XIIl] OVULE OF GYMNOSPERMS 119 



Here also the archespore arises from a sub-epidermal 

 cell, and although it usually becomes sunk deep in the 

 nucellus owing to the growth and repeated bipartition 

 of the cells above it, even this is not unknown in Angio- 

 sperms. Moreover the development of the tapetal cells, 

 and that of the superposed cells produced by the repeated 

 division of the archespore, occur here as before, and the 

 embryo-sac is developed from the lower of these cells as 

 described in the previous case. 



The really essential points of difference refer to the 

 processes in the maturation of the embryo-sac. 



The nucleus of the embryo-sac divides, and each 

 daughter-nucleus repeats the division, and so on, until 

 the enlarging embryo-sac, growing at the expense of 

 surrounding nucellus cells, is filled with a delicate thin- 

 walled tissue of nucleated cells, which has received the 

 name of endosperm. 



Then, while the endosperm and the embryo-sac are 

 still growing, two or three (or more) of the superficial 

 cells, at the micropylar end of this endosperm, begin to 

 be distinguishable by their larger size and more brilliant 

 protoplasm, and assume an oblong shape. 



Following the history of any one of these cells as 

 typical of the rest, it grows much larger than any of the 

 endosperm cells around, its upper end being separated 

 from the nucellus only by its own cell- wall, and sooner or 

 later undergoes a transverse division above, cutting off a 

 small cell which then undergoes division by two walls at 

 right-angles to the transverse wall and to each other. 

 Thus the oval large cell is crowned by four very small 

 ones in a rosette. Next a very small cell is cut off from 

 the large oval cell below this rosette, and grows up like a 

 wedge between the four rosette-cells, driving them apart 

 in the form of a sort of chimney, in the canal of which 



