The Cellular Basis 



135 



ments it is artificial parthenogenesis. Natural parthenogensis is 

 relatively rare and in the vast majority of animals and plants the 



FIG. 42. DIAGRAMS OF THE MATURATION AND FERTILIZATION OF THE EGG 

 OF A MOLLUSK (Crepidula}. A, B, First maturation division (ist Mat. 

 Sp.). C, Second maturation division (2d Mat. Sp.) and first polar body 

 (ist PB) resulting from first division. $N, Sperm nucleus. $C, Sperm 

 centrosome. D, Approach of sperm nucleus ($N) and sphere ($S) to 

 egg nucleus ( 9 A/") and sphere ( $ S) ; the second polar body (2d PB} has 

 been formed and the first has divided {ist PB}. E, Meeting of egg and 

 sperm nuclei and origin of cleavage centrosomes. F, First cleavage of 

 egg showing direction of currents in the cell. 



