34 2 Descriptive Zoology. 



from the color of the contained eggs, while the spermaries 

 are white. The ovaries and spermaries are similar in form, 

 resembling small bunches of grapes ; and if it were not for 

 the difference in color, it would require microscopic exami- 

 nation to distinguish the two sexes. Both the eggs and 

 sperms are discharged into the water. The sperms are 

 microscopic, tadpole-like bodies, swimming actively by the 

 vibration of their tails. If the sperms do not gain access 

 to the eggs, the eggs do not develop, but soon die. But 

 usually the sperms surround the eggs, there being, ordi- 

 narily, many sperms to each egg. One sperm gains en- 

 trance to an egg, at least the head fusing with part of 

 the egg. The egg is now said to be fertilized. 



After fertilization, the egg mass contracts, leaving a 

 clear space around it inside the outer coat, or cell wall. 

 Soon the egg mass within divides into two equal parts, 

 each of these halves again divides into two, the four then 

 become eight, sixteen, thirty-two, and so on until the num- 

 ber can no longer be counted and the egg looks like a 

 spherical mulberry. This process of division is known as 

 segmentation. The berrylike mass now becomes hollow, 

 consisting of a single layer of cells. Next one side is 

 pushed in like a rubber ball with one side punched in; 

 it now has a wall made of two layers of cells. On the 

 outside are little hairlike projections of the cells, called 

 cilia, which by their vibrations propel the body through the 

 water. A set of needlelike rods develop within the 

 embryo, which soon make a skeleton, shaped somewhat 

 like a common chair. This skeleton has a covering of soft 

 tissue, and the projections, which correspond to the legs of 

 the chair, are covered with strong cilia for locomotion. 

 The digestive tube has at first but one opening, that made 

 by the doubling in of the outer wall, as above mentioned, 



