CEPHALIC FLEXION AND GENERAL BODY BENDING 511 



It is obvious, therefore, thai the fields of influence which govern the de- 

 velopment of specific organs may be much more extensive in cellular area 

 than the actual cellular contributions which take part in the formation of 

 the specific organ structures. Experiments on the forming limb of Ambystoma 

 also have demonstrated that a particular area of the field is stronger in its 

 limb-forming potencies than other regions of the field. This property probably 

 is true of other fields as well. 



(For a detailed discussion of the field concept in embryonic development, 

 reference should be made to Huxley and DeBeer, "34, Chaps. 8 and 9; Weiss, 

 '39, p. 289 ff.) 



H. Cephalic Flexion and General Rod\ Bending and Rotation in 

 \ ertebrate tmbr>os 



The anterior end of the neural tubulation is prone to assume a bent or 

 flexed contour whereby the anterior end oi the neural tube is directed down- 

 ward toward the ventral aspect of the embryo. This general behavior pattern 

 is strong in vcrlcbrate embryos uiih the exception of the telcost fishes. In 

 teleost fishes this bending habit is slight. As the later development of the 

 head progresses in other vertebrate embryos, the neural tube shows a pro- 

 nounced cephalic (cranial) flexure in the region of the midbrain, in some 

 species more than in others. (See Chap. 19.) An additional bending occurs 

 in the posterior hindbrain area. The latter flexure is the cervical or nuchal 

 flexure (figs. 231. 238, 240, 244, 246). 



Aside from the acute bending which takes place in the formation of the 

 cephalic and the nuchal flexures, there is a definite tendency for many verte- 

 brate embryos to undergo a general body bending, with the result that the 

 anterior part of the body and the caudal portion of the trunk and tail may 

 be depressed in a ventral direction (figs. 222C-E; 227; 229F; 238; 240; 

 244; 246). In the frog embryo, at hatching, the opposite tendency may 

 prevail for a brief period, and the dorsal trunk region may appear sagging 

 or hollowed (fig. 226A, C). 



In addition to these bending movements, in the embryos of higher verte- 

 brates, a rotation or twisting (torsion) of the developing body along the 

 antero-posterior axis is evident. In the chick embryo, for example, the head 

 region begins to rotate toward the right at about 38 hours of incubation. 

 Gradually this torsion continues caudally (figs. 237, 238, 239, 260). At 

 about 70 to 75 hours, the rotational movement reaches the tail region, and 

 the embryo then comes to lie on its left side throughout its length (fig. 240). 

 In exceptional embryos, the rotational movement is toward the left, and the 

 embryo comes to lie on its right side. Similar movements occur in the pig 

 and other mammals. 



This rotational movement is advantageous, particularly in long-bodied 

 Amnioia, such as the snakes, where it permits the developing embryo to coil 



