. Principles of modern biology. Biology. nd DORSAL LIP 1st DORSAL LIP. 2nd TAIL 2nd VERTEBRA NEURAL TUBE 2nd EAR YOUNG EMBRYO OLDER EMBRYO Fig. 27-8. Left, an embryo at the beginning of gastrulation. An organizer from another embryo of the same stage has been transplanted, in the position shown. Right, the same embryo at a later stage. an entirely foreign region of the embryo. Thus the dorsal lip of the blastopore serves as the primary organizer in the vertebrate em- bryo. In fact the organizing capacity of the lip region can be traced back before gastrula- tion. If the corresponding part of a
. Principles of modern biology. Biology. nd DORSAL LIP 1st DORSAL LIP. 2nd TAIL 2nd VERTEBRA NEURAL TUBE 2nd EAR YOUNG EMBRYO OLDER EMBRYO Fig. 27-8. Left, an embryo at the beginning of gastrulation. An organizer from another embryo of the same stage has been transplanted, in the position shown. Right, the same embryo at a later stage. an entirely foreign region of the embryo. Thus the dorsal lip of the blastopore serves as the primary organizer in the vertebrate em- bryo. In fact the organizing capacity of the lip region can be traced back before gastrula- tion. If the corresponding part of a fertilized egg is removed, such an egg never develops into anything but a ball of cells; whereas if other small parts of the egg are taken away, the egg still gives rise to a normal or approxi- mately normal embryo. After gastrulation, other parts of the em- bryo, especially those lying near the primary organizer, gradually become determined. Soon a bit of ectoderm, transplanted from the dorsal surface quite some distance an- terior to the blastopore, will develop into nerve cord regardless of its new location. Moreover, this ectoderm acts as a secondary organizer in that it determines not only its own development, but also that of nearby parts. And in like manner, other parts of the embryo, once they have been determined and launched upon some special line of inde- pendent development, may act as organizers of such parts as may still remain in a non- determined state. Precisely how an organizer "organizes" is quite unknown. Perhaps it produces special substrates that, transmitted to neighboring cells, change the enzyme induction pattern of these cells. The dorsal lip tissue will in- duce the formation of a neural tube even if the lip tissue is killed before it is introduced into the host embryo. In fact, supernumerary neural tubes have been induced by a variety of chemical and physical agencies, some quite foreign to the normal embryonic environ- ment. At present, a
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