FERTILIZATION 113 



All agree that the two most important features of 

 fertilization are a stimulus to development and a trans- 

 mission of hereditary characters. We shall take more 

 latitude than any definition is likely to allow, and shall 

 begin by describing associated structures and the 

 behavior of the sperm and egg before they come into 

 contact. Since we have made our most detailed observa- 

 tions upon Dioon edule we shall use this species as a type. 

 The principal features are about the same in other forms 

 which we have studied. 



The relations of the various structures at the time of 

 fertilization are shown in Fig. 60. The outer fleshy coat 

 of the ovule is highly differentiated, the stony layer has 

 become so hard that it is diflicult to cut it with a pocket- 

 knife, and the inner fleshy layer has been reduced to a 

 thin, papery membrane, which in the figure appears as a 

 dark border lining the inner surface of the stony layer. 

 The nucellus, with its conspicuous beak and pollen tubes, 

 has begun to sag. The tissue of the female gametophyte 

 has become quite firm, while the depression above the 

 archegonia, called the archegonial chamber, has reached 

 its maximum depth, and the nucleus of the central cell 

 of the archegonium has divided to form the egg nucleus 

 and the ventral-canal nucleus. 



Sperms within the pollen tube measure about 200 ju' 

 in diameter and about 275 /Li from apex to base. After 

 leaving the tube they increase somewhat in size, reaching 

 a diameter of 230 /x and a length of 300 ju. Consequently 

 they are easily visible to the naked eye, appearing as 



' The micron, u, Is of a millimeter, about • of an inch. 



1,000 25,000 



All measurements of microscopic objects are expressed in microns. 



