50 GEGENBAUR, ON SAGITTA. 
correspond in its curvature with a superficial area of the same 
size on the surface of the yelk, but forms the segment of a 
far smaller sphere, so that on a superficial view of the ovum, 
the mulberry-like stage of segmentation appears to be simu- 
lated. 
On the second day the whole vitellus is subdivided into 
numerous pyramids which are in close contact, and form the 
boundary of a central cavity which has now attained a con- 
siderable size. 
Before describing the subsequent formation of the embryo, 
Gegenbaur refers to some particulars, only briefly noticed 
before, respecting the formation of the products of the seg- 
mentation and their true nature, as well as to the relations 
of the primordial germinal vesicle to the nuclei of the latter, 
The fact of the segmentation taking place beneath the vitel- 
line membrane, which does not become involved in the process 
of development, may at first sight, he says, perhaps, excite 
astonishment—a proceeding of this kind appearing to be 
opposed to our theory of the process of segmentation as well 
as to our notions respecting the multiplication of eels in ge- 
neral. It might therefore be supposed that I have regarded 
some accessory structure as the vitellme membrane, whilst 
the true vitelline membrane either participates im the seg- 
mentary process, or, as many observers have stated, disap- 
pears at the commencement of the process. In the present 
case, however, Gegenbaur says, it is self-evident that he does 
not, under the term vitelline membrane, intend any egg-case 
(Fihiille), but only that membrane which is formed origi- 
nally with the yelk, and surrounds it while still in the ovary ; 
and which, in that situation, already exists at a time when 
the interspace between it and the germinal vesicle scarcely 
exceeds the diameter of the latter. This condition of the 
vitelline membrane recalls the observations of De Quatre- 
fages in Hermella, and of O. Schmidt in Amphicora, in which 
cases segmentation also takes place beneath the yitelline 
membrane. 
The question whether the products of segmentation in the 
ovum of Sagitta, after the primitive vitelline membrane has 
become detached from them, be still provided with a mem- 
brane, and consequently represent true cells or not, cannot 
be answered until what is a ‘“cell-membrane” has been 
defined ; and, in Gegenbaur’s opinion, this can be comprehen- 
sively done by our regarding as a membrane the outermost 
thickened layer of a cell, whatever thickness it may possess, 
or in whatever physico-chemical relation it may stand 
towards the cell-contents. 
