DEVELOPMENT. 205 
Protozoans differ from all higher forms in having no 
true eggs. 
The egg of the Hydroid, after segmentation, becomes a 
hollow, pear-shaped body, covered with cilia. Soon one 
end is indented; then the indentation deepens until it 
reaches the interior and forms the mouth. The animal 
fastens itself by the other end, and the tentacles appear 
as buds. In the Sea-anemone, the stomach is turned in, 
and the partitions appear in pairs. 
In the Oyster, the egg segments into two unequal parts, 
one of which gives rise to the digestive tract and its de¬ 
rivatives, while from the smaller part originate the skin, 
gills, and shell. It is soon covered with cilia, by whose 
help it swims about. 
The embryo of an Insect shows from the first a right 
and left side; but the first indication that it is an Articu¬ 
late is the development of a series of indentations divid¬ 
ing the body into successive rings, or joints. Next, we 
observe that the back lies near the centre of the egg, the 
ventral side looking outward; i. the embryo is doubled 
upon itself backward. And, finally, the appearance of 
three pairs of legs proves that it will be an Insect, rather 
than a Worm, Crustacean, or Spider. 
The Vertebrate embryo lies with its stomach towards 
the yolk, reversing the position of the Articulate; but the 
grand characteristic is the medullary groove, which does 
not exist in the egg of any Invertebrate. This feature 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 Amphibians, that of True Reptiles and Birds, and 
that of Mammals. The amnion and allantois are wanting 
in the first group; while the placenta (which is the allan¬ 
tois vitally connected with the parent) is peculiar to Mam¬ 
mals. In Mammals, the whole yolk is segmented; in 
