174. BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
to the surface and joins the follicle. Here there is a conical, or some- 
times hour-glass shaped, passage through the whole thickness of the 
zona, which is sometimes drawn out into a sort of chimney-like eleva- 
tion around the orifice (Plate 7, figs. 94, 95). 
The zona when fully developed is about 4 » thick, in eggs which 
have been sectioned, but in fresh ova it becomes by the absorption of 
water 20-30 » in thickness. Like the zona radiata generally, it is 
transparent, colorless, and marked with radiating lines; it is not 
stainable with carmine, but takes haematoxylin and methyl green 
readily, so that it can be easily distinguished from the follicle. By 
macerating a well-grown ovum the zona is found to consist, not of a 
homogeneous substance pierced with pore canals,—as Hamann believes 
is the case in Holothuria tubulosa,— but of an inner transparent, 
homogeneous yolk membrane, which makes up about a third of the 
thickness of the zona, and of an outer layer composed in the main of 
highly refractive and elastic rods, which can easily be separated from 
one another, but which adhere closely to the yolk membrane (Plate 7, 
tig. 96). Semper may have recognized this fact, for, in figuring the 
zona of the egg of Caudina (Taf. 10, fig. 8), he shows two layers of 
nearly equal thickness ; and he calls the outer the “ faserige Schicht.” 
In the text, however, he states that the Aihaut as a whole is 
pierced with numerous pore canals. 
Is the zona radiata secreted by the ovum or by the follicular cells ? 
Semper in discussing this question suggests that in general the 
follicular cells, at the time that the zona first appears, are so far 
modified as to have probably ceased to be actively metabolic, but he 
adds that in Caudina arenata it seems probable that a part, rf not all, 
of the Hihaut is the product of the follicular cells. He does not 
state, however, on what he bases this conclusion. 
Examination of sections shows clearly that at the time when the 
zona radiata begins to appear the follicular cells are already very 
much moditied, being flattened into a thin transparent membrane, 
while the nuclei, which alone can be considered to be actively meta- 
bolic, are widely scattered. Furthermore, the fact that, just before a 
definite zona can be said to be present, the cortical portion of the yolk 
stains more deeply than the rest of it, seems to indicate that the 
secretion of the zona by the ovum is already in preparation, 
There is reason for believing that only the outer or cortical layer of 
the egg cytoplasm is concerned in the secretion of the zona, because 
at the point where the intravitelline membrane comes to the surface, 
