168 CARROLL— ON DEVELOPMENT OF 



remains after the destruction of the peripheral nucellar layer and con- 

 sequent increase of the size of the embryo sac after fertilization. (Fig. 

 28) After the embryo has filled the embryo sac, this layer appears as a 

 densely staining but somewhat crushed investment (Fig. 35). Miss 

 Riatt (39) reports a similar jacket layer in the innermost layer of the 

 integument of pallida. To judge from her figure 1, H, however, there 

 is one point of difference from fulva in the differentiation of the two 

 integuments. The hne of separation between the two integuments is 

 shown reaching well down to the basal end of the ovule. 



The development of the megaspore and embryo sac presents nothing 

 unusual. The archesporial cell is recognizable as a hypodermal cell 

 soon after the ovular swelling becomes evident (Fig. 16). No parietal 

 cell is cut off; and so the archesporial cell becomes the megaspore mother- 

 cell. The megaspore mother-cell divides into two (Fig. 17), and then 

 four (Fig. 18), forming a hnear tetrad of megaspores. The innermost 

 megaspore of the tetrad rapidly enlarges and the others rapidly degen- 

 erate. By the time of the first division of the megaspore nucleus the 

 outer megaspores appear as crushed degenerating masses (Fig. 19). 

 Divisions into four and eight nuclei follow quickly (Figs. 20 and 21). 

 Three antipodal cells are cut off and rapidly degenerate (Fig. 22). 

 The two synergid nuclei move to positions side by side in the upper end 

 of the sac. A vertical wall separates the two, then horizontal walls cut 

 them off as separate cells. (Figs 22 and 23). They rapidly degenerate 

 and, at the time of entrance of the pollen tube, appear only as crushed 

 remnants. The two polar nuclei move to the center of the sac and lie 

 in contact, closely appressed, until fertlization (Figs. 22, 25, and 26). 

 The egg lies in position immediately above the polars. Miss Riatt (39) 

 states that no antipodals were found in Inipatiens pallida. She states 

 that the synergids lie below the egg toward the center of the sac. In 

 her figure at the time of fertilization, when she says that no polar nuclei 

 are shown, the structures which she marks synergids resemble the polars 

 oi fulva. 



There are some interesting points in the history of the nucellus. 

 The archesporial cell arises as a hypodermal cell. No parietal tissue 

 arises, so that never more than one layer of nucellar tissue overUes the 

 embryo sac. By the first division of the megaspore the nucellar cells 

 overlying the apex may show signs of shrinkage (Fig. 17). At the stage 

 of fertilization the cells over the apex are disappearing, and cells along 

 the side of the embry^o sac, never more than one layer in thickness, show 

 signs of disintegration. The cells below the embryo sac, generally three 



