1914] MCALLISTER— EMBRYO SAC OF CONVALLARIACEAE 139 



My work on the embryo 



(19) has 



shown that in closely related species of the same genus a marked 

 difference may exist in the origin of the nuclei entering into the 

 structure of the embryo sac. In S. racemosa the mother cell 

 divides unequally, forming a large outer and a small inner cell. 

 Later stages show the division membrane to be permanent. From 

 the outer cell the 8 nuclei of the mature embryo sac are formed 

 by three divisions. Split cell membranes separate for a short 

 time the two pairs of nuclei resulting from the division of the 

 heterotypic daughter nuclei (figs. 28, 29). The nucleus of the 

 small inner cell divides but once, and the 2 resulting nuclei gradu- 

 ally degenerate. The embryo sac in S. racemosa is thus formed 

 from the 2 outer of the 4 potential megaspores. S. stellata (18), 



forms its embryo sac from 4 megaspores 



merged 



partition membranes. 



Smilacina sessifolia 



formed 



megaspores which become m 



division of the 4 nuclei forming the 8-nucleate embry 



(figs. 26, 27). Three divisions of the nucleus 

 mother cell thus produce the necessary number 

 mature embryo sac, as in cases of the so-calle< 

 embryo sac formation. 



megaspore 



ifolia the 4 nuclei resulting from 



me 



membranes 



come to occudv a large common 



5. stellata. 



Fig. 3 shows the 4 cells arranged bilaterally, while in fig. 4 a 

 linear row of 4 is to be seen. The bilateral arrangement here 

 is much the more abundant. The cell membranes of the second 

 division, as is the case in 5. stellata, are the first to disappear 

 (fig. 5), forming thus 2 binucleate cells. Later the middle mem- 

 brane disappears, giving rise to a tetranucleate cell (fig. 6). This 



