332 



NA TURE 



iFcb. 3, 1887 



morphosis, are as truly, if not as remarkably, distinct from eacli 

 other in tlieir larval as in their imago form ; as much so as is the 

 case in insects, or any other of those invertebrate types that are 

 truly metamorphic. 



\{ many of the existing Vertebrata are metamorphic noT.ti, is it 

 not very probable that they were all metamorphic once ? 



The fact that we have even now such forms as the larval 

 lamprey (or Ammocoete), the lirv.-e of Ganoids and Dipnoi, and 

 the tadpoles of newts and frogs, suggests to me the possibility of 

 the existence of huge swarms of low proto-Vertebrata in the 

 early ages of the inhabited planet. 



If such proto-vertebrate forms existed, then it is quite suppos- 

 able that a metamorphosis may, from time to time, have taken 

 place, of this and that jtiasiAatYnX form into archaic reptile, 

 ancestral bird, or primitive mammal. 



I am not afraid that anyone familiar with the development, 

 structure, and habits of the existing Amphibia will see any diffi- 

 culty in the passage of a metamorphic into a so-called non- 

 metamorphic type, during time, and under the pressure of new 

 outward conditions — when the dilemma offered to the supposed 

 low vertebrate was Transforni or perish. 



To me it seems that the creature's necessity was Nature's 

 opportunity ; and that, during long ages, the morphological 

 force had accumulated in those low forms an enormous surplus- 

 age of unused energy, which, in the ripeness of time, blossomed 

 out into this and that new and noble type. 



Of all the types of Vertebrata, there is none like the bird of 

 high degree for illustrating what Prof. Huxley calls " the three- 

 fold law of evolution,"^ namely, overgrowth of 'ome parts, 

 starvation and even death of others, and fusion of parts origin- 

 ally distinct. 



No kind of vertebrate whatever presents to the os'eologist so 

 hopeless an enigma in the adult skeleton, as that of the bird ; in 

 the overgrowth of certain parts, the abortion or suppression of 

 others, and the extensive fusion of large tracts of skeletal 

 elements. 



Hence this class has largely acted upon the morphological 

 mind ; the "comparative anatomist" has, of necessity, under- 

 gone evolution into the " morphologist," and the latter has had 

 to be refined and developed into the "embryologist. " 



In the bird class we meet with this remarkable phenomenon, 

 namely, that the swiftest creatures by far that inhabit the earth 

 have had, for the purposes of their most consummate mechan- 

 ism, the greatest loss of freedom of the individual parts of the 

 skeletal framework. 



Between the pigeon, on one hand, above, and tlie emu, on 

 the other, below, there are several families of related birds ; liut 

 there is no direct superposition — they are obliquely above or 

 below each other. 



Amongst the Carinatx, which lie in the interm^di.ate space, 

 there is none better for the purposes of study than the common 

 fowl ; to this type I have devoted most attention, and have now 

 worked out the limbs in as many stages as I formerly did tlie 

 skull. 



I can now give an account of the vertebral column with the 

 ribs and sternum, the limb-girdles and limbs, from the end of 

 the seventh day of incubation ; by which time the hyaline car- 

 tilage is perfect, and cert.ain even of the bony tracts are begun. 



The fowl is an intermediate form between the emu and the 

 pigeon, but most akin to the latter. I shall now confine myself 

 to what is seen in the development of the skeleton (excluding 

 the skull) in this medium type. 



The vertebral column, at the end of a week's incubation, is 

 formed of hyaline cartil.age ; up to the end of the true sacrals, 

 the notochord is completely invested with cartilage ; but, behind 

 those four segments, only at the sides. 



The notochord has its constrictions in the middle of each 

 centrum, and is most dilated at the intercentra. 



The neural arches do not nearly meet above ; the atlas is in 

 four pieces — a superficial and an inner piece to the centrum, and 

 a pair of arch-rudiments ; the inner segment of the centrum 

 becomes the odontoid process of the axis. 



Between the axis and the first true sacral, all the vertebrii: 

 have separate tibs ; in the cervical region, except near the dorsal 

 region, there are small styloid cartilages lying horizontally, 

 which have their head, or thick end, wedged in between the 

 upper and lower transverse processes. Near the dorsals they 



' See his paper "On the Application of the La-.vs of Evolution to the 

 Arrangement of the Vertebrata. and more especially uf tlie M.immalia ' 

 (Zool. Soc. Proc, December 14, 18S4, pp. 649-66i0- 



are transversely placed, and then begin to develop a descending 

 process . 



The first vertebra of this stage with complete ribs becomes, 

 by absorption of the lower part of the arch, the last cervical in 

 the adult. Behind the twenty pre-sacrals there are fifteen sacrals, 

 and this series has its subdivisions. 



The first develops ribs (it is dorso-sacral), the ne.Kt three 

 develop minute but distinct ribs, like those near the lower part 

 of the neck ; these are lumbosacral. Then come the four 

 sacrals with no ribs, and then the seven uro-sacrals, the first two 

 of which have rib-bars that ossify sep.irately, below the upper 

 transverse processes, which latter form a complete series from 

 the third cervical to the last free caudal segment. 



Of those there are five ; then come five more paired imperfect 

 rudiments, clinging to the terminal part of the notochord. 



At the end of the eighth day there are six of these, w ith the 

 last elong.ated, and the notochord projecting behind far enough 

 for three or four more rudiments. 



At the end of the tenth day the vertebral chain has undergone 

 a great change. The atlas is still composed of four distinct 

 pieces of cartilages, but the ribs have become fused above and 

 below with the transverse processes, and the notochord is now 

 most constricted at the intercentra. 



Besides this, in the pre-sacrals, it is constricted in two places 

 within each centrum ; so that each centrum in the modern bird 

 corresponds to three subdivisions of this axial chord. 



For two or three days there is evidence of an archaic sub- 

 divisinn of the notochord into three times as many vertebral 

 divisions as are made now in the modern bird. 



In the sacral the constrictions are fewer ; they are only at the 

 intercentra, and in the middle of the centrum. 



The only absolutely necessary part of the sternum is that 

 where the sternal ribs are attached ; that is a very small part, 

 and the rest is for the attachment of the huge muscles that act 

 upon the wings, and for the obliqui and recti abdominis. 



The limb-girdles are each in three pairs of distinct cartilages. 

 In front, the scapula, the minute pre-coracoid, the coracoid ; 

 behind, the ilium, pubis, and ischium ; the pre-pubis is part of 

 the ilium, and that has two regions, the pre-ilium and the post- 

 ilium. 



These parts in the bird are not continuous tracts of cartilages, 

 ossified by several centres, but are distinct, first as cartilages, 

 then as bony tracts ; those of the shoulder keep distinct ; those 

 of the hip soon coalesce. 



The wings at the end of the seventh day are three-toeJ 

 webbed paws, with all the digits turned inwards. The rods 

 that compose the main part of it are composed of solid cartilage : 

 the humerus, radius, ulna, and first and second metacarpals 

 have a bony sheath round their middle part ; the ends of the 

 digits and the carpals are but partly chondrified. Five carpal 

 nuclei, however, can be made out, and the two proximal nuclei 

 are known to be further subdivided, each into two, in other 

 types ; hence we can already account for seven carpals in the 

 bird, which has only two in the adult, in a free state. 



Moreover, the first digit has two, and the secoiid three pha- 

 langes, the normal number, as in lizards ; the third, which 

 should have four, but in birds has as a rule only one, has now 

 two, as in the ostrich, and a few other birds ; there is no sign at 

 the end of the seventh or even of the eighth day of incubation 

 of any more than three digits, but we have in the wrist an inter- 

 medio-radiale, a centralo-ulnr.re, and three distal carpals, 

 answering to the three developed metacarpals. The digits up 

 to the end of the eighth day are rounded and flattish, and are 

 quite like those of a young newt or fi og. But in two days more, 

 at the end of the tenth day, the wing has almost acquired the 

 adult form ; and one more bony centre, that of the first meta- 

 carpal, has appeared. The overgrowth of the second distal 

 carpal and the second metacarpal, with its large and dilated 

 digit, has arrested the distal carpal of the first or short digit, the 

 "pollex." This is the last nucleus tochondrify. It isstillavery 

 small, limpet-like disk of cartilage, and is now only to be seen on 

 the flexor face of the manus, inside the top of the second meta- 

 carpal ; the distal carpal of the third ray is also small as com- 

 pared with the large crescentic second distal nucleus. It is thrown 

 on to the ulnar or outer side of the manus by the overgrowth of 

 the middle rod and its carpal. The curve of the digits at their 

 end is now, not inwards, or to the radial side, but outwards ; 

 and the two developed distal segments form now the core of 

 two claws, that of the first, or pollex, being of considerable 

 length. 



