Module
7
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Embryonic disk

The intraembryonic coelom

The intraembryonic coelom first appears in the lateral plate mesoderm in the form of several isolated vacuoles. During the lateral unfolding of the embryo in the 4th week these vacuoles fuse and form a U-shaped cavity: the intraembryonic coelom. In the beginning a connection exists between the intra- and extraembryonic coeloms.
With the progress of the unfolding, though, the merging of the ectoblast layers along the medial line has the effect that the intraembryonic coelom is separated from the extraembryonic one and remains enclosed in the lateral mesoblast.

Fig. 19 - Mesoderm before the vacuoles
form on roughly the 20th day
media/module7/h2ma_coelom.gif

  1. Paraxial mesoderm
  2. Intermediate mesoderm
  3. Lateral plate mesoderm
  4. Chordal process

Legend
Fig. 19

Transverse section through a 20-day-old embryo. The separation into the paraxial, intermediate mesoderm and the lateral plate is shown.

Fig. 20 - Coelomic vacuoles
roughly 23rd day
media/module7/h2mb_coelom.gif

  1. Lateral plate mesoderm
  2. Intermediate mesoderm
  3. Paraxial mesoderm
  4. Neural groove
  5. Coelomic vacuoles
  6. Intraembryonic coelom
  7. Extraembryonic coelom

Fig. 21 - Coelomic cavity
roughly 25th day
media/module7/h2mc_coelom.gif

8
Extraembryonic mesoblast
9
Notochord
10
Splanchnopleure with endoderm
11
Somatopleure with ectoderm
12
Dorsal aorta (paired)

Legend
Fig. 20

After the 23rd day coelomic vacuoles form in the lateral plate.

Fig. 21

On the 25th day the intraembryonic coelom of the lateral plate divides the mesoderm into somatopleural und splanchnopleural mesoderm.