Walter Holbrook Gaskell

“ The embryological and phylogenetic histories are in complete unison to show that the cranial skeleton is older than the spinal, and this primitive branchial skeleton is also in harmony with the laws of evolution, in that its structure, even in the adult lamprey (Petromyzon) , never gets beyond the stage characteristic of embryonic cartilage in the higher vertebrates.
The simplest and most primitive skeleton is that found in Ammocœtes and consists of two parts: (1) a prosomatic, (2) a mesosomatic skeleton. ”

“ These two groups of cartilage arise independently, so that at first the basi-cranial system is quite separate from the branchial, and only late in the history of the animal is a junction effected between the branchial system and the trabeculæ and parachordals, an initial separation which is especially striking when we consider that in this animal all the cartilaginous structures of any one system are continuous: there is no sign of anything in the nature of joints. ”

“ Embryology, then, would lead to the belief that the earliest form of cartilage in the vertebrate kingdom ought to be of this type, viz. large cells, each of which is enclosed in a simple capsule, so that the capsules of the cells form the whole of the matrix, and thus form a simple homogeneous honeycomb-structure, in the alveoli of which the cartilage-cells lie singly. ”

“ It is a striking fact, common to the whole of the group of animals to which our inquiries, deduced from the consideration of the structure of Ammocœtes, have, in every case, led us in our search for the vertebrate ancestor, that they do possess a remarkable internal semi-cartilaginous skeleton in the prosomatic region, called the entosternite or plastron, which gives support to a large number of the muscles of that region ”

“ The white connective tissue of Limulus, as already stated, resembles that of the vertebrate more than does the connective tissue of any other invertebrate, and, similarly to that of Ammocœtes, does not stain, or gives only a light-blue tinge with thionin. The tissue of the entapophysial ligament, on the contrary, just like muco-cartilage, takes on an intense purple colour when stained with thionin. It possesses a mucoid substratum, just as does muco-cartilage, and in both cases a perfectly similar soft cartilage is born from it. ”

“ Each entapophysis is hollow and its walls are composed of chitin; but from the apex of each spur there stretches from spur to spur a band of tissue, called by Lankester the entapophysial ligament (Ent.l.) (Fig. 58) , and in this tissue cartilage is formed. Isolated cartilaginous cells, or rather groups of cells, are found here and there, but a concentration of such groups always takes place at each entapophysis, forming here a solid mass of cartilage, from which the massive cartilaginous bar of each branchial appendage arises. ”

“ There in the free-living, still-existent Ammocœtes we find the manifest natural outcome of the embryological history in the shape of simple trabeculæ and parachordals, from which the whole complicated basi-cranial skeleton of the higher vertebrates arose.
The mesosomatic skeleton, which is formed before the prosomatic, consisted, in the first instance, of simple branchial bars segmentally arranged, which were connected together by a longitudinal subchordal bar, situated laterally on each side of the notochord. ”

“ Before searching for any evidence of a similar tissue in any invertebrate group, it is advisable to consider the other portion of the cartilaginous skeleton of Ammocœtes, which consists of the trabeculæ, parachordals and auditory capsules—the basi-cranial skeleton—and is composed of hard, not soft cartilage. ”

“ The answer to this question is most significant: only one animal among all those at present known possesses a cartilaginous skeleton, which is directly comparable with that of Ammocœtes, and here the comparison is very close—only one animal among the thousands of living invertebrate forms, and that animal is the only representative still surviving of the palæostracan group, which was the dominant race when the vertebrate first made its appearance. ”

“ The trabecular floor of the brain does not long remain simple. Its sides grow vertically upwards, forming a lateral wall for the brain, in which in the higher types, two regions may be distinguished, viz. an alisphenoidal region behind, growing out from what is known as the basisphenoidal region of the primitive trabeculæ, and an orbito-sphenoidal region in front, growing out from the presphenoidal region of the trabeculæ. ”

“ The structure of this hard cartilage of the trabeculæ and auditory capsules resembles that of the soft, in so far that it consists of large cells with a comparatively small amount of intercellular substance. Schaffer, who has described it lately, considers that it is a nearer approach to hyaline cartilage than the soft, but yet cannot be called hyaline cartilage in the usual sense of the term. Its peculiarities and its differences from the soft are especially well seen by its staining reactions. ”

“ I entirely agree with Schimkéwitsch that the nests of cartilage-cells are much more extensive in, and indeed nearly entirely confined to, these two lateral trabeculæ in the entosternite of Hypoctonus. Ray Lankester describes in the entosternite of Mygale peculiar cell-nests strongly resembling those of Hypoctonus, and he also states that they are confined to the lateral portions of the entosternite. ”

“ Among the whole of the invertebrates at present living on the earth, is there any sign of an internal cartilaginous skeleton that will give a direct clue to the origin of the primitive vertebrate skeleton? ”

“ The strings of the racquet are represented by a thin membrane, in the centre of which the position of the infundibulum (Inf.) of the brain can be clearly seen. In an earlier stage of Ammocœtes the two trabecular horns do not meet, but are separated by connective tissue, which afterwards becomes cartilaginous.
As far, then, as the topography of this basi-cranial skeleton is concerned, the striking points are—the shape of the trabecular portion, diverging as it does around the infundibulum, and the presence on the parachordal portion of the two large auditory capsules. ”

“ In no single animal is there any evidence that the foremost arch, the mandibular, is a true branchial arch. As low down as the Elasmobranchs it becomes divided into two elements which form respectively the upper and lower jaws ”

“ The somatic muscular sheet with the superjacent skin can be stripped off very easily owing to the vascularity and looseness of the tissue immediately underlying it. When this is done the branchial basket-work comes beautifully into view as is seen on the right-hand side of Fig. 54. It forms a cage within which the branchiæ and their muscles lie entirely concealed. ”

“ The rest of the primitive vertebrate skeleton arose in the prosomatic region, and formed a support for the base of the brain. This skeleton was composed of hard cartilage, and arose in white fibrous tissue containing gelatin rather than mucin. ”