276 



Cell as Basis of Organic Activity 



Fertilization. — The whole process of gametogenesis is but in 

 preparation for the important act of fertilization and the production of 

 new individuals. By this act of fertilization, the diploid number of 

 chromosomes is again attained and a remarkable adaptation for the new 

 combination of hereditary characters is achieved. The fertilized egg is 

 termed the zygote. 



ENTRANCE or SPERM 



FIRST MATURATION 

 DIVISION 



FORMATION OF 

 FIRST POLAR BObY 



FIRST POLAR BODY 

 SEPARATED 



-OYAD 



SECOND MATURATION 

 DIVISION 



FORMATION OF 

 SECOND POLAR BODY 



MALE AND FEMALE PRONUCLEI 



FIRST CLEAVAGE 

 SPINDLE FORMED 



Fig. S3 — Maturation of the egg, sperm entrance, and fertilization 

 in the roundworm, Ascaris. 



The exact mechanism by which the sperm penetrates the egg is 

 complicated and has actually been observed in only a few animals where 

 the fertilization is external, that is, outside the body. The sea urchin, 

 a marine invertebrate, has been the chief source of this information. It 

 has been shown that the eggs of this animal secrete a special substance, 

 fertilisin, which actually attracts the sperm. The sperm swims in a 

 spiral path, and by its random movements comes within the influence of 

 this material. If the sperm head touches the egg surface at a proper 

 angle, it adheres, and the egg cytoplasm actually engulfs the sperm 

 head. The head and middle piece enter, and the tail is left outside. 

 Following the entrance of the sperm, a membrane is separated on the 

 surface of the egg, the fertilisation membrane. This membrane pre- 

 sumably prevents the entrance of other sperms. 



