162 TYPE STUDIES 



pistil. That the carpels are megasporophylls is proved by the structure and 



development of the ovules (see U). 



D. The development of the ovule and embryo sac. These can best be stud- 

 ied from microtome cross sections of the ovule cases prepared as 

 described in Sec. 212. The ovule cases from open flowers contain 

 mature embryo sacs. Those collected from three to four days after polli- 

 nation may show stages in fertilization (the time varies in different 

 lilies). Stages in the development of the embryo sac must be sought 

 in unopened buds along with or somewhat later than stages in pollen 

 formation. The following stages are likely to be found in material of 

 graduated ages and should be drawn : 



1. Young ovules with the two integuments beginning to develop around 

 the nucellus. The end of the nucellus will be exposed, and at the 

 tip, under a layer of cells, is to be found a large cell with deeply 

 staining protoplasmic contents and conspicuous nucleus. This 

 becomes the embryo sac. 



2. Later stages show the inner integument grown beyond the nucellus 

 and forming the micropyle at the tip of the ovule. The outer 

 integument extends around on the outside nearly to the end of the 

 inner integument. 



3. The embryo sac, gradually enlarging with the growth of the ovule, 

 is the seat of three nuclear divisions, or mitoses, by which the number 

 of nuclei is increased to eight. The gametophyte number of chromo- 

 somes, twelve, appears in the first of these mitoses as in the pollen 

 mother cell (B, 2). The first two of these mitoses have peculiarities 

 (see Principles, Sec. 3()0 and footnote) which show that they are of 

 the same kind as those characteristic of pollen formation and spore 

 formation when tetrads are developed within mother cells. But 

 tetrads are no longer developed in the nucellus of the lily, although 

 present in the ovules of many other angiosperms (see Principles, 

 Fig. 304). The four nuclei resulting from the first two mitoses in 

 the embryo sac of the lily, although comparable to megaspore 

 nuclei, have all become included in the very much reduced female 

 gametophyte that is developed in the embryo sac. 



4. The eight nuclei of the mature embryo sac become distributed as 

 follows : (a) three nuclei form the egg apparatus at the micropylar 

 end of the sac, one being the egg nucleus and lying slightly below 

 and between the other two, which are termed synergids ; (b) three 

 nuclei fonn the group of antipodal nuclei at the opposite end of the 

 sac ; (c) the other two, called polar nuclei, approach one another in 

 the center of the sac. This is the structure of the mature female 

 gametophyte (see Principles, Fig. 306, B). 



