Between the upper and lower limb buds, an unsegmented tapering strip of mesenchyme can be distinguished under appropriate lighting in the whole embryo. This mesenchyme is derived from myotomes and probably later forms the ventrolateral body wall musculature. O’Rahilly R., Gardner E., (1975) A mastoid/trapezius pre-muscle mass appears (Carnegie embryo No. 7829). Thought to be of mixed (lateral plate mesenchyme and somite) origin. McKenzie J., (1962)
A hyoid pre-muscle mass is mentioned as being supplied by the chorda tympani branch of the facial VII nerve late in the stage. Barniville H.L., (1915) The primordium of the trapezius, which is still attached to the sternomastoid mass, is said to be innervated in Carnegie embryo No. 3385. McKenzie J., (1962)
Bardeen C.R., Lewis W.H., (1902) describe a mass of mesenchyme from upper cervical level to the mid-thoracic level. It is thought to be the latissimus dorsi pre-muscle mass, serratus anterior/levator scapulae pre-muscle mass, rhomboid, and pectoral pre-muscle mass and teres major/deltoid pre-muscle mass.
Lewis W.H., (1902) also describes a latissimus dorsi/ teres major/ pectoral / serratus anterior/ levator scapulae pre-muscle mass which is continuous with the arm pre-muscle sheath but is also innervated. The rhomboid pre-muscle mass is innervated by a branch of cervical nerve 5.(Carnegie Embryo No. 163) The mastoid/trapezius pre-muscle mass is innervated by spinal accessory and branches of cervical nerves 1-4. Lewis W.H., (1902) describes a pre-muscle mass on the medial side of the pectoral pre-muscle mass and caudal to the infrahyoid pre-muscle mass. The phrenic nerve ends close to it therefore it is thought to be the diaphragm pre-muscle mass. An illustration of Carnegie Embryo No. 163 is given (Fig.8)
Blechschmidt E., (1963) Plate 28, labels a splenius capitis, iliocostalis, longissimus dorsi transversospinalis group, external intercostals and external abdominal oblique.
According to Bardeen C.R., Lewis W.H., (1902), the teres major /deltoid (scapulohumeral pre-muscle mass) described at stage 16 in Carnegie Embryo No. 163, now possesses fibres in Carnegie Embryo No.109. It can be distinguished from infraspinatus by its nerve supply. The latissimus dorsi muscle, levator scapulae muscle and serratus anterior muscle also show evidence of fibres. Pectoralis major possesses fibres but no attachment to ribs. It is still united with pectoralis minor. The pectoral mass obscures biceps brachii and brachialis in an illustration of Carnegie embryo No. 109 (Plate IV). Trapezius is not attached to scapula but the spinal accessory nerve (XI) is seen on its median surface together with cervical nerves 3 and 4. A muscle mass superficial to trapezius in Carnegie Embryo No. 109 is thought to be the platysma according to Lewis W.H., (1902)
Lewis W.H., (1902) describes a latissimus dorsi/ teres major pre-muscle mass which is continuous with the arm pre muscle sheath. The future teres major mass now has a scapular origin and it is inserted into the humerus. No vertebral origin of the latissimus dorsi has yet developed. An illustration of Carnegie Embryo No. 109 is provided and depicts the subscapularis, infraspinatus, and supraspinatus. The latter two are innervated by the suprascapula nerve.
The rhomboid pre muscle mass is described in Carnegie embryo No. 4430 by McKenzie J., (1962) . According to Lewis W.H., (1902) it is connected to thoracic and cervical vertebrae via fascia but has no scapula origin (Carnegie embryo No. 109) The coracobrachalis pre-muscle mass is associated with biceps. Lewis W.H., (1902) mentions that two heads of biceps are distinguishable and it is innervated. (Carnegie embryo No. 109). Likewise, three heads are beginning to appear on triceps and the muscle is part of a mass innervated by the radial nerve. Brachialis is innervated by the musculocutaneous nerve and possesses fibres. It is inserted into the ulna. Supinator is described as arising in conjunction with the extensor mass. It was identified by the direction of fibres and position. Brachioradialis, extensor carpi radialis longus, extensor indicis, extensor carpi radialis brevis are mentioned by Bardeen C.R., Lewis W.H., (1902) as part of a muscle mass and Lewis W.H., (1902) observes innervation by the radial nerve (Carnegie embryo No. 109). Lewis W.H., (1902) also describes a superficial flexor pre-muscle mass that is supplied by the median nerve. From this mass pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis (sublimini) are thought to arise, and a deep flexor pre-muscle mass that is supplied by the median and ulna nerves from which pronator quadratus, flexor carpi ulnaris, flexor pollicis longus, flexor digitorum profundus, interossei, and lumbricals will form. Lewis W.H., (1902) describes a pre-muscle mass supplied by (musculospiral) radial nerve branches which is thought to later form extensor pollicis brevis, abductor pollicis longus and extensor pollicis longus Carnegie Embryo No. 109. A superficial extensor muscle mass, supplied by branches of the posterior interosseus nerve, probably represents the future extensor digitorum, extensor carpi ulnaris and extensor digiti minimi. The internal abdominal oblique, transversus abdominis, external abdominal oblique, external intercostals and internal intercostals are mentioned by Bardeen C.R., Lewis W.H., (1902) in a description of Carnegie Embryo No. 109 as a ventro-lateral muscle mass. Rectus abdominis is mentioned as a mass situated more ventrally.
Longissimus dorsi, spinalis and iliocostalis are mentioned by Bardeen C.R., Lewis W.H., (1902) in a description of Carnegie Embryo No. 109 a dorsal muscle column. In an illustration of Carnegie Embryo No. 109 some muscles of the lower limb are labelled; obturator internis, quadriceps femoris, hamstring, soleus, gastrocnemius, peroneus longus peroneus brevis. Bardeen C.R., Lewis W.H., (1902) Plate V Fig.E.
Lewis W.H., (1902) describes the muscles in the shoulder and scapula region of Carnegie Embryo no. 43 in great detail. Coracobrachalis is still associated with the short head of biceps. it possess a humeral insertion but arises from coracoid process together with biceps. Biceps inserts into condensed tissue associated with the radius. Three heads are recognisable on triceps and the muscle is inserted into the olecranon process of ulna. The supinator is takes origin on the lateral condyle of humerus and ulna and has developed an insertion into radius. Infraspinatus is still united with teres major but origin on scapula and insertion into humerus has developed. Supraspinatus origin on scapula and insertion into humerus established. Subscapularis has origin on scapula and insertion into humerus and has separated from teres major. Serratus anterior has developed attachments to ribs and scapula. By this stage the pectorals have separated into minor and major and attachments to the clavicle, ribs, coracoid process and humerus have developed. Trapezius has developed insertions into clavicle, acromion and origin in fascia attached to cervical and thoracic vertebrae.
Lewis W.H., (1902) described a superficial flexor pre-muscle mass that is supplied by the median nerve in an embryo of the previous stage. The muscles within this mass have become now become more defined by the development of some origins and insertions. Pronator teres extends to the middle of the radius shaft, from flexor carpi radialis a tendon of condensed tissue fuses with tissue at the end of the second metacarpal. The muscle primordia within deeper, flexor, pre-muscle mass supplied by the median and ulna nerves have also become more defined. Pronator quadratus connects the distal ends of the radius and ulna. Flexor carpi ulnaris is distinct with its origin on the medial condyle of humerus and olecranon and insertion into pisiform. Flexor pollicis longus is closely associated with flexor digitorum profundus from which five tendons have formed in the carpal region which unite with condensations at the ends of digits. The lumbricals have distinct, short fibres ending in tendons that fuse with condensed tissue on digits.
An extensor muscle mass supplied by the radial nerve represents extensor carpi radialis longus and extensor carpi radialis brevis. It is still united with the brachioradialis at the origin on humerus and the muscle mass ends in condensed tissue near proximal ends of 2nd and 3rd metacarpals Lewis W.H., (1902). United with the common extensor at its origin on humerus is extensor carpi ulnaris, supplied by branches of the posterior interosseus nerve. It inserts into condensed tissue of the 5th metacarpal. Also supplied by the posterior interosseus nerve are the fused extensor digitorum and extensor digiti minimi. The former has formed tendons which extend to the four ulna digits. Lewis W.H., (1902). A pre-muscle mass supplied by (musculospiral) radial nerve branches was observed in the previous stage and is described at this stage as having an ulnar origin is fused with the supinator and an insertion into condensed tissue of the1st digit. The mass is thought to represent the extensor pollicis longus and abductor pollicis longus.
Gasser R.F., (1975) has labelled various muscles in micrographs of sections of a stage 19 embryo (Hooker-Humphrey collection 142). Semispinalis capitis and mastication pre-muscle are found on p. 193. Trapezius and longissimus dorsi are found on p. 195, semispinalis cervicis and splenius on p. 197. Levator scapulae is found on p.201 and scalenes on p.203. Lower down the embryo the rhomboid ,pectorals, serratus anterior and deltoid muscle are labelled on p.205. On p.207 muscle groups of the spine are labelled: superficial erector spinae and the transversospinalis group. Lower still is latissimus dorsi, p. 209, external intercostals and internal intercostals on p. 215 and p. 213. The abdominal muscles are labelled on p. 223; rectus abdominis, transversus abdominis, internal abdominal oblique, external abdominal oblique. Quadratus lumborum is found on p.225. The gluteal (gluteus maximus, gluteus medius, gluteus minimus) and iliopsoas muscle groups are labelled on p. 229 The extensor and flexor thigh pre-muscle is labelled on p. 231 and the extensor and flexor leg pre-muscle on p. 233. The hamstring muscle group are labelled on p. 235
Blechschmidt E., (1963) Plate 38 has labelled an illustration of a reconstructed 17.5mm embryo thought to be stage 20. In the scapular region the serratus anterior, trapezius are mentioned. In the shoulder region the deltoid, pectoralis major, teres major, latissimus dorsi are labelled. The spinal muscles, superficial erector spinae, are shown and the abdominal rectus abdominis and external abdominal oblique may be found. The external intercostals are labelled. In the head and neck region the splenius capitis, longissimus capitis, sternocleidomastoid, digastric, masseter, temporalis and lateral pterygoid may be found. Various muscles of the upper limb are indicated; flexor carpi ulnaris, brachialis, triceps, brachioradialis, extensor carpi ulnaris, extensor pollicis longus, extensor pollicis brevis, abductor pollicis longus, extensor digitorum. A few muscles of the lower limb are labelled; Iliopsoas, adductor group (longus/brevis/ magnus), gluteus maximus, quadriceps femoris, sartorius, tibialis anterior, extensor digitorum longus.