Cuap. I. MEMBRANES OF THE EGG. 487 
their oily appearance, their separation from each other, and their position within the 
vitelline sac. In an older egg, about one sixteenth of an inch in diameter, (Pl. 9a, 
fig. 16, 15,) their contents are granular, and the mesoblast very darkly and thickly 
outlined, evidently by reason of its oiliness, which, by its highly refracting powers, 
produces also a dark centre resembling, and no doubt often mistaken for, an ento- 
blast. In an egg one tenth ofan inch in diameter, these cells (Pl 9a, fig. 20, 
20a, a, b,) hardly differ from the last, excepting that their mesoblasts are less in 
size; an irregularity, according to age, noticed elsewhere in regard to the constit- 
uents of other membranes. In another egg of this size we have represented this 
membrane in profile, (fig. 18a, ¢,) as bounded by two lines, the outer being the 
original yolk membrane, and the imner the line of demarcation between the yolk 
and the membrane in question. 
The manner in which the cells of this layer overlap each other, in an egg 
about one quarter of an inch in diameter, (PI. 9a, fig. 21, 21a,) shows that they 
do not as yet all lie in one plane. Here their size, and also that of the meso- 
blast, is considerably increased. The latter has moreover a dot, the entoblast, in 
its centre, and in some instances two dots, with a corresponding elongation of the 
mesoblast, apparently indicating that a selfdivision is in progress, which, as_ will 
be seen hereafter, (Pl. 9a, fig. 27,) is finally accomplished. Upon opening the egg, 
this layer is found to have such a consistency as to restrain the yolk from spread- 
ing rapidly; and moreover it is recognizable by its much lighter color. 
In a full-grown egg the cells of the embryonal membrane are considerably 
larger and more transparent than in the last egg, and exhibit the same double 
entoblasts (Pl. 9a, fig. 22a). By fixing the focus of the microscope at the hori- 
zon of their greatest diameter it will be seen that they are sharply polygonal, 
(fig. 22,) the broad light bands between them representing the thickness of two 
juxtaposed walls, the superficies of which are obscured by mutual fusion, and by 
the absence of refraction consequent upon the loss of curvature. 
Thus far, the embryonal membrane has been traced in its development as a 
feature of the interovarian ege; whatever else may be said of it hereafter, refers 
to its more or less intimate connection with the changes of the embryonic envel- 
opes, of which it becomes at least a prominent part, if not conspicuously an effi- 
cient member. 
In an ege in which the cephalic hood has commenced to form, (PI. 11, fig. 1, 
a',) the cells of this membrane (Pl. 9a, fig. 24) are very transparent, especially 
where they rest closely upon the back of the embryonal area; yet, excepting per- 
haps the slightly smaller size of the mesoblast and the apparent presence of a dot 
1 See note 1, p. 486. 
