2 26 Vertebrate Embryology 



of the head, and the maxillary process of the 

 mandibular arch is also becoming well devel- 

 oped (Fig. 64, MN). The nasal pit is deep, 

 and is connected with the mouth invagination 

 or stomodaeum by the groove that will form 

 the posterior nares, in the way described on 

 page 187. The bottom of the stomodaeum 

 is now separated from the front end of the 

 alimentary canal by only a thin partition, and, 

 by the end of this day, this partition becomes 

 perforated and puts the stomodaeum into 

 communication with the rest of the digestive 

 tract. As the mouth is formed by the pushing 

 in of the ectoblast in front of the anterior end 

 of the pharynx, and by the upgrowth of the 

 parts surrounding this invagination, it is 

 evident that the buccal cavity must be lined 

 with ectoblast instead of with entoblast, as is 

 the rest (except the cloaca) of the digestive 

 tract (compare with frog, page 46). 



By gently pressing the head of the fresh 

 embryo the cranial nerves and their ganglia 

 may be seen, on each side of the auditory 

 vesicles. 



The vertebral column. — Although the forma- 

 tion of the vertebral column does not proceed 

 very far during this day, it will be a convenient 



