V.] THE WOLFFIAN DUCT. 139 



On the third day, in consequence of the continually 

 folding in of the somatopleure and especially of the splanch- 

 nopleure, as well as owing to the changes taking place in the 

 protovertebrse, the Wolffian duct undergoes a remarkable 

 change of position. Instead of lying as on the second day 

 immediately under the epiblast (Fig. 20, W.d.), it is soon 

 found to have apparently descended into the middle of the 

 intermediate cell-mass (Fig. 41, w.d,) and at the end of 

 the third day occupies a still lower position and even pro- 

 jects somewhat into the pleuroperitoneal cavity. (Fig. 44, 

 W.d.) 



Towards the end of the day the rudiments of the Wolffian 

 bodies (Fig. 44, W. b.) begin to make their appearance in con- 

 nection with the ducts, but the consideration of these may 

 conveniently be reserved to the next chapter. 



27. The chief events then which take place on the 

 third day are as follows : 



1. The turning over of the embryo so that it now 

 lies on its left side. 



2. The cranial flexure round the anterior extremity of 

 the notochord. 



3. The completion of the circulation of the yolk-sac ; 

 the increased curvature of the heart, and the demarcation of 

 its several parts ; the appearance of new aortic arches, and 

 of the cardinal veins. 



4. The formation of four visceral clefts and five visceral 

 arches. 



5. The involution to form the lens, and the formation 

 of the secondary optic vesicle. 



6. The closing in of the otic vesicle. 



7. The formation of the nasal pits. 



8. The appearance of the vesicles of the cerebral hemi- 

 spheres ; the separation of hind-brain into cerebellum and 

 medulla oblongata. 



9. The completion of the fore-gut and of the hind-gut ; 

 the division of the former into oesophagus, stomach and 

 duodenum, of the latter into large intestine and cloaca. 



10. The formation of the lun^s as two diverticula from 

 the alimentary canal immediately in front of the stomach. 



11. The formation of the liver and pancreas: the former 

 as two diverticula from the duodenum, which subsequently 



