FACTS AND FACTORS OF DEVELOPMENT 17 



usually broken off and left outside (Figs. 3, 4) . 

 The nucleus in the head of the sperm then be- 

 gins to absorb material from the egg and to 

 grow in size and at the same time a minute 

 granule, the centrosome, appears, either from 

 the middle-piece or from the head of the sperm, 

 and radiating lines run out from the centro- 

 some into the substance of the egg. The sperm 

 nucleus and centrosome then approach the egg 

 nucleus and ultimately the two nuclei come to 

 lie side by side (Figs. 3, 4) . Usually when one 

 spermatozoon has entered an egg all others are 

 barred from entering, probably by some 

 change in the surface layer of the egg or in the 

 chemical substances given out by the egg. 



This union of a single spermatozoon with an 

 egg is known as fertilization. Whereas egg 

 cells are usually, but not invariably, incapable 

 of development unless fertilized, there be- 

 gins, immediately after fertilization, a long 

 series of transformations and differentiations 

 of the fertilized egg which leads to the develop- 



(1st PB) formed by this division; F the second maturation 

 spindle (2d Mat. Sp.) and the sperm nucleus and spindle 

 (<jW) ; O the division of the male and female nuclei in the 

 first cleavage spindle (1st CI Sp.). (After F. R. Lillie.) 



