. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. TUBULATION AND ORGANIZATION OF BODY FORM 513. Fig. 251. Dependency of neural tube formation upon surrounding tissues. (A) Effect of notochord without myotomes. (B) Effect of myotomes without notochord. (C) Absence of notochord and myotomes. placed at head level in the host but can only organize trunk and tail structures when placed in trunk and tail areas of the host. In other words, there exists a mutual relationship between the level of the host
. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. TUBULATION AND ORGANIZATION OF BODY FORM 513. Fig. 251. Dependency of neural tube formation upon surrounding tissues. (A) Effect of notochord without myotomes. (B) Effect of myotomes without notochord. (C) Absence of notochord and myotomes. placed at head level in the host but can only organize trunk and tail structures when placed in trunk and tail areas of the host. In other words, there exists a mutual relationship between the level of the host tissues and the transplanted organizer material of the trunk organizer in effecting the formation of a head at the head level. Another forceful example of the interrelationship of developing parts and formative expression of body structures is shown by the work of Holtfreter ('33) on the development of the neural tube. This work demonstrates that the form of the neural tube is dependent upon influences in its environment, as shown in figure 251. The presence of the later developing notochord de- termines a thin ventral floor of the neural canal, whereas the contiguous myotome determines a thick wall of the neural tube. Normally, in development, the notochord lies below the neural tube, while the somites with their myotomic parts come to lie lateral to the tube. That is to say, the normal bilateral sym- metry of the neural tube is dependent upon the relationship, in their normal positions, of the notochord and the myotomes. The behavior of the developing neural tube, relative to the notochord and the myotomes, demonstrates the importance of the migration of the somitic mesoderm from a position contiguous and lateral to the notochord at the beginning of neuralization to one which is lateral to the forming neural tube as neuralization and differentiation of the neural tube progresses. A further illustration of the probable influence of the notochordal area in morphogenesis and or
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