262 Vertebrate Embryology 



fifth day, let us briefly summarize the more 

 important events of the fourth day. They 

 are : 



r. Increase in the body and cranial flexure. 



2. Growth of the allantois. 



3. Growth of the tail-fold. 



4. Appearance of the limbs as local thicken- 

 ings of the Wolffian ridge, just back of the 

 heart and just in front of the tail. 



5. Opening of the mouth, by the absorp- 

 tion of the partition between the stomodaeum 

 and the pharynx. 



6. Development of the olfactory grooves. 



7. Vacuolation of the cells of the notochord. 



8. Beginning of the vertebral column. 



9. Development of the ureter. 



10. Development of the duct of Muller. 



11. Development of the primitive germ 

 cells in the genital ridge. 



1 2. Appearance of a fourth and a fifth pair 

 of aortic arches, and the partial disappearance 

 of the first and second arches. 



13. Growth of the septa dividing the heart 

 into right and left sides. 



