CHAPTER X 



THE CHANGES BETWEEN FORTY AND FIFTY HOURS 

 OF INCUBATION 



FLEXION AND TORSION; THE COMPLETION OF THE VITELLINE 

 CIRCULATORY CHANNELS; THE BEGINNING OF THE CIRCU- 

 LATION OF BLOOD. 



Flexion and Torsion. Until 36 or 37 hours of incubation the 

 longitudinal axis of the chick is straight except for slight for- 

 tuitous variations. Beginning at about 38 hours, processes are 

 initiated which eventually change the entire configuration of the 

 embryo and its positional relations to the yolk. These proc- 

 esses involve positional changes of two distinct types, flexion 

 and torsion. As applied to an embryo, flexion means the bend- 

 ing of the body about a transverse axis, as one might bend the 

 head forward at the neck, or the trunk forward at the hips. 

 Torsion means the twisting of the body, as one might turn the 

 head and shoulders in looking backwards without changing the 

 position of the feet. 



In chick embryos the first flexion of the originally straight 

 body-axis takes place in the head region. Because of its loca- 

 tion it is known as the cranial flexure. The axis of bending in 

 the development of the cranial flexure is a transverse axis pas- 

 sing through the mid-brain at the level of the anterior end of the 

 notochord. The direction of the flexion is such that the 

 fore-brain becomes bent ventrally toward the yolk. The proc- 

 ess is carried out as if the brain were being bent about the 

 anterior end of the notochord. Until the cranial flexure is well 

 established it is inconspicuous in dorsal views of whole-mounts 

 but even in its initial stages it appears plainly in lateral views 

 (Fig. 24). 



To appreciate the correlation between the processes of flexion 

 and torsion it is only necessary to bear in mind the relation of 

 a chick of this stage to the yolk. As long as the chick lies with 

 its ventral surface closely applied to the yolk, the yolk consti- 

 tutes a bar to flexion. Before extensive flexion can be carried 



