512 DEVELOPMENT OF PRIMITIVE BODY FORM 



in spiral form within the extra-embryonic membranes. The coiUng tendency, 

 however, is not alone confined to the snake group, for the habits of general 

 body bending, referred to above, essentially is a coiling tendency. Viewed 

 thus, the rotation or torsion of the developing body along its median axis is 

 a generalized behavior pattern which permits and aids the coiling habit so 

 prevalent among the embryos of higher vertebrates. It may be observed further 

 that the coiling behavior is a common attitude during rest not only among 

 snakes but also among the adults of many higher vertebrates. 



I. Influences Which Play a Part in Tubulation and Organization of 



Body Form 



In Chapter 9, it was pointed out that the pre-chordal plate material, 



that is, organizer material which invaginates first during gastrulation and 

 which comes to lie in the future head region, induces the organization of 

 certain head structures and itself may form a part of the pharyngeal wall 

 and give origin to head mesoderm, etc. On the other hand, the trunk-organizer 

 material (notochord and somitic mesoderm) which moves to the inside, fol- 

 lowing the pre-chordal plate material, organizes the trunk region. The follow- 

 ing series of experiments based upon work by Spemann, '31, sets forth the 

 inductive properties of these two cellular areas: 



Experiment 



1. Head-organizer material, taken from one embryo and placed at head level of a 

 host embryo, will induce a secondary head, having eyes and ear vesicles 



2. Head-organizer material, transplanted to trunk and tail levels in host embryos, 

 induces a complete secondary embryo, including head 



3. Trunk-organizer material (i.e., notochord and somitic mesoderm), placed at head 

 level in host embryo, induces a complete secondary embryo, including the head 

 structures 



4. Trunk-organizer material, placed at future trunk or tail levels in host embryos, 

 induces trunk and tail structures only 



The many influences which play a part in the organization of the vertebrate 

 head and body constitute an involved and an unsolved problem. The extreme 

 difficulty of this general problem has long been recognized. (See Kingsbury 

 and Adelmann, '24. ) The above-mentioned work of Spemann represents a 

 beginning attempt to analyze this aspect of development and to understand 

 the factors involved. It demonstrates that the organization of the neural tube 

 and other axial areas is dependent upon specific cellular areas which migrate 

 inward during gastrulation. However, this is but one aspect of the problem. 

 As observed in the series of experiments above, trunk-organizer material 

 is able to organize a complete secondary embryo, including the head, when 



