60 



DEVELOPMENT OF THE FROG'S EGG 



[Cii. V 



seen in Fig. 21, A. The anterior end of the medullary tube 

 shows on each side a lateral protrusion, the eye-bulb. At the 

 anterior end of the body we can see the sense-plate and on 

 each side the broad gill-plate. Lying in the sense-plate on 

 each side is a deeply pigmented area which is the forecast of 

 the "suckers," S. There is a depression in the middle line, 

 in the centre of the sense-plate. This depression marks the 

 mouth-depression, and indicates the point at which, later, the 

 stomodseal invagination will take place. (See Fig. 20, F.) 



Fig. 22. — Development of embryo. Anterior view, showing sense-plate, nasal pits, 

 stomodamm, and gills. (After Ziegler.) 



A later stage is shown in Fig. 21, B. The relation of the 

 parts is much as in the last figure. The anterior end of the 

 medullary tube is larger than before, and the protuberances of 

 the eye-evaginations are more apparent. In the gill-plate on 

 each side appears a vertical depression and later another de- 

 pression behind it, GS, GS'. These depressions mark the 

 external gill-slits. The anus has shifted to a more ventral 

 position. The suckers have each elongated ventrally, and have 

 fused into a pigmented V-shaped figure. The outer medullary 



