404 XXIX. THE GYNCEC1UM OF ANGIOSPERMS. 



nucleus of the embryo- sac and its descendants at first divide free, 

 and only at a later stage of development the formation of partition 

 walls between these nuclei commences. The process, as seen 

 here, takes place in general in such embryo-sacs as show slow, and 

 on the whole inconsiderable, increase in size. Where, on the 

 other hand, the embryo-sac increases very rapidly in size after the 

 fertilisation of the oosphere, there nuclear division without cell- 

 division first takes place, and cell-formation, i.e., the formation 

 of the partition walls, first begins when the embryo-sac is ap- 

 proximately fully developed. As a result of fertilisation the 

 oospore has acquired a delicate cellulose membrane, and soon 

 begins to elongate into a sac, and after some time penetrates with 

 its apex into the body of the endosperm, where the apex of the 

 sac produces a few celled embryo. We have thus far examined 

 these ovules only in pure water or in sugar solution ; if we wish 

 to see the nuclei show up clearly, we must treat the ovules with 

 2 per cent, acetic acid. In this way we obtain very sharply- 

 defined figures in most ovules, and at the same time fix dividing 

 phases of the nuclei, although into these processes we do not 

 propose at present to go more deeply. Staining media cannot be 

 recommended, since they stain also the nuclei in the integument, 

 and in this way disturb the view into the interior. 



Embryo-Sac of Orchidece. Instead of Monotropa various Or- 

 chids (Orchis and other genera) can be used for study. Fertilisa- 

 tion takes place in these a good while after pollination, from three 

 to ten days, according to species, and in ovaries which are already 

 greatly enlarged. These are cut open, ovules removed with 

 the needles from a placenta, and transferred to water or 3 per 

 cent, solution of sugar. We can at once recognise the structure 

 of the fully-formed ovule (Fig. 147), which is very like to that of 

 Monotropa ; but there are two integuments, as commonly in 

 Monocotyledons, and an air-cavity in the neighbourhood of the 

 chalaza. This air-cavity makes observation more difficult, for 

 it is filled with air, which also penetrates between the integu 

 ments. The ovule, in water or in 3 per cent, sugar solution, 

 must therefore be freed from air under the air pump. Often even 

 a slight pressure upon the cover-glass serves to remove the most 

 disturbing air, found between the integuments. The nucellus in 

 Orchideae also is quite displaced by the embryo-sac ; as a relic 

 of the nucellus a strongly refractive cap of substance is often still 

 to be seen at the apex of the embryo-sac. The egg-apparatus (os) 



