EMBKTOGENT IN GYMNOSPEEMS. 



293 



filled with cellular tissue or endosperm cells, and at the same time 

 enlarges. This development of endosperm cells occupies frequently 

 a long time, especially in the Abietinese, which require two years to 

 ripen their seeds. After the embryo-sac has become filled with 

 cellular tissue, certain cells at the micropylar end of the sac enlarge 

 and form the corpuscles of Brown, the secondary embryo-sacs of 

 Mirbel and Spach (fig. 523 d). Each corpuscle is at first separated 



Kg. 521. 



Fig. 622. 



Fig. 623. 



from the inner surface of the embryo-sac by a simple cell, which after- 

 wards divides into four by the formation of two septa crossing each 

 other ; then a passage is formed between the inner angles of these 

 cells leading to the corpuscle. In the cavity of each corpuscle free 

 cells appear. After the corpuscles become evident, the pollen tubes 

 resume their growth, pass through the tissue of the nucleus, and reach 

 the outside of the embryo-sac, one over each corpuscle. The tubes 

 then perforate the membrane of the embryo-sac, reach the canal be- 

 tween the four cells, and come into contact with the corpuscle (fig. 

 523 d). A cell at the lower end of the corpuscle then enlarges, and 

 forms the embryonal vesicle. A free cell in the vesicle divides into 

 eight cells by vertical and transverse septa, and these together consti- 

 tute a short cyclindrical cellular body (fig. 524), the pro-embryo, as 

 it is called by Hofmeister. The four lower cells of this pro-embryo, 

 by the elongation of the upper ones (fig. 525), are finally pushed 



Fig. 621. Vertical section of the ovule of the Austrian Pine {Films amstriaca), showing 

 the nucleus, a, consisting of delicate cellular tissue containing deep in its substance an 

 embryo-sac, &, formed before impregnation by the coalescence of a vertical series of a few 

 cells. The micropyle, m, is very wide, and through it the pollen-grains come into contact 

 with the summit of the nucleus, into the substance of which they send their tubes. Fig. 

 622. Vertical section' of the ovule of the Scotch Fir,(Pi?iii5 sylvestris) in May of the second 

 year, showing the enlarged embryo-sac, h (full of endospermal cells), and poUen-tubes, c, 

 penetrating the summit of the nucleus after the pollen has entered the large micropyle of 

 the ovule. Fig. 623. Vertical section of the embryo-sac, &, and of part of the nucleus, a, 

 of the ovule of the Weymouth Pine (Pvnus Strohus). At the micropylar end of the embryo- 

 sac, two cells called corpuscles, d, have made their appearance. Each of these is at first 

 separated from the inner surface of the micropylar end of the sac by a single cell, which 

 afterwards divides intd four, leaving a passage from the surface of the sac down to the 

 corpuscle. The pollen-grain, c, on the summit of the nucleus, then sends down a tube 

 which perforates the embryo-sac, and reaches the corpuscle through the intercellular canal. 



