Cap. I. MEMBRANES OF THE EGG. 485 
cate, beyond the possibility of a doubt, that the zona, of which they are the 
constituents, is not a part of the yolk, but an envelope of the egg.’ 
In the next stage, in an ege about one sixteenth of an inch in diameter, we 
find the cells of the zona so pressed against each other that their outlines are 
not recognizable, (Pl. 9a, fig. 16, 16a, ¢,) and only a clear, thick band represents 
the presence of this membrane. Again: in an egg about one tenth of an inch in 
diameter, this layer is found much thickened and more conspicuous, by reason of 
the transverse strie which extend from its outer to its inner contour (PI. 9a, 
fig, 18, 18a, e). 
up the zona; when it becomes evident that they are the outlines of columnar 
The nature of these striw is readily demonstrable, by breaking 
cells? (Pl. 9a, fig. 19,) flattened in a direction opposite to that m which we found 
them in the beginning (Pl. 9a, fig. 17, 4’). 
and flexible yet tender layer, exhibiting a considerable degree of tenacity between 
At this age the zona is a very elastic 
the cells of which it is composed. This structure remains unchangeable for the 
rest of the interovarian life of the egg, (Pl. 9d, fig. 2, ¢,) as may be demonstrated 
by resorting to a full-grown ovum. 
Subsequently to the entrance of the egg into the oviduct, the zona would 
seem to be gradually resorbed, as the “investing membrane,” developing close 
upon the surface and from the substance of the yolk, becomes more and more 
defined as a membrane, and takes the place of the former. At what period the 
zona disappears altogether it is not possible now to say; but, in some instances 
certainly, not till after the embryo has commenced to form its amnios, when it 
appears to be very thin, and, in addition to other characteristics, is remarkable for 
This latter 
At a later period it has not been 
turning suddenly to a nacreous white upon the application of water. 
peculiarity we have not noticed previously. 
possible to find the least trace of the zona, excepting perhaps a mere film lining 
the shell, as late as during the middle period of incubation. 
The Vitelline Sac. In a previous section we have already discussed the origin 
1 See, below, note 1, p. 486. this? We ean hardly believe that these cells have 
>I y 
2 Thompson, loc. cit., page 83, says he is inclined such a far-reaching power. 
, D Fn) 
to believe that these striaw, as seen by Remak (Miil- 8 Dr. Martin Barry (Researches in Embryology, 
ler’s Archiy, vol. 4, p. 252) in the ovum of the rab- Phil. Trans., 1838, p. 316) says, “In the ovary of 
bit, do not depend on any structure of the zona itself, 
but rather on the markings produced by the adhesion 
of the pediculated cells of the tunica granulosa, which, 
under pressure, leave a radiated appearance on the 
zona; but we would ask, how are the cells of the tu- 
nica granulosa enabled to produce transverse striz in 
the considerable thickness of such a membrane as 
Birds, Amphibia, and Fishes, it is, I believe, allowed 
that there is no membrane formed external to the 
membrana vitelli,” and denies that the zona pellucida 
(“Chorion,” as he calls it, but in Phil. Trans., 1839, 
p- 310, he says “ Zona,”) has its parallel in other Ver- 
tebrata than the Mammalia. 
4 See Sect. 1 of this Chapter, p. 454-456. What 
