FORMATION OF THE EMBRYO. 



315 



mere hollowing out, produced by absorption, and having no evident 

 lining membrane. 

 Usually, however, 

 in this cavity filled 

 with fluid, or de- 

 veloped with it so 

 as to form its spe- 

 cial parietes, a 

 large cell appears 

 and expands into 

 a bladder or clos- 

 ed sac of consid- 

 erable size. This 

 is the sac of the 

 amnios of Mr. 

 Brown, the em- 

 bryo-sac (sac embryonaire) of the French botanists.* In this sac 

 the embryo is formed. 



576. From Linnaeus downwards, until recently, it was univer- 

 sally supposed that the embryo originated in the ovule, which was 

 in some way or other fertilized by the pollen. Since the discovery 

 of the pollen-tube in 18:24 by Amici, and its actual penetration to 

 the nucleus of the ovule by Mr. Brown, however, the late Professor 

 Horkel, and his nephew, Schleiden, who traced it quite to the 

 embryo-sac, have propounded a very different view. Schleiden 

 and his followers strongly maintain, as the result of direct observa- 

 tion, that the apex of the pollen-tube itself becomes the embryo ; 



* " The ovule is produced by the development of one cell of the placenta 

 into a cellular body, which essentially consists of a central row of cells, in- 

 rluM-il by a variable number of layers of cells. One of the cells of the central 

 row riilarges and displaces a varying quantity of the rest of the tissue of the 

 ovule. This is the embryo-sac." Hoflhieister, as rendered by Henfrey, Bot. 

 Gazette, I p. r,'7. 



FIG 43) A back view of a stamen of the common Milkweed (Asclepias), the appeodaf* 



; A M.-unrn in .re magnified, with the two pollen- masues cohering by their 



MMfiefet, each to a eland from the summit of the atigmatk body, to which a pollen mwa torn 



.'.her ;> .iiu-.uU .. iiiereot. 422. A pair of detached yolUa IIUIIMI (each front a 



: caudiclea from the gland. 423. Some of the pollen 

 with their tut- tW stisma (after Brown). 424. A section through the large anig- 



niaiic. body and a part of the summit of one of the styles, showing the coon* of the poBeo* 

 tube*. -. highly magnified. (The structure of these 



singular Mowers will be more fully explained under lh ortfer Ju&pMutocor,) 



