176 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
follows that the position of the micropyle is determined by the 
point at which the membrane becomes attached. T his membrane 
is the same as that which, during the early stages in the growth of 
the egg, separates a circumnuclear, slightly stainable layer of the 
yolk from a more deeply stainable peripheral layer of it, and soon 
becomes attached to the periphery of the ovum. : 
A membrane somewhat similar to that in Caudina has been 
described by Shiifer (80) in the ovum of the rabbit and by Van 
Bambeke (°83) in the eggs of certain bony fishes (Leuciscus, Lota). 
But in Leuciscus, as well as in the rabbit, the peripheral end of the 
pocket formed by the membrane is occupied by a yolk nucleus, and 
it is improbable that the membrane described by these observers has 
any connection with the micropyle. 
The micropyle of the ovum of Caudina has a striking superficial 
resemblance to the striated appendage of the ova of Sagartia and 
other actinians, as described by O. und R. Hertwig (79), which 
extends from the ovum— which lies imbedded in subepithelial con- 
nective tissue 
through the entodermal epithelium of the septum 
to the surface next the gastro-vascular cavity. But no connection 
between the nucleus and the periphery was observed in this ease ; 
besides, the appendage in question is a solid structure, not a tubule, 
These authors regard the structure as an organ connected with the 
absorption of nutriment from the fluids of the gastro-vascular cavity. 
But it would seem unnecessary to attribute to it this function, since 
the surrounding epithelial cells contribute largely to the nourishment 
of the ovum. May it not be, primarily at any rate, concerned with 
the fertilization of the ovum ? 
The egg stalk of mussels, such as von Jhering (77) has described 
in Scrobicularia, resembles the striated appendage of the ovum of 
Sagartia, in that it isa solid protuberance of cytoplasmic material. 
In this case, however, the appendage is a means of attachment of 
the ovum to the wall of the ovary and, as von Jhering has shown, it 
determines the position of the future micropyle. But in neither 
actinians nor mussels is the egg appendage to be regarded as the 
homologue of the membranous funnel in the ovum of Caudina, 
Two attempts have been made to explain the method of forma- 
tion of the micropyle in holothurians. The first is the theory of 
Johannes Miiller 754"), who supposed this to be the point at which 
the egg is attached by a sort of stalk to the ovarian wall, as in 
ophiurans. This view was also held by Leydig (754), who re- 
