THE DEVELOPMENT OF THE EMBRYO ITSELF. 105 



crease in the curvature of the heart and its complexity of struct- 

 ure by divisions ; the appearance of additional aortic arches 

 and of the cardinal veins ; the formation of four visceral clefts 

 and five visceral arches ; a series of progressive changes in 



AA.L 



Fig. 109.— Diagram of circnlation of yelk-sac at end of third day (Foster and Bal- 

 four). Blastoderm seen from below. Arteries made black. H, heart; AA, sec- 

 ond, third, and fourth aortic arches; AO, dorsal aorta; L.of.-A, left vitelline 

 artery; S. of. A. right vitelline artery; S. T, sinus terminalis; L. of, left vitelline 

 vein; B.oj, right vitelline vein; S. Y, sinus venosns; S. C, ductne Cuvierl; 

 5. Ca. V, superior cardinal or jugular vein; V. Ca, inferior cardinal vein. 



the organs of the special senses, such as the formation of the 

 lens of the eye and a secondary optic vesicle ; the closing in of 

 the optic vesicle ; and the formation of the nasal pits. In the 

 region of the future brain, the vesicles of the cerebral hemi- 

 spheres become distinct ; the hind-brain separates into cere- 



