THE EMBRYO-SAC IN ANGIOSPERMS. 525 



and pressure, the cells of the other groups in the fore part of the 

 nucellus are seen to gradually lose their outlines and become 

 finally absorbed into a common cavity (PL XVII. fig. 10, PL 

 XVIII. fig. 1, and PL XIX. fig. 5, &c.) as the ovule attains 

 maturity. This enlarging remnant of the " embryo-sac mother 

 cell," which has so important an influence on the surrounding 

 cells, is the embryo-sac. 



The processes which take place in the developing embryo-sac 

 are extremely interesting, but somewhat difficult to follow in 

 detail from the rapidity with which they occur, and also the length 

 of the sac, which makes it by no means easy to obtain longitudi- 

 nal sections through its whole course. The examination of several 

 thousands of ovules has yielded the following results, which are 

 in the main similar to those described bv Strasburger in Orchis 

 and others. The nucleus divides, each new nucleus passing to 

 an opposite end of the sac, and being imbedded in a mass of pro- 

 toplasm (PL XIX. figs. 1-6) ; as these separate a vacuole-like clear 

 space* forms between, and at last occupies the greater portion of 

 the sac, driving the protoplasm to the walls, where it forms a more 

 or less continuous lining. 



Each of these nucleated masses of protoplasm then undergoes 

 further division in its respective end of the sac ; in PL XIX. 

 fig. 7 the upper one has formed four nuclei arranged in a tetra- 

 hedron, while the lower one only appears to have formed two 

 masses, though in other cases four appear here also (PL XIX. 

 fig. 9). At first these are all alike, but very soon become 

 altered in size and position. 



First, one of the upper four nuclei passes down into the cavity 

 of the sac, either in the protoplasm of the lining wall, or sus- 

 pended by "bridles" from this — a process which is imitated in 

 the reverse direction by a nucleus from below (PL XIX. fig. 8). 

 These two nuclei approach one another about the middle of the 

 sac (PL XIX. fig. 9), and come into contact (PL XIX. fig. 10), 

 finally blending into one large, sharp, and refractive nucleus sus- 

 pended somewhere about the central part of the sac. This is 

 the " nucleus of the embryo-sac " (PL XIX. fig. 11). Meanwhile 

 a sister nucleus of the upper group enlarges, and becomes very 

 bright and sharply spherical as the "egg-cell" (oosphere, em- 



* I shall suggest later that this may be a cell-wall even more deliquescent 

 than those preceding it in the "embryo-sac mother cell." 



