VIII.] THE SPINAL CORD. 251 



plate stretching right across the mouth and dividing 

 it into two cavities — an upper and a lower one. 



In the front part of the mouth their union is quite 

 complete, so that here there is no communication between 

 the two cavities. Behind, however, the partition is not 

 a complete one, so that the two divisions of the buccal 

 cavity communicate at the back of the mouth. The 

 external opening of the mouth passes into the lower of 

 these two cavities, which may therefore be called the 

 mouth proper. Into the upper chamber the nasal 

 ducts open ; it may be called the respiratory chamber, 

 and forms the commencement of the chamber of the 

 nose. In birds generally the upper nasal cavity be- 

 comes subsequently divided by a median partition into 

 two chambers, which communicate with the back of 

 the mouth by separate apertures, the posterior nares. 

 The original openings of the nasal pits remain as the 

 nostrils. 



The spinal cord. — On this day important changes 

 take place in the spinal cord; and a brief history of 

 the development of this organ may fitly be introduced 

 here. 



At the beginning of the third day the cavity of the 

 neural canal is still of considerable width, and when 

 examined in vertical section its sides may be seen to be 

 nearly parallel, though perhaps approximating to each 

 other more below than above. 



The exact shape varies according to the region of 

 the body from which the section is taken. 



The epiblast walls are at this time composed of 

 radiately arranged columnar cells. The cells are much 

 elongated, but somewhat irregular; and it is very 



