6 MARGARET C. FERGUSON 



He concluded that the embryo-sac remained for a long time 

 as a single cell, its nucleus finally dissolving to be replaced 

 by a number of free nuclei ; in a few days the sac was filled 

 with long cells reaching to the middle ; at the beginning of 

 winter, the walls of this transitory endosperm were greatly 

 thickened ; in the spring, the thickened walls of the endosperm 

 were absorbed and the cells liberated. Each primordial cell 

 thus made free contained, somewhat later, three or four 

 daughter-cells which were, in their turn, liberated by the disso- 

 lution of the mother-wall. Thus the number of cells within 

 the embryo-sac was greatly increased, the embryo-sac itself 

 growing to more than twenty times its previous volume. The 

 cells of the nucellus also multiplied rapidly except in the region 

 previously penetrated by the pollen-tubes. In the middle of 

 May, a layer of cells lined the embryo-sac, cell layers in- 

 creased until they met in the center, then the corpuscula were 

 differentiated. The corpuscula were always separated in the 

 AbietinccB by one or more layers of cells, and the walls enclos- 

 ing the corpuscula were thought to be channelled, thus afford- 

 ing open communication with the surrounding cells. In Pinus 

 from 3 to 5 corpuscula were developed in each ovule, and a 

 corresponding number of funnel-shaped openings occurred in 

 the upper part of the endosperm. When the pollen-tube reached 

 the corpusculum it contained free spherical cells in its lower 

 end. The tube either flattened itself out upon the corpusculum 

 or penetrated a short distance into it. After fertilization the 

 impregnated germinal vesicle increased in size, its nucleus dis- 

 appeared, and soon a large daughter-cell was seen at the base 

 of the corpusculum. By repeated divisions of this cell the pro- 

 embryo was formed. 



In 1858 Hofmeister found the usual number of neck-cells in 

 Pinus Strobus to be four, exceptionally three, five, or six, all 

 lying in the same plane. He further demonstrated the vacu- 

 olate character of the contents of the corpusculum during its 

 development. These vacuoles disappeared before impregna- 

 tion, being replaced by free cells — the germinal vesicles, or 

 Keimblaschen. A pit was figured in the apex of the pollen- 

 tube after its entrance into the corpusculum, but it was said that 



