THE SEA-URCHIN. 165 



In the dark-colored sea-urcliin (Arbacia punctulata) the 

 ovaries are red, from the color of the contained eggs, 

 while the testes are white. Through the genital open- 

 ings already observed, there passes out into the water 

 from the female a multitude of red spherical eggs. From 

 the male there passes into the water a white liquid, which 

 on examination with a high power of the microscope is 

 seen to be composed of myriads of little bodies, the 

 spermatozoa, like slender tadpoles, and swimming by the 

 active vibration of their tails. 



If these two elements meet in the water the egg may 

 be fertilized; otherwise, the egg does not develop, but 

 soon dies. This process of the fertilization and the 

 changes that the egg undergoes in consequence, have 

 been studied in the following manner : — 



Live sea-urchins were opened, the ovaries and testes 

 removed, and torn open to let their contents escape. The 

 ova and spermatozoa were mixed in a watch crystal of sea 

 water and watched under the microscope ; the actively 

 swimming spermatozoa surround the ova; just how the 

 fertilization is accomplished is not fully known ; it is 

 believed that a spermatozoon enters the ovum. After this 

 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, six- 

 teen, thirty-two, and so on till the number can no longer 

 be counted and the egg looks like a spherical mulberry. 

 This berry-like mass now becomes hollow, next one side is 

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

 on the outside are little hair-like projections of the cells, 

 called cilia, which by their vibrations propel the body 



