. Analysis of development. Embryology; Embryology. Amphibians 247 which themselves tended to become elongate structm-es. Adopting the terms of Spemann ('31) one may thvis distinguish between head and trunk-tail organizers. (Their regionally specific effects in a host are illustrated in Figs. 92a, b.) The latter extends far into the lateral marginal zone, but fades out toward the ventral sector. The ventral entoderm and the ectoderm of the gastrula showed no in- ductive capacity. This deserves special men- tioning because in later stages certain deriva- tives of these two germ layers do possess
. Analysis of development. Embryology; Embryology. Amphibians 247 which themselves tended to become elongate structm-es. Adopting the terms of Spemann ('31) one may thvis distinguish between head and trunk-tail organizers. (Their regionally specific effects in a host are illustrated in Figs. 92a, b.) The latter extends far into the lateral marginal zone, but fades out toward the ventral sector. The ventral entoderm and the ectoderm of the gastrula showed no in- ductive capacity. This deserves special men- tioning because in later stages certain deriva- tives of these two germ layers do possess inductive faculty. It is interesting to note that small explants from the upper blastoporal lip induced out of their own material ectodermal structures which showed again regional specificity. In these instances head structures, such as single eyes, olfactory pits, otocysts or balancers, could appear independently of each other, and from different, though somewhat over- lapping, districts of the marginal zone (Fig. 84^). This seems to indicate that even at the gastrula stage the head organizer is not ac- tually an equipotential entity but is subdi- vided into specialized inductors although distinct boundaries between them do not seem to exist. THE ORGANIZATION OF THE MESODERM MANTLE AND THE TAIL BUD IN THE NEURULA MESODERM MANTLE The following chapters are devoted to the organization of the neurula and, in particu- lar, to the mesoderm mantle and the medul- lary plate. These primordia represent sig- nificant intermediate stages in a gradually increasing stabilization of differentiations. Of particular interest are the interactions be- tween these two structures and the complex relationships between the different compo- nents of the mesoderm. For orientation see Figure 85. Notochord. During neurulation, the dorsal median strip of the mesoderm mantle be- comes separated from the lateral parts. Its differentiation tendency for notochord is es- tablished from early neurula stages
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Keywords: ., bookcentury1900, bookpublisherphiladelphi, booksubjectembryology
