404 A TEXTBOOK OF GENERAL BOTANY 
Likewise, the term pollen sac is simply another name for the 
microsporangium of the spermatophytes, while a pollen grain is a 
microspore of a spermatophyte. The spermatophytes are divided 
into the two groups gymnosperms and angiosperms. In the — 
angiosperms the megasporangia are inclosed in ovaries, while 
in the gymnosperms they are exposed as in the pteridophytes. 

Fia. 452. Megaspores of pine 
Left, row of four megaspores of pine ; the basal one will give rise to the female 
prothallus. Right, three apical megaspores are disorganizing, while the basal 
one has enlarged considerably. (Redrawn after Miss Ferguson) 
Megasporangium and female gametophyte. The megasporan- 
gium (ovule) consists of a central mass, the nucellus, or sporan- 
gium proper, inclosed in one (Fig. 451) or two envelopes called 
enteguments. ‘The integuments arise as collarlike outgrowths 
which grow up over the nucellus and inclose it completely except 
at the apex, where there is a small opening, the mécropyle. A mega- 
spore mother cell is formed within the nucellus (Fig.451). In most 
species there is only one megaspore mother cell, but in some eases 
there are more. ‘Typically the megaspore mother cell divides: 
to form a row of four megaspores (Fig. 452). Three of these” 
