200 THE METAZO A DIVISION OF LABOR 



union of a sperm and an egg cell. A common bath sponge, an 

 earthworm, a fish, or a dog, each and all of them begin life in 

 precisely the same way. Animals which are thus composed of 

 many cells are known as the Metazoa, as distinguished from the 

 Protozoa, which are made of but a single cell. 



Sexual Development of a Simple Animal. In a many-celled 

 animal the life history begins with a single cell, the fertilized egg. 

 This cell, as we remember, has been formed by the union of two 

 other cells, a tiny (usually motile) cell, the sperm, and a large cell, 

 the egg. After the egg is fertilized by a sperm cell, it splits into 

 two, four, eight, and sixteen cells ; as the number of cells increases, 

 a hollow ball of cells called theblastulais formed; later this ball sinks 



Stages in the segmentation of an egg, showing the formation of the gastrula. 



in on one side, and a double-walled cup of cells, now called a gas- 

 trula, results. Practically all animals pass through the above 

 stages in their development from the egg, although these stages 

 are often not plain to see because of the presence of food ma- 

 terial (yolk) in the egg. In the sponge the gastrula, which swims 

 by means of cilia, soon settles down, a skeleton is formed, other 

 changes take place, and the sponge begins life as an animal 

 attached to some support on the water. The early stages of life, 

 when an animal is unlike the adult, are known as larval stages ; 

 the animal at this time being called a larva. 



The young sponge consists of three layers of cells : those of the 

 outside, developed from the outer layer of the gastrula, are called 

 ectoderm; the inner layer, developed from the inner layer of the 

 gastrula, the endoderm. A middle almost structureless layer, 

 called the mesoderm, is also found. In higher animals this layer 

 gives rise to muscles and parts of other internal structures. 



