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 organization of body form is the behavior of the develop- 

 ing metenteron or enteric tube. As observed previously, the gut tubulation 

 tends to invaginate or arch upward toward the notochord not only in embryos 

 developing from flattened gastrulae but also in amphibia. The movement of 

 the entoderm toward the notochord strikingly resembles the behavior of the 

 neural plate ectoderm during the formation of the neural tube. This com- 

 parison becomes more striking when one considers the manner of enteron for- 



