The appearance of somites, paired wedge-shaped segmentations in the paraxial mesoderm on either side of the notochord, is the defining criteria of this stage. 1-3 pairs are distinguishable at the level of the rhombencephalon although the number may differ on the two sides of the body. Some somites possess cavities within them called myocoeles
It is difficult to estimate the somite count from study of histological sections, therefore, somite number in an embryo, as an indication of age, must be treated with caution.
The defining feature of this stage is the appearance of somitic pairs 4-12.
Differentiation of the somites has begun and sclerotomal cells can be seen migrating from the ventromedial side of the somites in Carnegie Embryo No. 5074. Corner G.W.(1929) Plate 6, Fig 23-26.
The defining characteristic of this stage is the development of somites 13-20. In the 20-somite embryo, Carnegie Embryo No. 2053, myocoeles are present from the 4 to the 16th somite. The somites has differentiated into myotome, dermamyotome and sclerotome up to the 10th somite and somite 13 shows differentiation of sclerotome.
The defining characteristic of this stage is the development of somites 21-29. Myotome cells can be distinguished as cuboidal and eosinophilic.
Somite 1 is being transformed into loose mesenchyme but may retain the dermatome. None of the occipital somites possess a myocoele and their myotome has not yet migrated around the notochord.
An apparent ventromedial rotation of the somites takes place between stages 11 and 12. O'Rahilly R., Müller F., (1984)
According to Sensenig E.C., (1949) no myocoeles are found above upper thoracic segments, and the first migratory sclerotome cells may be seen.
The defining characteristic of this stage is the presence of 30+ somites. The highest number recorded at this stage is 39 according to Müller F., O'Rahilly R., (1988b)
Four somites are occipital, eight cervical, twelve thoracic, five lumbar and a further eight to ten coccygeal somites will differentiate within the caudal eminence. However it is difficult to accurately count somites by the analysis of histological sections alone.
The first somite has virtually disappeared, four occipital myotomes being present. The epithelioid character of the dermatomes of the occipital somites is lost and the sclerotome has formed a continuous column on each side. O'Rahilly R., Müller F., (1984)
By stage 14, the first four somites (occipital) can no longer be described as such., four occipital myotomes are visible rostral to the first cervical nerve in sagittal sections. The dermatomes lost their epithelioid character in the previous stage when the sclerotomes formed a continuous bilateral column
By this stage it can be seen that the dermamyotomes of the first three somites have given rise to a dense mesenchymal structure, the hypoglossal cord and the sclerotomal columns have become divided into a rostral and caudal part which unite with their contralateral counterpart in the median plane. O'Rahilly R., Müller F., (1984)
Eight cervical, twelve thoracic, five lumbar are present by this stage. The first few sacral, and a further eight to ten somites will differentiate within the caudal eminence.
Sclerotome has formed down to the level of the sacral somites and is migrating towards the notochord in the upper sacral region.
The myotome in the lumbar region is beginning to grow ventrally along the medial surface of the dermatome. In the thoracic region, the myotome has grown ventrally to meet the ventral edge of the dermatome, the cells have become spindle shaped.
In the cervical region, cells appear to migrate to the limb bud from the myotomes. Keibel F., Mall F.P., (1912) Vol. I, Chapter XII, pp. 470-474
Eight cervical, twelve thoracic, five lumbar, five sacral and 5-6 coccygeal somites are present. O'Rahilly R., Müller F., (1984)
At this stage the cells derived from the sclerotome of the somites have migrated around the notochord and condensed in the area between the neural tube and the aorta and also projections into the body wall area. These condensations are the primordial intevertebral discs, the future arch neural arch and costal processes of the vertebra. The condensation only occurs in the caudal half of each segment. e.g. Carnegie embryo No. 2
The occipital myotomes have fused into a mass of myoblasts lateral to the hypoglossal nerve. Myotomes in other regions have formed a dorsal 'epaxial division' and a ventrolateral 'hypaxial division' The former will form the extensor muscles of the vertebral column while the hypaxial part will give rise to lateral and ventral flexors of the spine.
The sclerotomes in each segment have united around the notochord, each sclerotome consists of a dense caudal part and looser rostral part. Caudal to the occipital region, the cranial part of the sclerotome forms the precursor of the neural arch.
Sclerotomes in the occipital region lose their segmentation, and fuse with the chordal part of the parachordal condensation contributing to occipital bone formation.
The outlines of somites in the lumbar and sacral regions may still be distinguished O'Rahilly R., Müller F (1987) Fig 17.