76 



EARLY EMBRYOLOGY OF THE CHICK 



out the chick must twist around on its side, i.e., undergo tor- 

 sion, as a man lying face down turns on his side in order to 

 flex his body. 



Torsion begins in the cephaHc region of the embryo and pro- 

 gresses caudad. The first indications of torsion appear almost 

 as soon as the cranial flexure begins and the two processes then 

 progress synchronously. , In the chick, torsion is normally car- 

 ried out toward a definite side. The cephaKc region of the 



mesencephalon 



metencephalon 



myelencephalon 

 auditory pit 



sinus region 



lateral mesoderm 



lateral body fold 



unsegmented dorsal 

 mesoderm 



prosencephalon 



optic vesicle 



margin of 

 head fold of amnion 



bulbo-conus arteriosus" 



ventricular region 



atrial region 

 .**-»- ■ omphalomesenteric vein 



extra-embryonic 

 vascular plexus 



oinphalo mesenteric 

 artery 



primitive plate 



Fig. 29. — Dorsal view ( X 14) of entire chick embryo having 19 pairs of 

 somites (about 43 hours incubation). Due to torsion the cephalic region appears 

 in dextro-dorsal view. 



embryo is twisted in such a manner that the left side comes to 

 lie next to the yolk and the right side away from the yolk. 

 The progress of torsion caudad is gradual and the posterior 

 part of the embryo remains prone on the yolk for a considerable 

 time after torsion has been completed in the head region. Fig- 

 ure 22 shows the head of an embryo of about 38 hours in which 

 the cranial flexure and torsion are just becoming evident. In 

 chicks of about 43 hours (Fig. 29) the further progress of both 

 flexion and torsion is well marked. 



The processes of flexion and torsion thus initiated continue 



