124 



GENERAL CONCEPTS 



■TalL 



-Entra-ncc Cone 



Hca.d ajid middle 

 .piece of sperm. 



-Entra-ncc pa-tK 



Copula-tion patK 



t and 2n(i pola.i' 

 bodies 



^^l^ 



division- 

 Figure 6.6. Diagram of the stages in the process of fertiUzation, the union of the 

 egg and sperm. 



In Other animals, fertilization occurs within the body of the female, 

 usually in the oviduct, after the sperm have been transferred from the 

 male to the female by copulation or by some other means. This method 

 of internal fertilization requires some cooperation between the two 

 sexes, and many species have evolved elaborate patterns of mating be- 

 havior to insure its occurrence. The male salamander, for example, 

 mounts and clasps the female, stroking her nose with his chin. He then 

 dismounts in front of her and deposits a spermatophore, a packet of 

 sperm. She picks up the spermatophore and stuffs it into her cloaca, 

 where the packet breaks, the sperm are released, and fertilization fol- 

 lows. 



Fertilization involves not only the penetration of the egg by the 

 sperm, but the union of the egg and sperm nuclei and the activation of 

 the egg to undergo cleavage and development (Fig. 6.6). The egg may 

 be in any stage from primary oocyte to mature ovum at the time of 

 sperm penetration, but the fusion of the sperm and egg nuclei occurs 

 only after the egg has matured. There is experimental evidence that 

 the eggs of some species secrete a substance, fertilizin, which is an im- 

 portant constituent of the jelly coat surrounding the egg. Fertilizin 

 causes the sperm to clump together and stick to the surface of the egg. 

 Other extracts of the egg jelly, which may be identical with fertilizin, 

 stimulate sperm motility and respiration and prolong sperm viability. 



After the entrance of one sperm, a fertilization membrane forms 

 around the eggs of some species which prevents the entrance of other 

 sperm. This prevents polyspermy and the possibility of the fusion of 

 more than one sperm nucleus with the egg nucleus. It can be shown 

 experimentally that such fusion of two or more sperm nuclei with one 

 egg nucleus leads to abnormal development. 



Eggs can be stimulated to cleave and develop without fertilization. 

 The development of an unfertilized egg into an adult is known as 

 parthenogenesis (virgin birth). Some species of arthropods have been 

 found which apparently consist solely of females which reproduce 

 parthenogenetically. In other species, parthenogenesis occurs for several 

 generations, then some males are produced which develop and mate 

 with the females. The queen honeybee is fertilized by a male just once 



