worms of the simplest type, of the platodes).

VIII. In the further development of the various tissue-forming animals from the gastrula we have to distinguish two principal groups. The earlier and lower types (the coelenteria or acoelomia) have no body cavity, no vent, and no blood; such is the case with the gastræades, sponges, cnidaria, and platodes. The later and higher types (the caelomaria or bilateria), on the other hand, have a true body cavity, and generally blood and a vent; to these we must refer the worms and the higher types of animals which were evolved from these later on, the echinodermata, mollusca, articulata, tunicata, and vertebrata.

Those are the main points of my “gastræa theory”; I have since enlarged the first sketch of it (given in 1872), and have endeavored to substantiate it in a series of “Studies on the gastræa theory” (1873-84). Although it was almost universally rejected at first, and fiercely combated for ten years by many authorities, it is now (and has been for the last fifteen years) accepted by nearly all my colleagues. Let us now see what far-reaching consequences follow from it, and from the evolution of the germ, especially with regard to our great question, “the place of man in nature.”

The human ovum, like that of all other animals, is a single cell, and this tiny globular egg cell (about the 120th of an inch in diameter) has just the same characteristic appearance as that of all other viviparous organisms. The little ball of protoplasm is surrounded by a thick, transparent, finely reticulated membrane, called the zona pellucida; even the little, globular, germinal vesicle (the cell-nucleus), which is enclosed in the protoplasm (the cell-body), is of the same size and the same qualities as in the rest of the mammals. The same applies to the active spermatozoa of the male, the minute, threadlike, ciliated cells of which millions are found in every drop of the seminal fluid; on account of their lifelike movements they were previously taken to be forms of life, as the name indicates (spermatozoa—sperm animals). Moreover, the origin of both these important sexual cells in their respective organs is the same in man as in the other mammals; both the ova in the ovary of the female and the spermatozoa in the spermarium of the male arise in the same fashion—they always come from cells, which are originally derived from the cœlous epithelium, the layer of cells which clothes the cavity of the body.

The most important moment in the life of every man, as in that of all other complex animals, is the moment in which he begins his individual existence; it is the moment when the sexual cells of both parents meet and coalesce for the formation of a single simple cell. This new cell, the impregnated egg cell, is the individual stem cell (the cytula), the continued segmentation of which produces the cells of the germinal layers and the gastrula. With the formation of this cytula, hence in the process of conception itself, the existence of the personality, the independent individual, commences. This ontogenic fact is supremely important, for the most far-reaching conclusions may be drawn from it. In the first place, we have a clear perception that man, like all the other complex animals, inherits all his personal characteristics, bodily and mental, from his parents; and, further, we come to the momentous conclusion that the new personality which arises thus can lay no claim to “immortality.”

Hence the minute processes of conception and sexual generation are of the first importance. We are, however, only familiar with their details since 1875, when Oscar Hertwig, my pupil and fellow-traveller at that time, began his researches into the impregnation of the egg of the sea-urchin at Ajaccio, in Corsica. The beautiful capital of the island in which Napoleon the Great was born, in 1769, was also the spot in which the mysteries of animal conception were carefully studied for the first time in their most important aspects. Hertwig found that the one essential element in conception is the coalescence of the two sexual cells and their nuclei. Only one out of the millions of male ciliated cells which press round the ovum penetrates to its nucleus. The nuclei of both cells, of the spermatozoon and of the ovum, drawn together by a mysterious force, which we take to be a chemical sense-activity, related to smell, approach each other and melt into one. Thus, by the sensitive perception of the sexual nuclei, following upon a kind of “erotic chemicotropism,” a new cell is formed, which unites in itself the inherited qualities of both parents; the nucleus of the spermatozoon conveys the paternal features, the nucleus of the ovum those of the mother, to the stem cell, from which the child is to be developed. That applies both to the bodily and to the mental characteristics.

The formation of the germinal layers by the repeated division of the stem cell, the growth of the gastrula and of the later germ structures which succeed it, take place in man in just the same manner as in the other higher mammals, under the peculiar conditions which differentiate this group from the lower vertebrates. In the earlier stages of development these special characters of the placentalia are not to be detected. The significant embryonic or larval form of the chordula, which succeeds the gastrula, has substantially the same structure in all vertebrates; a simple straight rod, the dorsal cord, lies lengthways along the main axis of the shield-shaped body—the “embryonic shield”; above the cord the spinal marrow develops out of the outer germinal layer, while the gut makes its appearance underneath. Then, on both sides, to the right and left of the axial rod, appear the segments of the “pro-vertebræ” and the outlines of the muscular plates, with which the formation of the members of the vertebrate body begins. The gill-clefts appear on either side of the fore-gut; they are the openings of the gullet, through which, in our primitive fish-ancestors, the water which had entered at the mouth for breathing purposes made its exit at the sides of the head. By a tenacious heredity these gill-clefts, which have no meaning except for our fish-like aquatic ancestors, are still preserved in the embryo of man and all the other vertebrates. They disappear after a time. Even after the five vesicles of the embryonic brain appear in the head, and the rudiments of the eyes and ears at the sides, and after the legs sprout out at the base of the fish-like embryo, in the form of two roundish, flat buds, the fœtus is still so like that of other vertebrates that it is indistinguishable from them.

The substantial similarity in outer form and inner structure which characterizes the embryo of man and other vertebrates in this early stage of development is an embryological fact of the first importance; from it, by the fundamental law of biogeny, we may draw the most momentous conclusions. There is but one explanation of it—heredity from a common parent form. When we see that, at a certain stage, the embryos of man and the ape, the dog and the rabbit, the pig and the sheep, although recognizable as higher vertebrates, cannot be distinguished from each other, the fact can only be elucidated by assuming a common parentage. And this explanation is strengthened when we follow the subsequent divergence of these embryonic forms. The nearer two animals are in their bodily structure, and, therefore, in the scheme of nature, so much the longer do we find their embryos to retain this resemblance, and so much the closer do they approach each other in the ancestral tree of their respective group, so much the closer is their genetic relationship. Hence it is that the embryos of man and the anthropoid ape retain the resemblance much later, at an advanced stage of development, when their distinction from the embryos of other mammals can be seen at a glance. I have illustrated this significant fact by a juxtaposition of corresponding stages in the development of a number of different vertebrates in my Natural History of Creation and in my Anthropogeny.

The great phylogenetic significance of the resemblance we have described is seen, not only in the comparison of the embryos of vertebrates, but also in the comparison of their protective membranes. All vertebrates of the three higher classes—reptiles, birds, and mammals—are distinguished from the lower classes by the possession of certain special fœig;tal membranes, the amnion and the serolemma. The embryo is enclosed in these membranes, or bags, which are full of water, and is thus protected from pressure or shock. This provident arrangement probably arose during the Permian period, when the oldest reptiles, the proreptilia, the common ancestors of all the amniotes (animals with an amnion), completely adapted themselves to a life on land. Their direct ancestors, the amphibia, and the fishes are devoid of these fœtal membranes; they would have been superfluous to these inhabitants of the water. With the inheritance of these protective coverings are closely connected two other changes in the amniotes: firstly, the entire disappearance of the gills (while the gill arches and clefts continue to be inherited as “rudimentary organs”); secondly, the construction of the allantois. This vesicular bag, filled with water, grows out of the hind-gut in the embryo of all the amniotes, and is nothing else than an enlargement of the bladder of their amphibious ancestors. From its innermost and inferior section is formed subsequently the permanent bladder of the amniotes, while the larger outer part shrivels up. Usually this has an important part to play for a long time as the respiratory organ of the embryo, a number of large blood-vessels spreading out over its inner surface. The formation of the membranes, the amnion and the serolemma, and of the allantois, is just the same, and is effected by the same complicated process of growth, in man as in all the other amniotes; man is a true amniote.