EMBRYOLOGY AND CLASSIFICATION. 315 



In this condition it closely resembles the young 

 Snake as represented by Rathke, or the young 

 Bird as represented by Pander, or the young 

 Rabbit as represented by Bischofif, and the inex- 

 perienced student of Embryology would find it 

 difficult to detect any character by which these 

 different embryos could be referred to their re- 

 spective classes among Vertebrates ; for nothing 

 indicates in them as yet the Fish or the Reptile, 

 the Bird or the Mammal. But as they increase 

 in size and complication of structure, the young 

 Skate becoming prominent above the yolk from 

 which it is nourished, it may be perceived that, 

 while it retains its primitive connection with the 

 yolk, through the enlarged vessels first observed, 

 its body remains exposed above it, while in the 

 other three the body becomes enclosed in a bag 

 which gradually grows out of its own lower mar- 

 gin, and, bending over the back, closes upon it 

 to form a protecting envelope, the amnios, while 

 another bag, the allantois, now extends from the 

 lower side, covered with vessels, which increase in 

 number and extent as the bag grows larger, while 

 at the same time the vessels of the yolk and the 

 yolk itself are gradually drawn into the body. 

 This new bag, with its innumerable vessels, folds 

 also in every direction over the young already en- 

 closed in its first bloodless envelope, and so forms 

 a second protecting sac. From this time forward 



