ORGANS OF REPKODL'CTION. 331 



the Mistletoe, two or three embryo-sacs are formed. This sac is 

 either formed by a simple hollowing out of the nucleus, and the 

 consequent formation of a p- .^ 5^ jr-^^ . ^ e_ 



cavity of variable form and 

 size ; or it is produced (as 

 appears to be generally 

 the case) by the special 

 development of one of its 

 cells, and which as it con- 

 tinues to increase in size, 

 presses upon the sun-ound- 

 ing cells, and thus causes 

 their more or less complete 



absorption. This sac some- ^ifl- 715- Unrlivided ovue of the Mistletoe 



*,',„^„ «„ „„ 4.i,„ „i.^ 4. (^Viscian album), consisting of a naked 



times causes the almost nucleus Fig.nc. The same ovule cut 



entire absorption of the vertically to show the embryo-sac, c. 



1 1 . ^ n. Nucleus. 



nucleus, and even projects 



beyond it, either through the opening in its coats afterwards 

 to be described, called the micropyle, or through its sides 

 in various directions, by which one or more saccate processes 

 are formed. The embryo-sac is surrounded by a thin layer of 

 cells, which has received the name of tercine. The sac contains 

 at first, an abundance of protoplasmic matter, in which are de- 

 veloped towards, or at the period of fertilization, endosperm 

 cells and germinal vesicles. The formation of these will be 

 alluded to hereafter, under the two heads of Albumen and Fertil- 

 ization. The protoplasmic semi-fluid matter is by some called the 

 liquor amnios. Some ovules, as those of the Mistletoe (^^f 715), &c. 

 consist simply of the nucleus and embryo-sac as above described, 

 in which case the nucleus is termed naked. In almost all plants, 

 however, the nucleus becomes enclosed in one or two coats: 

 thus in the Walnut, there is but one coat, and that appears at first 

 as a little circular process around its base, this gradually in- 

 creases in size, and by growing upwards, ultimately forms a 

 sheath or cellular coat to the nucleus, which it entirely closes 

 except at the apex, where a small opening may be always ob- 

 served {fig. 717). The coat thus formed, where there is but one, 

 is called the integumentum simplex, s, and the orifice, end, at 

 the apex of the nucleus, n, is termed the micropyle ox foramen. 

 Besides the Walnut, there is only one coat formed in the Com- 

 posite, Campanulacea;, Lobehaceoe, &c. 



In most plants, however, the ovule has two coats, in which 

 case we observe two circular or annular processes around the 

 base of the nucleus, the inner one being first developed, and 

 consequently projecting at this time beyond the outer. These 

 processes continue to grow upwards as before described, until 

 they also ultimately form two sheaths or coats, which entirely 



