Chap. 18 REPRODUCTION 337 



or internal within the body of the female as in some fishes, in salamanders, 

 reptiles, birds, and mammals (Fig. 18.4). Fertilization can occur only in a 

 wet place since sperm cells are swimming cells and all cells are essentially 

 aquatic. 



The eggs of sea-urchins and sand-dollars are beautifully translucent and 

 beneath the microscope much of the process of fertilization can be seen. 

 During the spring breeding season the common eastern sea-urchin (Arbacia 



Fig. 18.4. Courtship of brook sticklebacks 

 (mature fish, two-and-a-half inches long); ex- 

 ternal fertilization of the eggs. In spring the 

 male leaves the school, stakes out his territory 

 and builds a nest, and at the same time appears 

 in breeding colors. The females are now ready 

 to lay from 50 to 100 eggs. The courtship be- 

 gins. The male (left) zigzags toward the female, 

 swims toward his nest, and repeatedly thrusts 

 his head into it. The female enters the nest and 

 lays the eggs. She leaves the nest. The male 

 enters and sheds the milt (sperm cells) over 

 them. The sperms and eggs meet in the open 

 water. The male fans the water over the eggs 

 and thus increases their supply of oxygen; lines 

 indicate currents in the water. (Courtesy, Tin- 

 bergen: Social Behavior in Animals. London, 

 Methuen, 1953.) 



pimctulata) naturally deposits its eggs and sperm directly into the sea. If the 

 mature male and female animals are placed in separate dishes of sea water, 

 the sex cells are discharged. A few eggs, the size of coarse sand grains, can 

 then be slipped onto a glass slide with a little sea water. Under the micro- 

 scope the nucleus of the living egg appears as a rounded body about one-sixth 

 the diameter of the whole egg and the membrane surrounding the cell and 

 the grainy cytoplasm are clearly visible. If now a droplet of the sea water 

 containing sperm cells is added to the eggs, thousands of sperms can be seen 

 swimming toward one or another of the eggs. At once, the surface of almost 



