262 SPERMATOGENESIS AND OOGENESIS LESS. 



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

 p.c. i). 



It was mentioned in a previous lesson (p. 200) that in 

 some cases 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 takes 

 place only 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 pronucleus (D, 

 pr. nu. 9 ). 



The formation of both polar cells takes place by a 

 reducing division, so that, while the immature ovum con- 

 tains double the number of chromosomes found in the 

 ordinary cells of the species, the mature ovum, like the 

 sperm, contains only one-half the normal number. 



In some animals the first polar body has been found to 

 divide after separating from the egg. In such cases the egg- 

 mother-cell or immature ovum gives rise to a group of 

 four cells the mature ovum and three polar-cells ; just 

 as the sperm-mother-cell gives rise to a group of four cells, 

 all of which, however, become sperms. 



Shortly after, or in some cases before maturation the 



