Development of the vertebral column.-The vertebral column is developed around the notochord, except at the anterior end of that structure, which is imbedded in the basis cranii. It is formed from protovertebral mesoblast. The outer part of each protovertebra is transformed into a muscle-plate, and thus the original mesoblastic segmentation is maintained. The inner parts of the protovertebræ do not, however, remain distinct, but blend with one another on each side of the neural canal to form a longitudinal mass, which extends to the side of and subsequently encloses the notochord, and finally sends dorsal prolongations over the neural canal, so that this also receives a continuous mesoblastic investment, forming the membrana reuniens superior of Remak. The investment is only incomplete opposite the points where the nerve roots are connected with the spinal cord, and in it there is no sign of any differentiation into vertebræ. It is continuous with a similar investment within the cranium, which extends in front of the notochord into the fronto-nasal process. The mass of mesoblast, which thus encloses the notochord and neural canal, is often spoken of as the membranous vertebral column and cranium, but it represents much more than the cartilaginous and bony structures of those parts, all the investing membranes of the cord and brain and the ligaments of the vertebræ being also derived from it. From it septa pass between the muscle-plates and serve to give attachment to the developing muscle-fibres.

The first appearance of the permanent vertebræ is in the form of cartilage, which becomes formed in this mesoblastic investment on either side of the neural canal, nearly opposite the interval between each two muscle-plates, to form the neural arch. This part of the vertebra therefore alternates with the original mesoblastic somites as represented by the muscle-plates.

According to Froriep, the lateral halves of each cartilaginous neural arch become united below the notochord before the appearance of the rudiments of the cartilaginous bodies, and the latter appear as median accumulations of cartilage, immediately posterior to the hypochordal part of the cartilaginous arch. In most of the vertebræ this hypochordal part of the arch soon disappears as a distinct structure, but in the atlas vertebra the primitive condition is maintained.

The serial arrangement of the musculature represents phylogenetically the original segmentation of the vertebrate body. The segmentation of the vertebral column, on the other hand, has been arrived at later, and has been carried out in dependence upon the muscular segmentation.1

The cartilage makes its appearance on the fourth day in the chick, on the eleventh or twelfth day in the rabbit, and in the fourth or fifth week in man (Kölliker). It is completed by the sixth or seventh week, soon after which ossification commences. To form the intervertebral discs, the mesoblast between the bodies of the vertebræ acquires a fibro-cartilaginous character, while at the same time the notochord, which gradually elsewhere becomes reduced in size and eventually disappears, here undergoes enlargement, and its cells form an irregular network in the central intervertebral pulp. Its remains are found at all periods of life in the middle of the discs (Luschka).

In the osseous fishes there is an intervertebral dilatation of the notochord, the growth of which proceeds to such a considerable extent as to give rise to a mass of soft gelatinous substance, which occupies the conical hollows of the biconcave vertebral bodies. But in birds, reptiles, and amphibia, where synovial articulations are developed between the vertebral bodies, the notochord soon disappears from the intervertebral spaces, although its remains are seen for some time in the bodies themselves (Gegenbaur).

In mammals, the notochord is constricted within the cartilaginous vertebral body, but dilated in the intermediate parts of this rod, the whole cord being moniliform. At a somewhat later period small dilatations are also to be seen in the epiphysial cartilages (fig. 193).

1 A. Froriep, "Zur Entwickl. der Wirbelsäule, &c.," Archiv f. Anatomie, 1883 and 1886.

Ribs and Sternum.-The ribs are formed by separate cartilaginous transformation in extensions of the protovertebral mesoblast between the muscle-plates. According to some, they grow out from the cartilaginous vertebræ, but become separate before ossification begins. Similar deposits are formed in connexion with the other vertebræ (except the coccygeal in man), but they here become united by ossification with and form parts of the vertebræ (see Osteology, Vol. II.). At their ventral

Fig. 192. SECTIONS OF THE VERTEBRAL COLUMN OF A HUMAN FETUS OF EIGHT WEEKS.

(From Kölliker.)

A, transverse longitudinal section of several vertebræ. 1, 1, chorda dorsalis, its remains thicker opposite the intervertebral discs; 2, is placed on one of the bodies of the permanent vertebra; 3, on one of the intervertebral discs.

B, transverse horizontal section through a part of one dorsal vertebra. dorsalis in the middle of the body; 2, arch of the vertebra; 3, head of a rib.

1, remains of the chorda

SHEEP'S EMBRYO.

(Kölliker.)

l.a, l.p, anterior and posterior ligaments; l.i, intervertebral ligament; k, k', cartilaginous ends of two vertebral bodies, w, w': c, enlargement of notochord in the ligament; c', c", enlargements in the cartilaginous ends of the vertebræ.

extremities the first seven (thoracic) cartilaginous ribs become united on either side into a longitudinal cartilaginous plate, and this afterwards joins its fellow of the opposite side to form the sternum (manubrium and body). The xiphoid is of later formation (Parker). This mode of development of the sternum explains many of the malformations in the shape of fissures of the sternum of different gradation which have been observed.

The Limbs. The limbs arise as outgrowths from the lateral part of the trunk in the thoracic and pelvic regions in the third day in the chick and in the third and fourth week in the human embryo. They appear as flattened semilunar thickenings of the parietal mesoblast covered by epiblast, budding out from a lateral ridge which is seen in the early embryo near the line of cleavage of the mesoblast and close to the outer margins of the muscle-plates, and several of which subsequently send prolongations into each limb (1); they are therefore connected with several mesoblastic somites, as is also indicated by their nerve supply.

1 This is the case in elasmobranchs (see fig. 194, from Balfour), but, according to Paterson,

Each limb consists of a part which is sunk in the substance of the lateral ridge, and in which the thoracic or pelvic girdle becomes developed, and of a free or

Fig. 194. TRANSVERSE SECTION THROUGH AN ANTERIOR PART OF THE TRUNK OF AN EMBRYO OF

SCYLLIUM. (Balfour.)

sp.c, spinal cord; sp., ganglion of posterior root; ar, anterior root; dn, dorsal; sp.n, ventral branch of spinal nerve; mp, muscle plate; mp', part of muscle plate already converted into muscle; mp.l, part of muscle plate extending into the limb; nl, nervus lateralis; ao, aorta; ch, notochord; 83.9, sympathetic ganglion; ca. v, cardinal vein; sd, segmental duct; st, segmental tube; du, duodenum; hp.d, junction of hepatic duct with it; pan, rudiment of pancreas connected with another part of duodenum; ume, opening of umbilical canal (vitelline duct).

projecting part (fig. 195). This is at first quite simple, and represents the distal segment of the limb (hand or foot). The other two segments (forearm and leg; arm and thigh) are successively marked off between it and the girdle by the development of transverse furrows representing the joints (fifth and sixth week). At about the same time four notches appear in the flattened distal extremity, marking off the intervals between the fingers and toes, and the middle segment (fore-arm or leg) begins to be flexed upon the proximal (arm or thigh), the

the limbs do not receive such prolongations from the muscle-plates in birds and mammals; the muscles develop in situ from previously indifferent mesoblast.

concavity looking forwards in the upper limb and backwards in the lower limb. The limbs also come to be folded ventrally against the body of the embryo.

From the manner in which the flattened limb-bud grows out from the lateral ridge, it is obvious that its surfaces must at first be dorsal and ventral. The dorsal surface afterwards becomes extensor and the ventral flexor. The anterior edge is respectively the radial and tibial; the posterior, the ulnar and fibular. As

Fig. 195.-OUTLINES OF THE ANTERIOR EXTREMITIES

OF HUMAN EMBRYOES AT DIFFERENT AGES. (After A
His.)

A, at four weeks; B, at five weeks; C, at seven weeks;
D, at nine or ten weeks.

development proceeds, a half rotation occurs in opposite directions in the two limbs, resulting in the middle flexure (elbow, knee) being directed forwards in the upper, backwards in the lower limb.

The bones of the limbs are laid down as cartilages which appear as separate differentiations of the more centrally placed mesoblast,

B

D

a portion of mesoblast remaining for a time undifferentiated opposite each synovial articulation. Within this a cleft subsequently appears, and enlarges to form the synovial cavity, the mesoblast which bounds the cleft developing eventually into the synovial membrane and capsular ligaments of the joint.

The cranium.-In the head the notochord extends as far forwards as the mid-brain. Here also it is invested by a continuous mass of mesoblast, which sends lateral prolongations over the neural canal as in the trunk (membrana reuniens).

Fig. 196.-DIAGRAMS OF THE CARTILAGINOUS CRANIUM. (Wiedersheim.)

A, First stage.

Ch, notochord; Tr, trabeculæ cranii; P.ch, parachordal cartilages; P, situation of pituitary body; N, E, O, situations of olfactory, visual and auditory organs.

B, Second stage.

B, basilar cartilage (investing mass of Rathke); S, nasal septum and ethmoidal cartilage; Eth, Eth', prolongations of ethmoidal around olfactory organ, completing the nasal capsule; Ol, foramina for passage of olfactory nerve fibres; N, E, O, Ch, Tr, as before.

The main difference in development between the cranium and vertebral column consists in the fact that no separate cartilaginous deposits to form vertebræ occur

166

FORMATION OF THE VISCERAL SKELETON OF THE HEAD.

within the head, nor can any parts be distinguished which strictly represent vertebræ. The cartilage of the basis cranii makes its appearance in the form of two longitudinal bars lying on either side of the notochord (parachordal cartilages), and of two other bars (trabecula cranii of Rathke) which embrace the pituitary body, and which become united together in front and with the parachordals behind to form a continuous mass, which posteriorly completely invests the notochord (fig. 196). The cartilaginous basis cranii may therefore be distinguished into the parachordal and prechordal parts. Of these the first represents the basi-occipital and basi-sphenoid, the second the presphenoid and ethmoid portions. From the basis cranii continuous cartilaginous plates grow on either side over the cerebral vesicles to a greater or less extent in different animals, least in mammals, where only the occipital region becomes thus roofed in by cartilage. Anteriorly the united trabeculæ cranii stretch forwards into the fronto-nasal process, where they form the ethmoid cartilage and nasal septum, besides enclosing the nasal pits externally (cartilaginous nasal capsule). From the sides of the presphenoid portion the orbito-sphenoids (lesser wings), containing the optic foramina, are developed, and from the sides of the basi-sphenoid,

Fig. 197.-VIEW FROM BELOW OF THE CARTILAGINOUS

CRANIUM WITH ITS OSSIFIC CENTRES IN Α HUMAN

FETUS OF ABOUT FOUR MONTHS. (After Huxley.) The cartilage is dotted to distinguish it from the bone which is shaded with lines.

b.o, basi-occipital; a.o, lateral occipitals; f.m, foramen magnum; o.c, o'.c', bony deposits in the periotic capsule; p.s, post-sphenoid; pr.s, pre-sphenoid; 0.8, orbito-sphenoid; s.n, septum nasi.

the greater wings or alisphenoids. A cartilaginous capsule, connected with the parachordal portion of the basis cranii, invests the otic vesicle (periotic capsule). Within this, bony centres are eventually formed, which unite to form the petro-mastoid. In the human embryo, chondrification begins in the fourth or fifth week in the basilar portion of the skull, and is nearly completed by the eighth week.

Formation of the visceral skeleton of the head: cartilaginous bars cf the visceral arches.-A cartilaginous bar extends from the periotic capsule and basis cranii, within each of the first three visceral arches, and passes forwards to meet its fellow in the middle line. The bar of the mandibular arch is known as Meckel's cartilage. It is visible in all sections of the foetal jaw up to the seventh month. Its proximal end is attached at first to the basis cranii, afterwards to the periotic capsule; its distal end joins that of its fellow in the middle line of the lower jaw. Only near this conjoined part does Meckel's cartilage take part in the formation of the lower jaw bone, the greater part of this bone being developed by ossification at several places in the connective tissue around the cartilage. In some animals a short cartilaginous bar is formed in the maxillary process (palato-pterygoid bar, fig. 198, A, ppg). Close to it the palatine and pterygoid bones are formed in membrane, but the bar itself entirely disappears. The second or hyoid bar arises from the skull close behind the attachment of Meckel's cartilage, and passes along the second arch. It disappears in part, but in part is converted into the styloid process, stylo-hyoid ligament, and lesser cornu of the hyoid bone. The body of the hyoid bone (basi-hyal) is an intermediate formation between the second and third arches. The bar of the third arch is known as the thyro-hyoid. Its lower end forms the greater cornu of the hyoid bone; but its attachment to the skull early disap