EMBRYOLOGY AND CLASSIFICATION. 815 
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 Bischoff, 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 
