30 



NATURE 



\May 8, 1879 



five o'clock the do^ saunters leisurely down the road till he 

 meets the stage, he then bounds back to the poultry-yard, 

 catches chicken's, bites their heads off", and takes them to the 

 cook ! The number of chickens he kills bears a relation to the 

 number of passengers he saw in the stage. 

 ' A gentleman who was stopping at the hotel for a few days 

 went into the woods one afternoon with a gun. When he 

 returned the dog came to him in much excitement to see what 

 game he had taken. Finding his hands and his bag empty the 

 dog ran into the forest and returned in less than an hour with a 

 bird, which he gave with an air of compassion to the unskilful 

 hunter. W. D. Gunning 



Waltham, Mass., April i8 



ON THE EVOLUTION OF THE VERTEBRATA ' 



SEVERAL theories of the vertebrate skeleton have 

 been promulgated during the last century, some of 

 which have since been abandoned, and others greatly 

 modified. About a quarter of a century ago, three great 

 stumbling blocks were removed from the study of animal 

 forms, by the discovery that the cell- wall was not essential 

 as inclosing sarcode, by the removal of the old concep- 

 tions about the origin of species, and by the rejection of 

 the vertebrate theory of the skull in its older and grosser 

 form. In the present course, the lecturer wishes to give 

 both an analytic and synthetic account of the vertebrate 

 skeleton, to see if a consistent history cannot be given of 

 every cartilage, bone, and joint, in the higher types. 



A vertebrated animal is constructed of a chain of 

 segments similar to each other, which are obsolete in 

 the head, and in each of which there is a smaller dorsal 

 tube, through which the continuous neural axis runs, and 

 a larger ventral tube, which contains the digestive organs, 

 heart, and main blood vessels. The neural axis swells 

 into three main vesicles in the head, giving rise to the 

 fore, mid, and hind brain. The skeletal structures are 

 formed on a single median axis, the notochord, which 

 lies directly beneath the neural axis, and which is 

 arrested in the head close behind the fore-brain. The 

 barrier, however, which woukl stop the growth forwards 

 of the notochord, is not developed when its apex shrinks. 

 By the time the embryo is fairly formed, a fold of the 

 palatal skin has given rise to a sac which opens into the 

 lower and hinder part of the fore-brain. This sac is the 

 pituitary body, the manner of the development of which 

 has been clearly made out by Mr. Balfour, in the sharks 

 and skates, and corroborated by the lecturer in the snake, 

 lizard, and green turtle. 



The mouth and posterior aperture do not exist at first, 

 but are formed afterwards as involutions, which, in the 

 latter case, at any rate, are not terminal, the alimentary 

 tract extending behind the anus, and possibly in front of 

 the mouth also in ancient forms. The visceral clefts 

 appear as slits in the wall of the pharynx, and in the 

 aquatic forms, give rise to the gills, while in the higher 

 types (amniota), all of them close up but one, which 

 remains as the tympano-eustachian cavity. The vertebrae 

 alternate with the primary segmental masses. Each 

 centrum, as it chondrifies, constricts the notochord, but 

 there is usually some remnant of it to be seen in the adult 

 in the invertebral spaces. The walls of the head are 

 large and continuous, and its lower arches are generally 

 small, clefts appearing between them. Both the arches 

 and clefts become greatly modified in the adult, especially 

 in the higher types. Thus the upper jaw is probably due 

 to the modification and blending together of two or three 

 pairs of arrested arches. 



Besides the axial skeleton, a cartilaginous skeleton is 

 developed immediately under the skin, and thus there is 

 both a cartilaginous exo- and endoskeleton. The exo- 

 skeleton gives rise to the labial and extra-branchial carti- 

 lages, the limb arches and their limbs, and the inter- 

 calary cartilages of the median fins of fishes. 



» Abstract of Prof. Parker''; Hunterian Lecture^, delivered at the College 

 of Surgeon's, commenc ng r n February lo. 



The bony parts of the skeleton are classified according 

 to their relation to the axial or extra-axial cartilaginous 

 skeletons. All bony scales, scutes, or sub-cutaneous bony 

 plates, or tracts, are classified as exoskeletal ; ossifica- 

 tions of the endoskeletal cartilage or its perichondrium 

 are, of course, endoskeletal. Unfortunately for science, 

 the extinct lower forms of the Vertebrata had their endo- 

 skeletons but slightly ossified, and thus only the outworks 

 of their structure are left to us, as is the case with many 

 of the Ganoids of the old red sandstone. The lowest of 

 these, however, were half way up the vertebrate scale, if 

 we compare them with the lancelet. Of existing brain- 

 bearing fishes the lamprey and hag are the lowest, but 

 man scarcely stands at a greater distance from them than 

 they do from the lancelet, which, as far as we know at 

 present, stands alone in creation. 



Until we can connect the known Vertebrata, or at least 

 their embryos, with the worm-like Invertebrata, the former 

 will continue to be a very anomalous group. The diffi- 

 culty is not with man ; in him we have organ for organ 

 and part for part, and he is better than a beast only by 

 reason of something that cannot be demonstrated by the 

 anatomist as such. 



-/-' i'^:,*>^\\V?•^T 



A -«*• 



ir.'M ■ 



yj.i 



^,anus: Alt, auditory capsule; B^'T , branchial clefts; i^r*"*, branchial 

 arches; ^, eye ; e.^a, ethmo-palatine ; e.pg^, epi-p'erygoid ; /,b, fore- 

 brain; k.b, hind brain; H.br, hyo-branchial cleft; hy, hyoid arch 

 /,, lacrymal cleft; M, rnJUth; ?«.^, mid-brain; mn, mandible; My^ 

 myelon: N, nostril; Nc, notochord; p.rh^ pro-rhinal ; T, tympanic 

 cleft. The Roman figures indicate the nerves. 



The above diagram represents an ideal vertebrate, the 

 oral, lacrymal, and nasal clefts being taken as homologous 

 with the post-oral clefts. This theory seems probable 

 both from the author's researches on the visceral arches 

 and clefts, and those of Milnes Marshall on the nerves. 

 The seven branches of the vagus (x'-') are here shown as 

 separate nerves, and the hind brain as a series of enlarge- 

 ments. 



As the relation of the endoskeleton to the exoskeleton 

 does not usually seem to be properly understood, it may 

 be as well to say a few more words about it. On the 

 whole the foundations of the internal skeleton are laid in 

 cartilage, and of the external in bone, which is formed by 

 the ossification of fibrous tracts. The cartilage as a rule 

 also ossifies, and this inner or cartilage bone has, so to 

 speak, an organic affinity for the outer or membrane bone. 

 But there are several things in the vertebrate exoskeleton 

 that are formed of cartilage, as already mentioned ; and 

 in the endoskeleton the cartilage is often suppressed in 

 certain parts, bony substance, formed in fibrous tissue, 

 replacing it. Indeed, unossified fibrous tracts often take 

 the place of cartilage. The welding together of parts 

 originally distinct makes the matter much more compli- 

 cated. No inherited elements are rejected by the morpho- 

 logical force ; they are only kept from growing into special 

 tissues until needed. Thus the rich growth of the human 

 brain is covered in with a stout masonry that is merely 

 made up of the inner layer of old ganoid plates, and the 

 cartilages of the human nostril are inheritated from some 

 ancient sucking fish, while the outer ear once figured, 

 speaking morphologically, as the blow-hole of some 

 Silurian shark. 



A word or two must now be said about the different 

 1 kinds of ossification. When the perichondrium, or clothing 



