The classification into epithelial organs, connective tissues, and the more specialized muscle and nerve, was largely due to him; and he proved the presence of neuroglia in the brain and spinal cord, discovered crystalline haematoidine, and made out the structure of the umbilical cord.
It consists of two regions, an external epithelial layer and a more internal sub-epithelial layer.
The epithelial layer consists of (1) so-called " indifferent " cells secreting the perisarc or cuticle and modified to form glandular cells in places; for example, the adhesive cells in the foot.
Or myo-epithelial cells, with the cell prolonged at the base into a contractile muscle-fibre (fig.
Belonging primarily to the epithelial layer, the muscular cells may become secondarily sub-epithelial.
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