EXISTING ORGANISMS— EMBRYOLOGY 



49 



the blood passes forward into a pair of aortae. The aortae curve 

 dorsad and pass along the body, giving off branches which supply 

 both the body and the extraoni])ryonic regions whence the two 

 pairs of veins drain ])lood. The blood which these branches dis- 

 tribute to the body is collected by a pair of anterior or precardinal 

 veins, and a pair of postcardinals. On each side of the body these 

 vessels unite, to enter the heart as a common trunk. Such a plan 

 is common to all vertebrate embryos early in their development 

 (Fig. 28). Most significant of all is the series of aortic arches 

 formed between the ventral and descending aortae anterior to the 



Aortic arch 3 

 Aortic arch 2 



Aortic arch i 



Pulmonary artery 



Dorsal aorta 



Aortic arch 4 

 Aortic arch 6 



Esophagus 

 Trachea 



Ventral aorta Bulhus cordis 



Fig. 29. — Reconstruction of the aortic arches and pharyngeal pouches of a 

 5 mm. human emljryo. The pouches are indicated by Roman numerals. 

 (From. Arey's Developmental Annlomy, after Tandler, with the permission 

 of the W. B. Saunders Company.) 



heart. The connections by which they are joined originally are 

 the first pair of six. The remaining five pairs form as outgrowths 

 of the two aortae of the same side which meet, forming dorso- 

 ventral connections. While these arches are very different in the 

 adults of the several classes, they appear in the embryos with very 

 similar form, and the embryo of man is not excepted (Fig. 29). 



The Eye. The development of the eye in vertebrates is initiated 

 by the appearance of an evagination of the lateral walls of the 

 forebrain, the anterior of the three primitive expansions of the 

 neural tube (Fig. 30). The evagination expands at its outer end, 

 and here invaginates in turn to form a double cup, the optic cup, 

 connected to the forebrain by the more slender optic stalk. This 

 cup is destined to form the pigmented and nervous layers of the 

 retina in the adult Qye. The lens develops from an invagination 

 of ectoderm opposite to the optic cup, which is later freed from the 



