CHAPTER XII 



THE STRUCTURE OF CHICKS FROM FIFTY TO FIFTY- 

 FIVE HOURS OF INCUBATION 



I. External Features. 

 II. The Nervous System. 



Growth of the telencephalic region; the epiphysis; the 

 infundibulum and Rathke's pocket; the optic vesicles; 

 the lens; the posterior part of the brain and the cord 

 region of the neural tube; the neural crest. 



III. The Digestive Tract. 



The fore-gut; the stomodaeum; the pre-oral gut; the 

 mid-gut; the hind-gut. 



IV. The Visceral Clefts and Visceral Arches. 

 V. The Circulatory System. 



The heart; the aortic arches; the fusion of the dorsal 



aortae; the cardinal and omphalomesenteric vessels. 

 VI. The Differentiation of the Somites. 

 VII. The Urinary System. 



I. EXTERNAL FEATURES 



In chicks which have been incubated from 50 to 55 hours 

 (Fig. 34) the entire head region has been freed from the yolk 

 by the progress caudad of the sub-cephalic fold. Torsion has 

 involved the whole anterior half of the embryo and is completed 

 in the cephalic region, so that the head now lies left side down 

 on the yolk. The posterior half of the embryo is still in its 

 original position, ventral surface prone on the yolk. At the 

 extreme posterior end, the beginning of the caudal fold marks 

 ofl the tail region of the embryo from the extra-embryonic mem- 

 branes. The head fold of the amnion has progressed caudad, 

 together with the lateral amniotic folds impocketing the em- 

 bryo nearly to the level of the omphalomesenteric arteries. 



The cranial flexure, which was seen beginning in chicks of 

 about 38 hours, has increased rapidly until at this stage the 

 brain is bent nearly double on ; itself. The axis of the bending 



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