40 ARTHUR DENDY. 



form of a wide tube slightly bent on itself, made up behind by 

 the union of the two vitelline veins running across from the 

 yolk-sac in the folds of the splanchnopleure just where the 

 latter diverge, and ending in front in the short bulbus arte- 

 riosus (fig. 59, B.Ar.). In transverse sections (figs. 65, 66) 

 the heart appears as a saccular diverticulum of the splanchno- 

 pleuric mesoblast, here composed of short columnar cells, 

 beneath the front part of the alimentary canal, hanging down 

 into the pericardium, and containing two epithelioid tubules. 

 These tubules (figs. 65, 66, Ep. T.) are continuous behind with 

 the vitelline veins, while in front they unite to form the bulbus 

 arteriosus. The bulbus arteriosus very soon divides into two 

 primitive aortse (figs. 62 to 69, P. ^o.), which run forward 

 beneath the alimentary canal for some distance, and then, 

 curving upwards, turn back and run above the alimentary canal 

 towards the posterior end of the body, giving off the vitelline 

 arteries (fig. 69, V. A.) in the trunk at the level of the twelfth 

 mcsoblastic somite. Where the two primitive aortse turn up- 

 wards in front to reach the dorsal aspect of the alimentary 

 canal they dilate to form a large blood-space (fig. 62, P. Ao.) on 

 each side of the extreme anterior end of the alimentary eanalj 

 like the rest of the blood-vessels this space is lined by a distinct 

 epithelium. 



In the splanchnopleuric mesoblast of the yolk-sac, forming 

 the floor of the large pleuro-peritoneal cavity posteriorly, nu- 

 merous blood islands have made their appearance (figs. 58, 69, 

 70, B. I.), and the posterior half of the sinus terminalis is 

 distinctly visible (figs. 58, 69, 71, S. T.), marking the limit to 

 which the separation of the serous envelope from the yolk-sac 

 has extended. These vessels of the yolk-sac, as already noticed, 

 appear to be formed, at any rate in part, from the sheet of 

 mesoblast which grows out from the primitive streak. 



Nervous System and Sense-organs. — The medullary 

 canal has completely closed in above, except on what is now 

 the ventral aspect of the fore-brain, where the margins of the 

 inediilhiry groove overlap one another as in ihe jjreceding 

 stage, but are not yet fused. Reference to fig. 60, in which the 



