314 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 Kadiate. 

 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. 



