358 CHORDATA. 
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- 
tuial 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 Lire History OF THE Comvuon Froc. 
Showing the egg 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, 
