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

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defects seen ln fowls, human sacral agenesis is rare. This may be
because phylogènetlc reductlon of caudal segments, already evident in
chlckens, has been carried further in the hur¡an sprne (Hughes and Freeman,
19741 .
A revlew of the histological differences between experimentat and
control embryos was complîcated by shrlnkage of ,or" embryos during pro_
cesslng, producing sp!ittlng of the neural tube roof ln older embryos,
and wide separation of notochord, somites and neurar trssue rn earry embryos.
These art,¡ facts could probably be avoided by using dloxane for processing._
Examination of the control emb ryos by serîal sectlons revealed a co_
ordinated sequence of changes in chorda-mesoderm and neural tissue during
¡eufolation, though the description is stat¡c rather than dynamic.
Duríng Stages l0-12 at the posterior rhombold sinus, neural plate
dlfferentiated ín the region of Hensenrs node and the neural folds were
flattened or elevated, while the chorda-mesoderm was fused l¡to an undiffer_
entlated cell nass. At the anterior rhomboíd sfnus the neurar fords showed
further elevatíon, and accessory canars were present ín the tat-bud materiar;
The notochord was estabr ished and somitic mesoderm became separated, though
not segmented. lmmediately above the rhombold sinus the mesoderm showed
separation intå club-shaped protosom¡tes, while the neural plate was in_
vertèdr. closing, or crosed. cranially the brain was crosing or irosed.
During stages r3-20,Hensenrs node {ån¿ r"t.. the primitive streak)
gave bray to a tail-bud by Stage 16, from which the caudal regíon developed,
The rhomboíd sinus was crosed by stages l!-r6, but neurar materiar from the
ta¡ l-bud contributed ro the sp¡nal cord of the tail until Stages 1!_20.
l'llth the onset of neurulatlon, thickening, elevation, folding, and