MULTICELLULAR ANIMALS 193 



HYDRANTH (Fig. 1 76) is the organ that gives rise to buds that develop 

 asexually into free swimming forms. But the term reproductive 

 system in Animal Biology is usually confined to those structures 

 that are concerned in gametic reproduction. 



Fertilization. In animals the two cells that unite to initiate 

 the process of gametic reproduction are observably different. The 

 ovum is a relatively large, inactive cell (Fig. 133) containing more 

 or less stored-up food materials in the form of yolk. The indi- 

 vidual or organs that produce such cells are known as female. The 

 spermatozoa (Fig. 133) are relatively small, active cells, devoid of 

 yolk, produced in male organs. It is necessary to distinguish not 

 only individuals but also organs by sex, for in some animals both 

 types of sex cells are produced in the same individual. The sper- 

 matozoa, actively swimming about, come in contact with, and 

 penetrate the ova, the nuclei of the two cells joining. This union 

 is known as fertilization and the resulting fertilized tgg is the 



ZYGOTE. 



Fertilization may be brought about in two ways. In many ani- 

 mals, for example, the starfish, aquatic annelids, and jelly fish, 

 both eggs and spermatozoa are shed into the surrounding water; 

 the meeting of sperm with egg is therefore largely a matter of 

 chance. Assurance that sperm and eggs are in close proximity is 

 provided in most of these forms by the close association of males 

 and females during the period of tgg and sperm deposition. This 

 is the case in the fishes and Amphibia during the spawning season. 

 Fertilization occurring outside the female body is termed external 



FERTILIZATION. 



The second method of fertilization is accomplished by the deposi- 

 tion of spermatozoa within the body of the female and is known as 

 internal fertilization. Its nature requires that the female be pro- 

 vided with structures for the retention of eggs and the reception of 

 sperms. Internal fertilization much increases the probability of 

 meeting between egg and sperm; usually associated with the char- 



