VII.] THE MOUTH. 18 



f 



processes unite together to form the upper boundary of the 

 mouth, each superior maxillary process sends inwards a lateral 

 bud. These buds become flattened and form horizontal plates 

 which stretch more and more inward towards the middle line. 

 There they finally meet, and by their union, which is effected 

 first in front, and thence extends backwards, they con- 

 stitute a horizontal plate, stretching right across the mouth, 

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

 In the front 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 com- 

 municate 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 becomes subsequently divided by a median partition 

 into two chambers, which communicate with the back of the 

 mouth by separate apertures. The original openings of the 

 nasal pits remain as the nostrils. 



5. One important occurrence of the fifth day is the 

 appearance of the anus, which is formed very much in the 

 same way as the mouth. 



Beneath the tail an involution of the epiblast takes place 

 towards the cloaca. At this point the wall of the cloaca, 

 which has here taken no share in the cleavage of the meso- 

 blast, becomes thinner, and is finally perforated. An orifice 

 thus places the cloaca in communication with the exterior, 

 and constitutes the anus. 



6. On this day also 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 (Fig. 41) 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. 



"We say below and above, because a vertical section is naturally examined 

 with its dorsal side uppermost. In the ordinary terminology of the spinal cord, 



