368 FERTILISATION AND EMBRYOLOGY 



trates the overlying nucellar cap, and thus reaches the embryo 

 sac. Meanwhile the generative cell has divided to form two 

 male nuclei (Fig. 215, C, 3), which are extruded from the tip 

 of the pollen tube, and, entering the embryo sate, fuse respec- 

 tively 1 (a) with the egg, and (b) with the two polar nuclei 

 (cf. Fig. 215, D), or with the nucleus formed by the fusion of 

 the latter. The former fusion results, in the usual way, in the 

 production of an embryo, but the latter also leads to abundant 

 division, whereby a nutritive tissue, the endosperm, is formed. 

 In the utilisation of both male nuclei for these different purposes, 

 the Angiosperm exhibits a characteristic difference from the 

 Gymnosperm. 



The product of the second fusion, which is really built up of 

 three nuclei (one from either end of the embryo sac and one 

 male nucleus), divides repeatedly, till the sac becomes filled 

 with numerous free nuclei distributed uniformly throughout its 

 protoplasmic content. Formation of separating walls now takes 

 place almost simultaneously, so that the sac becomes occupied 

 by a continuous thin- walled tissue, the endosperm, which stores 

 food for the; developing embyro. It thus fulfils the same function 

 as in Gymnosperms, where, however, it is formed already prior 

 to fertilisation. 



During the ftuclear divisions leading to endosperm-formation, 

 the fertilised egg, now provided with a cell-wall, enlarges con- 

 siderably and commences to segment. The first division is 

 unequal, a smaller cell situated at the end away from the 

 micropyle being cut off from the large remaining portion. The 

 latter forms the sac-like basal cell (Fig. 216, I-VI, 6.) and plays 

 no further part in the development of the embryo. The smaller 

 segment undergoes successive transverse divisions, so as to give 

 rise to a short elongating suspensor (Fig. 216, S.). Hereby the 

 slightly larger hemispherical terminal cell (<?.), which subse- 

 quently produces the embryo proper, is carried down into the 

 heart of the developing endosperm. Young proembryos at this 

 stage of development, with a suspensor composed of a number 

 of flat cells, are readily squeezed out of very young seeds of 

 the Shepherd's Purse, such as can be removed from the ovaries 

 of flowers which have not yet faded ; the seeds are best mounted 

 * This process is often spoken of as " double fertilisation." 



