STRUCTURE OF THE BOLL 69 



lining the wall of the embryo-sac. The large central 

 cf.vity not occupied by protoplasm is filled with cell sap, 

 consisting of water with various salts and food substances 

 dissolved in it. At the end of the seed, where the embryo 

 is situated (corresponding to the tip or stalk end when 

 viewed from the outside), the nuclei form walls between 

 themselves, cutting the protoplasm into definite cells, in 

 the midst of which is the embryo embedded. Walling off 

 does not take place until rather later in the other parts 

 of the embryo-sac. By the eighteenth day the embryo- 

 sac is about one- quarter filled with endosperm tissue, 

 and three-quarters with cell sap, and in three or four 

 days more the endosperm has filled the whole sac, only 

 to be disorganized and obliterated by the embryo within 

 a week. 



THE EMBRYO (Fig. 11, B, and Fig. 13). We have 

 already mentioned the earliest origin and appearance of 

 the embryo. To the naked eye nothing of the nature of 

 an embryo is visible on cutting open the seed until about 

 the fifteenth day, for, although the embryo has been 

 growing steadily all the time, its initial dimensions are 

 so small that even rapid cell division does not give it any 

 great size until several days have elapsed. This makes 

 its ultimate behaviour all the more striking. On the 

 eighteenth day it can be seen clearly by the naked eye, 

 in three days more it is 2J millimetres long, and in a 

 week more it has practically filled the seed. The second 

 half of boll maturation is occupied, so far as the embryo 

 is concerned, by differentiating the internal structures, 

 with which we are not concerned. 



