*9«] SHARP— ORCHID EMBRYO SAC 377 



Bletia Shepherdii Hook. 



This form affords another example of the derivation of the female 

 gametophyte either from one or from four megaspores, the course 

 followed being apparently connected with the position of the wall 

 formed at the division of the megaspore mother cell, as pointed 

 out in Epidendrum variegatum. The nucleus of this cell goes into 

 synapsis (fig. 34) and at its division the spindle may lie near the 

 micropylar end of the cell or at its center. In the former case the 

 daughter cells are very unequal in size; the small micropylar one 

 degenerates, and the chalazal one divides to form two megaspores 

 (%• 35)- Of these the outer one disorganizes, while the inner one 

 enlarges and continues the development, by two successive divisions 

 giving rise to the 4-nucleate embryo sac (figs. 36-38). 



When the division of the megaspore mother cell nucleus occurs 

 at the center of the cell (fig. 39) the wall formed is evanescent, the 

 two nuclei thus being left free in the same cell cavity (fig. 40). 

 These nuclei divide simultaneously, as shown in fig. 41; here the 

 wall laid down at the first mitosis in the megaspore mother cell is 

 still visible as a remnant, and several chromosomes are seen lying 

 m the cytoplasm apart from the spindles. The four nuclei which 

 thus arise, being the product of two successive divisions from the 

 nucleus in which the heterotypic prophases occur, are to be regarded 

 as megaspore nuclei (fig. 42). 



Except for the absence of disorganized cells at the micropylar 

 end, the 4-nucleate sac formed as just described is similar in appear- 

 ance to that produced from a single megaspore (cf. figs. 38 and 42). 

 Since the active growth of the sac results in the complete oblitera- 

 tion of the disorganized cells, it is not possible to determine by 

 inspection of the later stages from which type of 4-nucleate sac 

 they have been derived, but there seems to be no reason why either 

 type or both should not continue the development, which from this 

 point onward is exceedingly irregular. Abnormalities of many 

 kinds were observed, and all that can be attempted here is to indi- 

 cate one or two of the common tendencies shown. 



In only three cases were there seen more than four nuclei in the 

 embryo sac before fertilization. In one of these (fig. 43) the two 

 micropylar nuclei had divided, resulting in a 6-nucleate sac like 



