424 



consisting sometimes of several million 

 eggs. The egg cells enter the oviducts 

 and are pushed along by cilia lining the 

 tubes. They leave the body by an open- 

 ing on the lower (ventral) surface. In 

 fish this releasing of egg cells is known 

 as spawning. Similarly, the male releases 

 the sperm cells, close to the eggs. 



If you could place fresh fish eggs and 

 sperms on a slide under the microscope, 

 you would see the sperms lash their tails 

 and move among the egg cells. Many 

 sperms gather around one egg as if in re- 

 sponse to a chemical stimulus. Soon one 

 sperm cell pushes through the cell mem- 

 brane into the cytoplasm of the egg. The 

 sperm nucleus moves tow^ard the nucleus 

 of the egg and fuses with it. Fertilization 

 has been accomplished; the egg is now a 

 fertilized egg. As soon as one sperm has 

 entered the egg, a tough membrane 

 known as a fertilization membrane 

 forms around the egg and no other 

 sperms can enter. The fertilized egg (or 

 zygote) develops into a new individual. 



Is fertilization necessary for develop- 

 ment? As a rule eggs do not develop into 

 mature organisms unless they have been 

 fertilized. But there are some exceptions. 

 Plant lice (aphids) have a complicated 

 life history. In the fall they lay fertilized 

 eggs. These winter over and in the 

 spring develop into young females which 

 bring forth live young. They form eggs 

 within themselves which are never ferti- 

 lized but which develop into new or- 

 ganisms within the parent. Reproduction 

 by means of unfertilized eggs is called 

 parthenogenesis (parth-en-oh-jen'e-sis) . 

 Bees also may reproduce by partheno- 

 genesis. You have read that the queen bee 

 lays all the eggs. Some of these eggs are 



Hoiv Animals and Plants Reproduce unit viii 



unfertilized and develop into male bees 

 (drones). Most of the eggs are fertilized 

 and develop into queens (true females) 

 or into workers (females with incom- 

 plete reproductive organs). Whether a 

 fertilized bee egg develops into a queen 

 or a worker depends upon the kind and 

 amount of food supplied by the workers. 



Parthenogenesis made to occur in the 

 laboratory. Jacques Loeb, a great biolo- 

 gist, performed many experiments to 

 learn whether esfffs could be made to 

 develop without fertilization. In one 

 series of experiments, he transferred the 

 eggs of a sea urchin (a salt water ani- 

 mal) from one particular salt solution to 

 another. This treatment caused some of 

 the eggs to begin their development, 

 even though no sperm had entered. The 

 developing sea urchins had no fathers. In 

 another experiment thousands of frogs' 

 eggs were pricked with a needle; about 

 200 developed into tadpoles and nearly 

 100 of these continued their develop- 

 ment and became adult frogs. These ex- 

 periments have been repeated with the 

 same results, and other experiments have 

 been performed. Unfertilized eggs have 

 been made to develop by the use of heat, 

 acids, various salts, electricity, and other 

 agents. Sometimes the eggs developed 

 only partially; sometimes they developed 

 into adult animals. Among the other ani- 

 mals produced from eggs by artificial 

 parthejio gene sis are starfishes and at least 

 one rabbit. 



Tlie fertilized egg becomes an embryo. 

 i.cr us study the development of a fish. 

 This will help us understand the devel- 

 opment of other animals. If the tempera- 

 tiu'c and other conditions are suitable, 

 the nucleus of the zygote (fertilized egg) 



