200 COMPARATIVE ZOOLOGY. 
mon to all animals. But every grand division of the Ani 
mal Kingdom has its characteristic method of developing. 
Protozoans differ from all higher forms in not undergo- 
ing segmentation. 
The egg of the Sea-anemone, after segmentation, be- 
comes a solid pear-shaped body, covered with cilia. Soon 
one end is indented; then the indentation deepens until it 
becomes a cavity, and the edge rolls inward till it extends 
half-way to the bottom. Little ridges are then seen in the 
interior, which finally become so many partition-walls; 
while minute protuberances around the opening, or mouth, 
are the beginnings of the tentacles. There is no distine- 
tion into nervous and di- 
gestive regions. The egg 
of the Oyster divides in 
two, as if to form two in- 
dividuals, but soon two tri- 
angular disks appear, the 
rudiments of shells, and 
Fia. 170. — Development of Sea- anemone, the two pulsating eal: 
slightly magnified: 1, first stage after seg- fused together into one 
mentation; 2, shows the beginning of the r 
digestive cavity; 6, its completion; a, ten- heart. Then the embryo 
bgeles +) Dypeniuens: revolves in the egg, and 
becoming fringed with cilia, it quits the egg, and closes 
the valves, which were wide open. The embryo of an In- 
sect shows from the first a right and left side; but the 
first indication that it is an Articulate is the development 
of a series of indentations dividing the body into succes: 
sive rings, or joints. Next, we observe that the back lies 
near the centre of the egg, the ventral side looking ont- 
ward; 7. 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 toward 
