| Definition: |
The mesoderm of segments T1-A9. It first becomes morphologically distinct during ventral furrow formation. Following invagination during stages 6 and 7, it remains a coherent structure with no morphologically apparent subdivisions, apart from transient metamery during stage 9, until stage 11. It undergoes a number of morphological changes during this period: as its cells divide following invagination, it loses its epithelial integrity and by stage 9 has rearranged into a monolayer of cuboidal cells. A further division happens during stage 10 leading to two distinct layers by stage 11. |