- 816 EMBRYOLOGY AND CLASSIFICATION. 
the Fish can no longer be confounded with the 
young Bird or Reptile or Mammal, and the blood- — 
vessels of the latter will soon enter into such con- 
nection with the parent as to distinguish it also 
from the young Bird or Reptile which forms no 
such connection. 1 will not pursue this trans- 
formation farther in all its details, which would 
require numerous figures to be well understood, 
but briefly allude to a few facts proving still more 
clearly the ‘unity of plan prevailing throughout 
the whole Vertebrate type. 
The young Skate up to the period already de- 
scribed does not differ from the young Shark ; 
but when the fins make their appearance, though 
exactly the same at first in both these animals, 
one pair in the Skate presently grows larger 
than the others, expanding upon the sides of the 
body and extending towards the tail and towards 
the front of the head. Thus the young Skate, 
as it advances in life, leaves behind the Shark 
character, common to both in their younger 
state, but permanent only in the Shark, in which 
the fins undergo no such change. This shows 
beyond a doubt that the family of Skates is su- 
perior to that of Sharks, —an inference which 
is confirmed by the order of their succession 
upon earth, the Shark family having preceded 
that of the Skates, in geological times. But it is 
not only among the lower groups that such core 
