DEVELOPMENT. 201 
the yolk, reversing the position of the Articulate; but the 
grand characteristic is the primitive stripe, which is nearly 
confined to the eggs of Vertebrates. This is connected 
with another, the setting apart of two distinct regions— 
the nervous and nutritive. There are three modifications 
of Vertebrate development: that of Fishes and Amphib- 
ians, that of True Reptiles and Birds, and that of Mam- 
mals. The amnion and allantois are wanting in the aquat- 
ic Vertebrates; while the placenta (which is the allantois 
vitally connected with the parent) is peculiar to Mammals. 
In Mammals, the whole yolk is segmented; in birds, seg- 
mentation is confined to the small white speck seen in 
opening the shell. 
At the outset, all animals, from the Sponge to Man, are 
indistinguishable from one another. They are mainly 
drops of fluid, a little more transparent on one side than 
the other; and, in all cases, this almost homogeneous glob- 
ule must develop three well-defined parts—a germinal 
dot, germinal vesicle, and yolk. But while Vertebrates 
and Invertebrates can travel together on the same road 
up to this point, here they diverge—never to meet again. 
For every grand group early shows that it has a peculiar 
type of construction. Every egg is from the first impress- 
ed with the power of developing in one direction only, 
and never does it lose its fundamental characters. The 
germ of the Bee is divided into segments, showing that it 
belongs to the Articulates; the germ of the Lion has the 
primitive stripe—the mark of the coming Vertebrate. The 
blastodermic layer of the Vertebrate egg rolls up into two 
tubes —one to hold the viscera, the other to contain the 
nervous cord; while that of the Invertebrate egg forms 
only one such tubular division. The features which de- 
termine the subkingdom to which an animal belongs are 
first developed, then the characters revealing its class. 
There are differences also in grade of development as 
