CH. V] 



EARLY DEVELOPMENT OF THE EMBRYO 



61 



plates take no part in ths rolling in to form the medullary tube, 

 but flatten out and seem to disappear. 



Ziegler ('92) has made several excellent figures of living 

 embryos of Rana temporaria (Figs. 22, 23). The first of these 

 shows young embryos as seen from in front, so that the sense- 

 plate is turned toward the observer (Fig. 22). A longi- 

 tudinal groove appears in the middle of the sense-plate, and 

 subsequently a transverse groove develops across the sense- 

 plate (Fig. 22, D, E). The depression that later forms the 

 mouth lies at the crossing-point of the longitudinal and trans- 



FIG. 23. Development of embryo, showing closure of blastopore and formation of 

 anus. (After Ziegler.) 



verse grooves. There is present on each side above the mouth 

 a thickened ridge that forms the superior maxillary process. 

 Below and behind the mouth a pair of ridges appear that meet 

 in the middle line. These are the sub-maxillary processes 

 which later form the lower jaw. A pair of depressions of the 

 surface ectoderm below the mouth-area mark very early the 



