258 SPERMATOGENESIS AND OOGENESIS 



division of the protoplasm, and the bud becomes separated 

 as a small cell distinguished as the first polar cell (c E, 

 p.c. i). 



It was mentioned in a previous lesson (p. 198) that in 

 some case development from an unfertilized female gamete 

 took place, the process which is not uncommon among 

 insects and crustaceans being distinguished as partheno- 

 genesis. It has been proved in many instances and may be 

 generally true that in such cases the egg begins to develop 

 after the formation of the first polar cell. Thus in partheno- 

 genetic ova it appears that maturation is completed by the 

 separation of a single polar cell. 



In the majority of animals, however, development only 

 takes place after fertilization, and in such cases maturation 

 is not complete until a second polar cell (D and E, p.c. 2) has 

 been formed in the same manner as the first. The ovum 

 has now lost a portion of its protoplasm together with three- 

 fourths of its chromatin, half having passed into the first 

 polar cell and half of what remained into the second : the 

 remaining one-fourth of the chromatin takes on a rounded 

 form and is distinguished as the female pronuclens (D, 

 pr. mi. $ ). 



Shortly after, or in some cases before maturation the 

 ovum is fertilized by the conjugation with it of a single 

 sperm. As we have found repeatedly sperms are produced 

 in vastly greater numbers than ova, and it often happens 

 that a single egg is seen quite surrounded with sperms all 

 apparently about to conjugate with it. It has however been 

 found to be a general rule that only one of these actually 

 conjugates : the others, like the drones in a hive, perish 

 without fulfilling the one function they are fitted to 

 perform. 



The successful sperm (A, sp) takes up a position at right 



