102 THE BIOLOGY OF THE FROG CHAP. 



blastopore was closed over. Later this invagination meets 

 and fuses with a diverticulum from the posterior part of the 

 archenteron, thus establishing an opening between the latter 

 and the exterior. The tail arises as an elevation of the 

 region in front of the blastopore, which grows backward and 

 pushes the anus to a more ventral position. Later it be- 

 comes flattened from side to side, and its upper and lower 

 edges become produced into a thin expansion, or tail 

 fin. 



The nostrils appear as a pair of external depressions or 

 pits a little above the rudiment of the mouth. These pits 

 deepen, and finally communicate with the buccal cavity. 

 Above and to the sides of the nasal pits the beginning of 

 the eyes is indicated as a pair of thickenings of the ecto- 

 derm. The outline of the enlarged anterior portion of the 

 medullary tube may be observed from the surface. It is 

 bent downward in front, and shows a division into three 

 regions, which become the three primary vesicles of the 

 brain. Near the posterior of these vesicles there is devel- 

 oped on either side an invagination or pit of the ectoderm, 

 which finally sinks in and becomes cut off from the surface 

 and forms the vesicle of the inner ear. 



At the time the neural tube is formed, the superficial cells 

 of the ectoderm become furnished in many places with cilia 

 by means of which the embryo slowly rotates within the 

 jelly. The general direction of the stroke of the cilia is 

 from before backward. The movement is strongest at 

 the anterior end of the body, and is weaker on the ventral 

 than on the dorsal side. "A tadpole of 6 or 7 mm. will 

 progress, if placed upon its side in water, along the bottom 

 of a flat glass vessel, at the rate of one millimeter in from 

 four to seven seconds." (Assheton '96.) After the tadpole 

 is hatched from the jelly the cilia gradually disappear. 



