THE CYCLE OF LIFE 



375 



structure, and 

 of the not less 

 characteristic 

 associated 

 faculties or 

 activities. 

 Often we can- 

 not tell one 

 kind of egg 

 from another, 

 but the one 

 will develop 



FIG. 58. Egg of Ascidian, Ciona intestinalis, after 

 Duesberg, to show distribution of organ- 

 forming substances in the egg, which is 

 about to divide into two. At the lower 

 pole there is a distinct crescent with very 

 crowded small granules (plastochondria) ; 

 around the division-spindle in the centre 

 there is a clear zone ; the upper portion of 

 the egg has numerous yolk granules and 

 few plastochondria. 



into a star- 

 fish and the 

 other into a 

 sea-urchin, 

 one will be- 

 come a rep- 

 tile and the 



other a bird. Development is the expression or realization 

 of an inheritance. 



The starting-point of an individual life is usually a 

 fertilized egg-cell a new unity formed by the intimate 

 and orderly union of paternal and maternal inheritances, 

 conveyed we know not how in the often microscopic egg- 

 cell and the extremely microscopic sperm- cell. There 

 may be development from a bud or from a fragment of a 

 parent organism this is the expensive process of asexual 

 reproduction. There are also many cases of partheno- 

 genesis, where the egg-cell develops without being ferti- 

 lized. Thus a drone-bee has a mother but no father, but 

 these modes of asexual reproduction and parthenogenetic 



