15. E M B R Y O G E N E S I S in
VlttO
559
regard to the undifferentiated somitic mesoderm, exist in the vicinity
of Hensen's node and are situated on both sides of, and a little behind,
the node (Fig. 14). Part of the chordal centre is made up of permanent
tissue, whereas the somitic centres seem to be formed of transient
matter.
One of the most important properties of these centres is that they
permit the differentiation of mesodermal organs at the expense of
undetermined tissues. Hence, in the absence of the presumptive chordal
anlage, the chordal centre can use cells of the somitic anlage. In the
v'1
PLp
PS
I I
I I
PCh
i i
i I
PS
•"
/
/
/
/
/ P L p
0-lmm
I
ii i
FIG. 14. Mesodermal organizing centres. Gh, Notochord (differentiated); ChC, chordal
centre; PCh, potential chord; S, somites; SC, somitic centres; Sp, unsegmented somitic
plate; PS. potential somites; Lp, lateral plate; PLp, potential lateral plate. (After Spratt,
1 9 5 5 . )