Popular Science Monthly/Volume 77/August 1910/Relation of Embryology and Vertebrate Paleontology
|RELATION OF EMBRYOLOGY AND VERTEBRATE PALEONTOLOGY
By Professor RICHARD SWANN LULL
THE problem of recapitulation among vertebrates gives by no means as accurate results as among invertebrate forms, for while a single adult shell, if perfectly preserved, will often display the entire life history or ontogeny of the individual, a bone, or even a complete skeleton, is rarely retrospective and if at all only in some minor detail. The vertebratist, therefore, in his study of ontogeny, for comparison with racial history must needs follow either the entire growth of one animal, a thing manifestly impossible when the embryonic stages are considered, or study a long series of individuals in various stages of development, the securing of which in the great majority of cases is largely the result of a number of happy accidents. When one comes to weigh the evidence offered by the actual embryos of fossil vertebrates he will find a very great dearth of material, for fossil embryos—that is, the stages in the life history before birth or hatching—are extremely rare.
Recent embryology, on the other hand, is more productive of results and the earlier stages of certain organs often suggest those of equivalent development in animals of the past. In his interpretation of a given structure, however, one has to bear in mind whether it may not have been modified to suit some modern need in the life history of the individual, and thus no longer give us a true image of bygone structure. These cœnogenetic organs are not historic, but as Wilder says, "have to do with such immediate environmental problems as nutrition or protection." Again, if the organ has approximately the same form and character in the ancestral type at the same stage of its development, it represents an actual repetition of past history and is therefore palingenetic. Sometimes it is not quite clear, however, under which caption the embryonic structure comes, and its interpretation must be attempted with caution.
Osborn in his lectures to his students speaks of the three-fold evidence for evolution which stands firmly like a tripod, the legs of which are comparative anatomy, embryology and paleontology; and the evidence of each should correspond, provided the interpretation be correct. Of these, however, embryology is manifestly the weakest member, while paleontology is a tower of strength!
The reptiles are so rare as embryos and withal so ancient a group that their ontogeny throws but little light upon paleontology. Among the fossil forms a number of specimens of Ichthyosaurus have been found with young contained within the body of the adult. Many of these are in the normal position for fœti-in-utero; others are displaced, with the head directed forward. These latter Branca thinks may be young that have been eaten. There is also, at times, a very great difference in the size of the contained young. Aside from a slight difference in proportions, especially that of head to trunk, and a less degree of hardness of the embryonic bones, as indicated by their being crushed over the parent's ribs, the young teach us nothing as to ancestral structure as they are in every way perfect ichthyosaurs. They do prove, however, when the evidence of viviparity which they offer is taken in connection with the supreme degree of aquatic adaptation indicated, that the ichthyosaurs were high sea-forms, never coming ashore even for egg-laying.
That certain of the dinosaurs were also viviparous may be proved by an embryo contained in the unique specimen of Compsognathus longipes from the Jurassic of Bavaria. So far as I am aware this embryo gives no other evidence of ontogenetic value.
The turtles have been made the subject of exhaustive study by Hay and from the embryological point of view by Clark under L. Agassiz. Anatomically they are the most remarkable of reptiles, having undergone during their career an extreme modification in many directions while retaining a number of very primitive characters. The most remarkable feature is the development of the shield or carapace, which contains what are generally considered as the homologues of the ribs of other vertebrates, but, strangely enough, here lying outside of the shoulder girdle, a feature wherein the turtles are utterly unique. The embryology, which is well known, ought to throw some light upon the origin of this important feature. In their earlier stages of development the Chelonia resemble the Lacertilia, the chief pecularity being caused by the development of this carapace which appears in the form of two longitudinal folds extending above the line of insertion of the fore-and hind-limbs which have already made their appearance. Hence the carapace grows outward and over the limb-girdles which come to lie within the rib-like osseous supports. This ontogeny shows us clearly how the ancestral carapace may have been formed. Paleontology has not as yet confirmed this, but doubtless will do so.
Among birds one of the most interesting features is the occurrence of vestigial tooth papillae in the jaws of certain embryo parrots and owls—reminiscent of Mesozoic days when birds were toothed in their adult state.
Mammalian evidence is very striking in many details and much of it has recently been summarized by Hubrecht, who makes much of the character of the placentation and derives from it and other features some remarkable conclusions. Hubrecht abandons the idea of the derivation of the mammalia from the reptilian-insectivorean stem, but on the contrary derives them from amphibia-like animals of the Carboniferous. The character of the placentation, moreover, places man, the Anthropomorphæ and the hedgehog among the most archaic of living mammalian types, an idea also borne out by comparative anatomy and one which paleontology may some day confirm.
The most primitive mammals, the Prototheria, are still very suggestive of their old reptilian ancestry in many ways, especially in the method of producing the young inclosed within an eggshell. The position of the Marsupials is surely low in the scale of mammalian life, although they show in many respects remarkable specializations. Wilder compares them with the Prototheria in that they also bring forth their young at a very early state of development, though not protected by an eggshell. The period during which they are permanently attached to the nipples within the pouch is actually post-embryonic and properly larval. Vestiges of placentation have been found in at least one marsupial, a fact which gives color to the belief that in this respect they may be degenerate rather than primitive. Broom has shown that the modern marsupial shoulder girdle passes through a prototherian stage implying a relationship which is strongly supported in other ways.
The fœtal Sirenia and Cetacea, so far as known, show no greater development of hind-limbs than do the post-natal individuals. They do show a marked neck construction and the head bent at right angles with the trunk in a normal quadrupedal posture. The head of the fcetal manatee is very suggestive of the modern ungulate. Ryder has tried to prove the homology of the caudal flukes in the Sirenia and Cetacea with the integument of the hind feet, drawing his evidence largely from comparison with the seals. In the embryo the flukes appear as lateral expansions near the end of the tail, giving it a lancelike form when viewed from above. These flaps by transverse expansion develop into the powerful swimming flukes of the adult. They may be compared with lateral flanges on the tail of the sea otter Enhydris, but in the latter the flaps are elongate, while in the Cetacea they are short and situated toward the end of the tail. Nevertheless, the homology of the two types of flange structures appears true, the posterior position and concentration in the whale being a mechanical adaptation which has become accelerated in its appearance so as to be embryonic. The presence of hair on the body of the fœtal whale and of distinct calcareous tooth germs in both upper and lower jaws of the unborn young of whalebone whales are both reminiscent.
The horses, our knowledge of which is so complete owing to the pioneer work of Marsh and later of Osborn, show some interesting points of comparison between foetus and ancestor. The skulls of prenatal modern horses resemble those of Mesohippus or even of Eohippus in the proportions of face and cranium, the short-crowned grinding teeth, lesser angle between basi-cranial and basi-facial axes and the fact that the orbit is incompletely ringed with bone. The feet of the unborn foal are also somewhat reminiscent of old-time conditions.
One of the most difficult points to be reconciled in the acceptance of the Cope-Osborn theory of the origin of molar cusps was the apparent non-agreement of cusp ontogeny with the interpreted phylogeny which this theory upheld. The difficulty has been met in two ways: by the supposition that ecenogenesis has entered into the embryogeny, or that the paleontological record as shown by the trituberculists is open to a different interpretation. The present great exponent of the idea claims that the matter is still sub judice and thus the problem stands.
In conclusion, the paleontological student of the higher vertebrates can hope to find in embryology a host of valuable suggestions, much verification of his work and sundry apparent inconsistencies which must in some way be reconciled. He should ever bear in mind the influence of nature and nurture, the latter often giving rise to perplexing conflicts between the two records. He will on the whole have in embryology a fair mirror of the past wherein, even though the image be somewhat distorted and the more remote reflections dimmed by time, he can view the striking features of the long procession of the ages.