486 DEVELOPMENT OF THE CHICK. 



developed and about to meet in the region of mid- and 

 hind-brains. Four to six pairs of somites have appeared. 

 The primitive streak is still clearly marked, and the vitelline 

 veins are established. 



At the thirty-sixth hour (figs. 86, 93). The vascular area 

 (AV fig. 93) has been organised and the heart and paired aorta 

 have developed in the embryo. The neural folds have met 

 along the mid-dorsal fine, except at the hinder end of the 

 body, and the brain occupies about half the total length of 

 the tubular nervous system. The first indications of the eye 



Fig. 93. The yolk of a Hen's Egg at the thirty-sixth hour from the 

 commencement of incubation. The structure of the embryo at this 

 stage is shown on a larger scale in figs. 86 and 87. xf. 



AD, area pellucida of the blastoderm. AK, area opaca. AV, area vasculosa. 

 EM, embryo. SM, vitelline membrane^ Y, Yolk-sac. 



and ear respectively are seen in the optic vesicles and auditory 

 vesicles (BO and EI fig. 86). The head-fold of the amnion 

 is rising up. From twelve to fourteen pairs of somites are 

 present. 



At the forty-eighth hour. The * cranial flexure ' and 

 twisting of the head over to the left side now render the 

 embryo asymmetrical. The vascular area has increased, 

 and the vitelline arteries are distinct and he immediately 

 behind the vitelline veins. The amniotic folds have increased 

 in extent and the number of somites is now about 27-28. 



