No. 454-3 STUDIES ON THE PLANT CELL. 743 



chromosomes (Fig. i$b, i, 2) show clearly the V-shaped forms 

 characteristic of this division. The second mitosis (Fig. 15 b, 3) 

 is homotypic. The lowest cell of the group of four (Fig". 15 a, 

 3) becomes the embryo-sac and the mitoses that take place 

 within it as the female gametophyte develops are all typical. 

 This account illustrates a simple history in megaspore mother- 

 cell development and is considered the first of three types in a 

 classification proposed by Schniewind-Thies (:oi). 



The second type of development is one in which two mega- 

 spores are generally developed from a mother-cell and one of 

 these becomes the functional embryo-sac. Schniewind-Thies 

 presents an excellent illustration of this type in Scilla. The 

 first mitosis in the megaspore mother-cell (Fig. 1 5 c] is hetero- 

 typic (Fig. 15^, i)and results in two cells (Fig. I5<r). The 

 second mitosis in both cells is homotypic (Fig. 15 d, 2). Either 

 the lower (Fig. 15 c, 3) or the upper (Fig. 15 c, 4) of the pair 

 may become the embryo-sac. The embryo-sac then includes the 

 homotypic or second mitosis within its development, making it 

 the first nuclear division of the gametophyte history. The typi- 

 cal mitoses of the gametophyte begin with the second nuclear 

 division in the embryo-sac. Three megaspores may be formed 

 in such a group when the cell of the pair that does not become 

 the embryo-sac divides again. 



The third type of development is illustrated by several forms, 

 of which the best known are Lilium (Mottier, '98 and : 03) and 

 Tulipa (Schniewind-Thies : 01). The lily has been much studied, 

 but Mottier presents the most detailed account of spindle for- 

 mation and the behavior of the chromosomes. He supports the 

 observations of Schniewind-Thies, based upon the tulip, and her 

 explanation of this type of development. The megaspore 

 mother-cell of the lily and tulip develops directly into the 

 embryo-sac. The first mitosis in this cell (Fig. 1 5 //) is heterc- 

 typic and the second (Fig. 152') homotypic. These divisions 

 give the four-nucleate embryo-sac and one more mitosis presents 

 the mature structure. This last is a typical mitosis, the only one 

 found in the embryo-sac before the development of the endo- 

 sperm and sporophyte embryo. Thus the two mitoses charac- 

 teristic of the spore mother-cell are here included within the 



