THE CELLULAR BASIS 107 



since that day, only about thirty-five years 

 ago, when Oscar Hertwig first saw the ap- 

 proach and union of the egg and sperm nuclei 

 within the fertilized egg. Indeed so rapid has 

 been the advance of knowledge in this field 

 that many of the pioneers in this work are 

 still active in research. 



1. Fertilization. The development of the 

 individual may be said to begin with the ferti- 

 lization of the egg, though it is evident that 

 both egg and sperm must have had a more re- 

 mote beginning, and that they also have un- 

 dergone a process of development by which 

 their peculiar characteristics of structure and 

 function have arisen; a subject to which we 

 shall return later. But the developmental 

 processes which lead to the formation of fully 



FIG. 23. DIAGRAMS OF THE MATURATION AND FERTILIZATION 

 OF THE EGG OF A MOLLUSK (Crepidula). A, B. First matura- 

 tion division (1st Mat. Sp.) C. Second maturation division 

 (2d Mat. Sp.) and first polar body (1st PB) resulting from 

 first division. <$N, sperm nucleus, gC, sperm centrosome. 



D. Approach of sperm nucleus (<ZV) and sphere (c?*S) to 

 egg nucleus ($JV) and sphere ($/S) ; the second polar body 

 (2d PB) has been formed and the first has divided (1st 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. 



