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Lendon (1968, 1975) and notos (1976) both described fiuid
accumulatlon deep to the neu¡:al plaque, which they regarded as a sequel of
separat¡on, leadîng to the later elevation of the plaque and stretchlng
of the spinal nerves.
Some indlcatlon of the lnfluence of the notochord on early neurogenesi's
ls provided by studîes of the rrovêrgrowthlr phenomenon. Bergquist
(1959) and rãlán (1965) found .that remova I of the fourth neuromere of the
chlck braln produced marked overgrowth of local braln tissue only when
the underlylng notochord was removed or damaged by the operatlon. They
suggested that an ¡ntact notochord may exert some controll îng influence
over the ôdjacent neural tube. Refinement of the technic' to allow
separatlon or remova I of the tip of the notochord and replacement of the
overlying rhombencephalon at stðge 11-12 (Burda, 1968), also produced
local overgrowth of brain tissue. Thís was accompanied by increased cell
dlvision and the distribution of mitotic figures throughout_ the braln
wall. Autoradlography showed that both experimental embryos with overgrovrth
and normal controls lost the abilîty to lncorporate H3 - thymldîne
by the fourth day'mafking the onset of differentiation (Bsrda-l,lilson, 19/1)'
Furthermore the ânterior notochord'efter experimental . s.eparatîon from
the rhombencephalon showed earlier vacuolation, nuclear pycnosls , and
accumulatÍon of P.A.S. - posl tive material than the notochord oí control
emb ryos .
Examlnatlon of somitic mesoderm in control embryos of the present
serles showed close contact of -g.j-!gg with the neural tube, whereas
the unsegmented and fused mesoderm of the rhomboid slnus was general ly
separated from the neural plate by a smal I. gap. Somites had formed down
to the t¡p of the tail by Stage 20.