EXPLANATION OF PLATES. XXI 



FIG. 



27. More advanced embryo, seen on its ventral side, and showing the first development of 

 the Circulating apparatus. Around the Vascular Area is shown the terminal sinus, 

 a, a, a. The blood returns from this by two superior branches, b, b, and two infe- 

 rior, c, c, of the omphalo-meseraic veins, to the heart, d; which is, at this period, a 

 tube curved on itself, and presenting the first indication of a division into cavities. 

 The two aortic trunks appear, in the abdominal region, as the inferior vertebral 

 arteries, e, c; from which are given off the omphalo-meseraic arteries, /,/, which 

 form a network that distributes the blood over the vascular area. In the cephalic 

 region are seen the anterior cerebral vesicles, with the two ocular vesicles, g. After 

 Bischoff( 938). 



