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814 EMBRYOLOGY AND CLASSIFICATION. 
If we now pass to the highest type of the Ani- 
mal Kingdom, the Vertebrates, there is no lack of 
evidence to show the identity in their mode of 
development, as well as the striking resemblance 
of the young in their earliest stages of growth. 
The young Fish, the young Reptile, the young 
Bird, the young Mammal, resemble one another 
to an astonishing degree, while they have not one 
feature in their mode of growth which recalls 
either the Articulate, the Mollusk, or the Radiate. 
It is, therefore, not true, though so often stated, 
that in their development the higher animals 
pass successively through the condition of all the 
lower ones; while it is emphatically true that in . 
each of the four great branches of the Animal 
Kingdom there is a common mode of develop 
ment. It is equally true that in certain features 
the higher classes of each branch in their younger 
condition recall the characteristic features of the 
lower ones, though each class has its own struc- 
tural character, and early diverges from the com- 
mon starting-point. One single case may suffice 
to demonstrate this general statement. When 
the young Skate begins to form upon the large 
yolk of its egg, it has an oblong form, somewhat 
club-shaped, the broader end representing the 
head, while the tapering end is the tail. It is 
early surrounded by a network of bloodvessels 
circumscribed upon the yolk by a circular vein. 
