206 
COMPARATIVE ZOOLOGY. 
Birds, segmentation is confined to tlie small white speck 
seen in opening the shell. 
At the outset, all animals, from the Sponge to Man, 
are structurally alike. All moreover, undergo segmen¬ 
tation, and most have one form or other of the gas- 
trula stage. 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 construc¬ 
tion. Every egg is from the first impressed 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 medullary fur- 
row—the mark of the coming Vertebrate. The blasto¬ 
dermic 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 determine the 
subkingdom to which an animal belongs are first devel¬ 
oped, then the characters revealing its class. 
There are differences also in grade of development as 
well as type. For a time there is no essential difference 
between a Fish and a Mammal: they have the same ner¬ 
vous, circulatory, and digestive systems. There are many 
such cases, in which the embryo of an animal represents 
the permanent adult condition of some lower form. In 
other words, the higher species, in the course of their de¬ 
velopment, offer likenesses, or analogies, to finished lower 
species. The human germ, at first, cannot be distinguished 
from that of any other animal: for aught we can see, it 
may turn out a Frog or a Philosopher. The appearance 
of a medullary furrow excludes it at once from all Inver¬ 
tebrates. It afterwards has, for a time, structures found in 
the lower classes and orders of Vertebrates as permanent 
