Universlty of Manitoba, ln Partîal Fulfiìlment - MSpace at the ...

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j) ênd prom¡nence of Hensenrs node and the primltive streak.
Howéver in two embryos (68 4l , 6E 45) there was a defînite appeêr_
ance of sllght eversion of the neurar fords at the posteríor rhomboid sinus,
rather than the wlde erevatron or frattenrng seen in the contro¡ and other
experlmental embryos. These two embryos still showed an orderly arrange_
ment of cells perpendicular to the well-preserved dorsal surface of the
open neural plate (Fi9. 47 and 48).'
6.3.3 Stage l1-.|2 Control Embryos (Grouo ll)
Neural closure extended from the brain down to the antèrior rhomboid"
slnus, with closing or elevated neural folds at the posterior rhomboid
sinus. The rhombic roof was thinner by Stage ll+ than at Stage tO
(rls. lo3).
The first signs of accessory neural canars were seen at the rhomboid
sinus. The overlap zone could in fact be traced up as far as the proto_
somite area when asymmetry of the neurar tube rather than the presence of
accessory canals was taken as a criterion..
As în Stage lO the areas of unclosed neura.l plate were continuous
wlth, but sharply distinguishable from, adjacent ectoderm, .ln the braln
regign, ectoderm was separated from the underlying neural tube by neural
crest cells,but not at this stage by migrating somit¡c mesoderm,
The notochord was in close contact with the neurêl tube or neural
plate over the somite, protosomíte, and anterior rhomboid sinus areês, but
was general ly not in contact with the midbrain and hindbrain. somites and
protosom¡tes were also in contact with the neurar tube,but this contact was
not maintained with unsegmented and fused mesoderm of the rhomboid sinus.
The somltes showed a normal sequence of changes, wlth no cyst¡c or hemo_
rrhagic areas and no reduction .in volume.