ELEMENTARY MORPHOGENESIS 31 



THE EGG : ITS MATURATION AND FERTILISATION 



We know that all the organs of an animal or plant con- 

 sist of cells, and we know what acts a cell can perform. 

 oSTow there is one very important organ in all living beings, 

 which is devoted to reproduction. This organ, the so-called 

 ovary in animals, is also built up of cells, and its single cells 

 are called the eggs ; the eggs originated by cell-division, and 

 cell-division is to lead from them to the new adult. 



But, with a very few exceptions, the egg in the ovary is 

 not able to accomplish its functions, unless certain typical 

 events have occurred, some of which are of a merely pre- 

 paratory kind, whilst the others are the actual stimulus 

 for development. 



The preparatory ones are generally known under the 

 name of " maturation." The egg must be " mature," in 

 order that it may begin development, or even that it may 

 be stimulated to it. Maturation consists of a rather com- 

 plicated series of phenomena : later on we shall have 

 occasion to mention, at least shortly, what happens in the 

 protoplasm during its course ; as to the nuclear changes 

 during maturation it may be enough for our purposes to say, 

 that there occur certain processes among the chromosomes, 

 which lead to an extension of half of them in the form of 

 two very small cells, the " directive cells ' ; or " directive 

 or polar bodies," as they have been somewhat cautiously 

 called. 



The ripe or mature egg is capable of being fertilised. 



Before turning to this important fact, which, by the way, 

 will bring us to our specially chosen type, the Echinus, a 

 few words may be devoted to the phenomenon of " partheno- 



