4I 6 COMPARATIVE ZOOLOGY 



brain, mid brain, and hind brains The fore brain throws 

 out two lateral hemispheres (cerebrum), and from these 

 protrude forward the two olfactory lobes. From the 

 mid brain grow the optic lobes ; and the hind brain is 

 separated into cerebellum and medulla oblongata. The 

 essential parts of the eye, retina and crystalline lens, 

 are developed, the former as a cuplike outgrowth from 

 the fore brain, the latter as an ingrowth of the epider- 

 mis. An infolding of the epidermis gives rise to the 

 essential parts of the inner ear, and from the same layer 

 come the olfactory rods of the nose and the taste buds 

 of the tongue. So that the central nervous system and 

 the essential parts of most of the sense organs have a 

 common origin. 



Modes of Development. The structure and embryol- 

 ogy of a hen's egg exhibit many facts which are common 

 to all animals. But every grand division of the animal 

 kingdom has its characteristic method of developing. 



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 ani- 

 mal 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 derivatives, 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- 



