FUNCTION.] EMBRYOLOGY OF TROP.EOLUM, 227 



placenta, is a portion of rather firm and dense cellular tissue, 

 inclosing a bundle of vessels, and forming the so-called 

 umbilicus : this, with the vessels it incloses, descends in 

 apposition with the placenta to form the raphe, and near the 

 point where it terminates in the base of the ovule, the vessels 

 are gradually lost, or rather terminate in closed extremities. 

 The nucleus has only one tegumentary membrane (primine ?) 

 at the apex of which is presented the exostome, or micropyle, 

 opening close by, and to the outside of the umbilicus : so 

 that the direction of the nucleus is exactly parallel with that 

 of the axis of the pistil. The conducting tissue of the style 

 may be traced between the columella, and that prolongation 

 of the carpellary leaf which forms the style into the carpellary 

 cavity, as far as the exostome, with which it is brought in con- 

 tact by the anatropous development of the ovule. The vessels 

 which proceed along the placenta to form the raphe, are 

 spiral vessels and annular ducts ; and at the point at which 

 they make a turn downwards towards the chalaza many of 

 them end in closed extremities, while the vascular structure 

 of the raphe usually terminates in a single vessel. These 

 vessels, together with an analogous set which run along the 

 dorsum of the carpel, proceed from a larger bundle of vessels, 

 which in the receptacle bifurcates into these two sets. 



Second Period. During the expansion of the bud, before 

 the dehiscence of the anther, and, therefore, before impreg- 

 nation, a small elliptical cavity appears near the apex of the 

 nucleus, having a delicate lining membrane formed by the 

 walls of the surrounding cells. This cavity is the embryo-sac 

 ('sac embryonnaire/ Brongniart and F. Gr. F. Meyen; 

 ' membrana anmii/ Malpighi ; ' quintine/ Mirbel.) From 

 the exostome a minute canal may be traced in the apex of 

 the nucleus, leading to the embryo-sac. The apex of the 

 embryo-sac incloses, at this period, a quantity of organisable 

 mucilage, containing many minute bodies having the appear- 

 ance and character of cytoblast (Schleiden.) 



Third Period. The apex of the nucleus, and of its tegu- 

 mentary membrane, is now inclined and approximated towards 

 the axis of the pistil. The embryo-sac is much enlarged and 

 lengthened; its mucilage has disappeared; and in its place 



