GEGENBAUR, ON SAGITTA, 51 
Regarded in this ight a membrane may be found to exist 
around the products of segmentation in the oyum of Sagitta, 
which membrane in the earliest stages of the development, 
it must be confessed, differs but little from the interior vitel- 
hne substance, and which (physically, at any rate) bears the 
same relation to the vitelline membrane that the primordial 
utricle in a plant-cell does to the cellulose membrane, 
Each segmentation-cell presents an oval nucleus, which at 
first is situated in the thicker or outer portion of the cell, 
and consequently near the surface of the ovum. What 
becomes of it in the process of division has, in the most 
important particular, escaped Gegenbaur’s notice; though 
it is to be remarked that a stage was often noticed at 
which many of the nuclei, much elongated, exhibited con- 
strictions ; so that, although a division of the nucleus was not 
seen, still such a division might be concluded to take place ; to 
which may be added the circumstance that in no case is the 
cell without a nucleus. Consequently there is nothing to 
support the notion of a disappearance of the nucleus before 
segmentation, and a new formation of nuclei after that pro- 
cess has been gone through. That this stage of nuclear 
division has escaped observation, may perhaps be explained 
by the rapidity with which it takes place. The same obser- 
vation holds good of the germinal vesicle, the nucleus of the 
ovum, regarded as a cell from which all the nuclei of all the 
tubsequent cells arise in the same way that the latter have 
shemselves arisen from the ovum-cell. In the earhest stages 
of segmentation the nature of the nucleus was shown In a 
more precise manner, inasmuch as at that time, and before 
complete diyision of the yelk, two nuclei were seen to exist. 
A peculiarity of the vitellus in the ovum of Sagitta was 
noticed at a later stage of segmentation. When more highly 
magnified, the contents of each cell were seen to be com- 
posed of spherical bodies somewhat flattened by mutual 
pressure, and which, at the situation of the nucleus retreating 
from it, left a cavity. From the nuclear cavity thus formed, 
radiating prolongations stretched out among the neighbour- 
ing vitelline granules. 
The first indication of the development of the embryo is 
shown in a division of the pyramidal vitelline cells, each of 
which is subdiyided im the middle of its longitudinal diame- 
ter, so that the central cavity of the vitellus becomes 
enclosed by an internal layer composed of smaller cells, 
which again is surrounded by a layer of larger cells, of 
which the surface of the vitellus is also constituted. The 
longitudinal axis of each of the outer cells coincides with 
