358 



CHORD AT A. 



stage culminates in the metamorphosis. The gills atrophy, the gill- 

 slits close up, the intestine shortens, the aortic arches assume the 

 adult condition and the tail commences to be absorbed. The animal 

 now begins to leave the water by degrees till, when the tail has com- 

 pletely disappeared and the limbs are completed, it becomes a terres- 

 tiial frog. 



Comparison of Amphioxus, Skate and Frog. — We now have 

 to compare the early stages of the skate, the frog and the Amphioxus, 



Fig. 256. — The Life History of the Common Frog. 



Showing the ^%% and larval or tadpole stages. (For description see Text.) 



The frog passes beyond the fish-stage to the amphibian, but in its early 

 stages, with external fertilisation, short embryonic period and small 

 amount of yolk, it is a nearer approximation to Amphioxus than is 

 the skate. 



Blastula. — In Amphioxus the segmentation is total and equal 

 and produces a (nearly) centro-symmetric blastula with equal cells. In 

 the frog there is some yolk aggregated in the future hypoblast cells, 



