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6.3 DESCRIPTION OF HISTOLOGICAL APPEARANCES
Embryos recovered 0 to lt2 hours after wíndowing (Section 6.1)
were examlned hîstological ly in serlal sections. Because the neural
defects observed after dífferent perìods of incubatìon showed a progresslon
of changes, the embryos were examined ln four groups of Stages
(see Sect ion 4.8.2) .
The general description of the histology typical of Group I - lV
embryos ís based on the appeêrance of each embryonic system.
ln some embryos there was dorsal splitting of the closed neural tube
(at later s tages ) ,...or- sepa ra t î on of notochord and somites from the neural
tissue (at earl ier stages). Both were caused by shrinkage of the embryos
during processing and were quíte different from the appearance of open
neural defects (figs. 89 - 94).
6.3.1 Stage !0 Control Embryos (Group l)
The neural plate was closed or closing cver the brain-and presomite
areas, closing or inverted in the somite area, inverted ln the protosomlte
area, elevated at the anter¡or rhomboid slnus, and elevated or
flattened at the posteiior rhomboid sinus,
The neural folds about to close showed swell ing and rounding of
the free edges, which were often inverted into the future neural- canal.
At thê rhomboid sinus differentîat¡on of the neural plate was revealed
by marginal foldîn9 and elevation above the level of the adjacent
ectoderm. This region of neural plate showed a regular êrrangement of
cells perpendicular to the exposed dorsal surface. After neural closure
(at more cranial levels), these cells retaîned the same orîentation to
each other and to the surface (which then enclosed the lumen of the neural
tube).