THE LIMBS. 45 
THE LIMBS. 
Though the body of the embryo begins to assume definite form as soon as it is 
folded and nipped off from the rest of the ovum, it does not present any distinguish- 
able human characteristics until the anterior and posterior limbs are formed. There 
are no traces of these before the third week of intrauterine lite when two longi- 
tudinal ridges, the Wolffian ridges, are developed, one on each lateral surface of the 
body, just external to the outer margins of the protovertebral somites, and opposite 
the line of the intermediate cell mass. The rudiments of the fore and hind linbs 
are discernible, almost from the first, as shght prominences of the Wolffian ridges, 
and in the fourth week they project as bud-like outgrowths in the thoracic and 
pelvic regions respectively. The development of the fore-limb or arm is through- 
out slightly in advance of that of the hind-limb or leg. At the fourth week each 
limb-bud is a flattened semi-lunar projection, as long as it is broad, with dorsal and 
ventral surfaces and anterior or preaxial, and posterior or postaxial borders. As 
growth proceeds, the elongating limb-buds are bent ventrally, and in the fifth 
week two transverse furrows, on the ventral aspect of each, indicate the positions 
of the joints and the division of each hmb into three segments— distal, middle, and 
proximal—representing the hand, forearm, and arm in the upper limb, and the foot, 
leg, and thigh in the lower limb. The terminal or distal segments are broad flat 
plates with rounded margins, but each is soon divided into a somewhat enlarged 
basal part, and a thinner and more flattened marginal part. It is where these two 
parts are continuous that the rudiments of the digits appear. They become 
distinguishable about the end of the fifth week as small lobes which gradually 
extend outwards. In the fore-limb the fingers project beyond the margin of the 
hand-segments in the sixth week, but the toes do not reach the margins of the foot 
till the early part of the seventh week. The nails appear at the third month, and 
reach the ends of the digits at the sixth month. 
In the primary position of the limbs the elbow and the knee appear alike to be 
directed outwards, but this is soon altered. At the end of the sixth week each 
limb undergoes a partial rotation, the direction of which is different in the fore and 
hind limbs respectively. In the former the elbow is turned backwards, the ventral’ 
surface therefore becomes anterior, and the preaxial (thumb) margin is directed 
outwards; in the hind-limb the knee is turned forwards and the ventral surface of 
the limb becomes posterior, whilst the preaxial (great toe) margin is directed 
inwards; thus in the adult the anterior surface and outer border of the upper 
extremity correspond with the posterior surface and inner border of the lower 
extremity, whilst obviously the posterior surface and inner border of the former 
are homologous with the anterior surface and outer border of the latter. 
Each limb-bud may be regarded as an extension from a definite number of the 
segments of the body; it contains a core of mesoderm, and the anterior or ventral 
primary divisions of the corresponding spinal nerve segments are apparently 
prolonged into it. 
The central part of the mesoderm, except in the regions of the joints where 
cavities appear, is condensed and then converted first into cartilage and afterwards 
into bone. The proximal part of the bony skeleton of each limb, the limb girdle, 
is not however developed in the limb-bud, but in the body wall at its base. The 
more superficially situated mesoderm is transformed into muscles and subcutaneous 
tissues, the extensor muscles appearing on the dorsal and the flexor muscles on the 
ventral aspect. 
As the nerve trunks pass into the free portion of the limb they bifurcate, the 
branches passing respectively to the dorsal or extensor aspect of the limb, and to 
its ventral or flexor aspect. 
Apparently in mammals the whole of the mesodermal core of each limb-bud is 
formed from the somatic mesoderm of the lateral plates. If this is the case the 
muscles of the limbs differ in origin from those of the back, for the latter are 
developed from the muscle-plates of the protovertebral somites. In lower verte- 
brates (cartilaginous fishes) buds are given off to the limbs from the muscle-plates 
Be * 
