MAMMALIA. 



191 



number to about twelve. The amniotic folds nearly meet above the 

 embryo. 



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The later stages in the development proceed in the main in the 

 same manner as in the Bird. The cranial flexure soon becomes very 

 marked, the mid-brain forming the end of the long axis of the embryo 

 (fig. 146). The sense organs have the usual development. Under the 

 fore-brain appears an epiblastic involution giving rise both to the 

 mouth and to the pituitary body. Behind the mouth are three well- 

 marked pairs of visceral arches. The first of these is the mandibular 

 arch (fig. 146 md), which meets its fellow in the middle line, and forms 



