776 PHYSIOLOGY. 



figure. Sometimes there is but one coat {fig. 720), and in rare 

 cases the nucleus is naked, or devoid of any coat {fig. 718). 



These coats completely invest the nucleus except at the 

 apex, where a small opening or canal is left, termed the micro- 

 pyle {fig. 1120, w). In the interior of the nucleus, but of 

 various sizes in proportion to it, the embryo-sac {fig. 1120, s) 

 is commonly seen. This sac is, however, liable to many modi- 

 fications; thus, in some cases {figs. 1121 and 1122), as in the 

 Orchidacea, the embryo-sac completely obliterates the cells of 

 the nucleus by its development, so that the ovule consists simply 

 of it and its two proper coats {fig. 1121, «, b). In the Legu- 

 minoscB, the embryo-sac increases still further, and causes the 

 absorption of the secundine or inner coat of the ovule also, so 

 that it is then simply invested by one coat (primine) ; while in 

 other plants, as in the Santalacea, the sac elongates so much 

 at the apex as to project out of the micropyle. The embryo- 

 sac contains at first a more or less abundant quantity of proto- 

 plasm ; in this nuclei afterwards appear {fig. 1121,5), which, by 

 the process of free-cell development, form a corresponding 

 number of cells (usually three), which are commonly termed 

 germinal vesicles {figs. 1120, c, and 1122, h). The vesicles are 

 situated at or near the summit of the embryo-sac. Henfrey 

 says, that these are not perfect vesicles with a cellulose coat 

 before impregnation, but merely corpuscles of protoplasm, or 

 rather free primordial utricles like the unfertilized spores of 

 Fucus (p. 767.) Hence he terms them germinal corpuscles, and 

 applies the term germinal vesicle only to the impregnated 

 corpuscle or rudimentary embryo. Whether these are simply 

 corpuscles of protoplasm or true vesicles is therefore doubtful; 

 but we shall in future, in accordance with the majority of 

 writers, consider them as true vesicles before impregnation. 



Such is the general structure of the unimpregnated ovule. 

 Much difference of opinion, until lately, existed amongst phy- 

 siologists, as to the contents of the embryo-sac previous to 

 impregnation. Schleiden, Schacht, and others, contended, that 

 no germinal vesicle existed in the sac until after the contact of 

 the pollen-tube with it in the ordinary process of impregnation; 

 in fact, they believed that the germinal vesicle was itself formed 

 from the end of the pollen-tube, which, according to their obser- 

 vations, penetrated the wall of the sac, and by subsequent de- 

 velopment produced the embryo. This view was, however, at 

 once combated by many accurate observers, who all agreed in 

 describing the presence of one or more germinal vesicles or cor- 

 puscles in the sac before impregnation. Indeed, Schleiden him- 

 self, who originated this view of the origin of the embryo, has 

 been convinced of his error, by Eaddlkofer, one of his own 

 pupils. 



"When the pollen falls upon the stigma (the tissue of which at 



