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 

 readies 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. e., 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 



