DEVELOPMENT OF BLOOD, &c., IN MAN AND MAMMALS 411 



the mode of construction being always the same. The long bones forming the arms and legs (as also those forming 

 the vertebral column) are separated by layers of cartilage, and this circumstance, taken in conjunction with the 

 successive appearance of the centres of ossification, favours the argument for design ; certain portions of the bones 

 becoming ossified to support weight, other parts remaining cartilage to prevent concussion or shock. Professor 

 R. Owen states the argument thus : " The young lamb or foal can stand on its four legs as soon as it is born ; it 

 Ufts its body well above the ground, and quickly begins to run and bound. The shock to the Umbs themselves is 

 broken and diminished at this tender age by the division of the supporting long bones, by the interposition of the 

 cushions of cartilage between the diaphyses and epiphyses." 



The formation of the vertebral column and its muscles is deserving of special notice because of its central 

 position and segmented nature. It consists in the embryo of symmetrical, square-shaped, cellular and cartilaginous 

 masses, which are virtually repetitions of each other. The vertebrae, as development proceeds, afford a protective 

 osseous canal for the safe lodgment of the spinal cord. They also afford support for the ribs, and attachment for 

 the muscles. The vertebrae and ribs divide the body into what are practically transverse layers. The young muscles 

 connected with the backbone emphasise this arrangement. " The muscles of the trunk are formed from the proto- 

 vertebrae. These are at first separate masses of mesoblast, the cells of which have at the periphery of the mass a 

 tendency to a radial disposition, whilst towards the centre they are loosely arranged, and may even leave a more 

 or less distinct space unoccupied by cells (proto-vertebral cavity). Whether there be originally a cavity or not in 

 it, the proto-vertebra presently becomes filled up with cells and then forms a fairly compact mass of cells which 

 are most irregularly arranged, but externally (next to the cutaneous epiblast) become regularly disposed into an 

 epithelium-like plate of columnar cells. This is known as the muscle-flate, and in it the original mesoblastic segmen- 

 tation continues to be exhibited. They do not long remain as a single epitheUum-hke layer, for the extremities 

 of this layer fold sharply round and become continuous with a cell-stratum, which immediately lines the internal 

 surface of the columnar layer and forms an inner muscle- plate. It is uncertain whether the cells of this inner muscle 

 plate were derived from part of the columnar layer which has folded over, or whether they spring from other cells 

 of the proto-vertebra. Soon after their appearance as a distinct layer of the muscle-plate they begin to elongate 

 in the sagittal (antero-posterior) direction, and it may presently be observed that they are becoming developed into 

 longitudinal groups or segments of muscle-fibres which stretch between the original intervals between the proto- 

 vertebrae. Although the muscle-plates are originally mainly concerned with the formation of the muscles which 

 move the central skeletal axis, it is probable that all the skeletal muscles both of the trunk and limbs are eventually 

 derived from them. 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 proto-vertebral mesoblast. The outer part of 

 each proto-vertebra is transformed into a muscle-plate, and thus the original mesoblastic segmentation is maintained. 

 The inner parts of the proto- vertebrae 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. The first appearance of the permanent vertebrae 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. The serial arrange- 

 ment 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." 



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 (KoUiker). It is completed by the sixth or seventh week, soon after which 

 ossification begins. To form the inter-vertebral discs, the mesoblast between the bodies of the vertebrae 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 

 inter-vertebral pulp. 



The hmbs, which ultimately form important auxiliaries of the back bone and body, " arise as outgrowths from 

 the lateral part of the trunk in the thoracic and pelvic regions, on the third day in the chick, and in the third and 

 fourth week in the human embryo. They appear as flattened semi-lunar 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 prolonga- 

 tions into each hmb ; they are therefore connected with several mesoblastic somites, as is also indicated by their 

 nerve supply." i 



1 Quain'3 "Elements of Anatomy," edited by E. A. Sehafer, LL.D., F.R.S., and George Dancer Thane, vol. i. part i. 1898, pij. ]i;9-163. 



