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Carnegie Stage 13 - First Trimester
First thin surface layer
of skin appears covering
the embryo.

< Somite

SIZE: 4.0 - 6.0 mm

TIME PERIOD: 26- 30 days post-ovulation

The arches that form the face and neck are now becoming evident under the enlarging forebrain. By the time the neural tube is closed, both the eye and ear will have begun to form. At this stage, the brain and spinal cord together are the largest and most compact tissue of the embryo.

A blood system continues to develop. Blood cells follow the surface of yolk sac where they originate, move along the central nervous system, and move in the chorionic villi, the maternal blood system.

More than 30 pairs of somites will have formed by now. Somites represent differentiated cells made up of various precursor cells.

In each somite, the area closest to the neural tube forms the epaxial dermamyotome region that produces epaxial myotome. The area farthest from the neural tube forms the hypaxial dermamyotome region that produces hypaxial myotome. The area in between these two regions is the central dermamyotome, which produces the dermis. The ventral-medial region of the somite forms sclerotome that migrates to become cartilage.

In the epaxial region, back muscle is formed from epaxial myotome and rib is formed of sclerotome. In the hypaxial region, intercostal muscle and the lowest part of the rib region are formed from hypaxial myotome. The middle part of the rib region is formed from epaxial myotome despite the fact that this region is in the hypaxial muscle region.

The digestive epithelium layer begins to differentiate into the future locations of the liver, lung, stomach and pancreas.


Gilbert, Scott. 2000. Developmental Biology, 6th edition. Sinaur: Sunderland.

Abstract: PNAS April 11, 1995 vol. 92 no. 8 3219-3223

Control of Somite Patterning by Signals from the Lateral Plate

The body musculature of higher vertebrates is composed of the epaxial muscles, associated with the vertebral column, and of the hypaxial muscles of the limbs and ventro-lateral body wall. Both sets of muscles arise from different cell populations within the dermomyotomal component of the somite. Myogenesis first occurs in the medial somitic cells that will form the epaxial muscles and starts with a significant delay in cells derived from the lateral somitic moiety that migrate to yield the hypaxial muscles. The newly formed somite is mostly composed of unspecified cells, and the determination of somitic compartments toward specific lineages is controlled by environmental cues. In this report, we show that determinant signals for lateral somite specification are provided by the lateral plate. They result in a blockade of the myogenic program, which maintains the lateral somitic cells as undifferentiated muscle progenitors expressing the Pax-3 gene, and represses the activation of the MyoD family genes. In vivo, this mechanism could account for the delay observed in the onset of myogenesis between muscles of the epaxial and hypaxial domains.

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