IOO 



THE BIOLOGY OF THE FROG 



CHAP. 



As the above changes are taking place the embryo 

 elongates in the direction of the neural tube, which marks 

 the longitudinal axis of the future animal. On either side 

 of the anterior end of the neural tube there appears a pair 



FlG. 23.— Development of the embryo. A, yolk-plug stage; B, showing 

 the medullary folds, the blastopore nearly closed, and below the latter 

 the invagination which is to form the anus; C, D, later stages; E, the 

 medullary folds have grown together and covered the blastopore. Above 

 the anus is the rudiment of the tail. (From Morgan, after Ziegler.) 



of thickenings of the ectoderm. The anterior members of 

 each pair, the sense plates, grow forward and meet in front 

 of the end of the neural tube ; a depression appears in each 

 plate and marks the beginning of the ventral sucker of the 



