'Researches in Embryology: Third Series, 529 
shown that in the embryo this mysterious centre is present until it 
has assumed the form of the cavity, including the sinus rhomboidalis, 
in the central portion of the nervous system. 
The process above described as giving origin to the new being in 
the mammiferous ovum, is no doubt universal. The author thinks 
that there is evidence of its occurrence in the ova of batrachian 
Reptiles, some osseous Fishes, and certain of the Mollusca ; though 
the explanation given of these has been of a very different character. 
It has hitherto been usual to regard the round white spot, or cica- 
tricula, on the yelk of the bird’s laid egg, as an altered state of the 
discus vitellinus in the unfecundated ovarian ovum. So far from 
thinking that such is the case, the author believes the whole sub- 
stance of the cicatricula in the laid egg to have its origin within the 
germinal vesicle, in the same manner as in the ovum of Mammalia. 
There is no fixed relation between the degree of developement of 
ova, and their size, locality, or age. The variation with regard to 
size is referable chiefly to a difference in the quantity of fluid im- 
bibed in different instances by the incipient chorion. Vesicles filled 
with transparent fluid are frequently met with in the Fallopian tube, 
very much resembling the thick transparent membrane of the ovarian 
ovum. These vesicles are probably unimpregnated ova, in the 
course of being absorbed. The so-called “ yelk” in the more or 
less mature ovarian ovum, consists of nuclei in the transition state 
and exhibiting the compound structure above described. The mass 
of these becomes circumscribed by a proper membrane. They and 
their membrane subsequently disappear by liquefaction, and are 
succeeded by a new set, arising in the interior, and likewise be- 
coming circumscribed by a proper membrane, and so on. This ex- 
plains why some observers have never seen a membrane in this 
situation. After the fecundation of the ovum, the cells of the 
tunica granulosa, that is, part of the so-called “ disc,” are found to 
have become club-shaped, greatly elongated, filled in some instances 
with cells, and connected with the thick transparent membrane by 
their pointed extremities alone. 
That the thin membrane described by the author in his second 
series as rising from the thick transparent membrane in the Fallo- 
pian tube, and imbibing fluid, is really the incipient chorion, was then 
shown by tracing it from stage to stage, up to the period when 
villi form upon it. There remained, however, two questions unde- 
cided ; viz., whether the chorion is formed of cells, and if so, 
whether the cells are those of the so-called “ disc,” brought by the 
ovum from the ovary. The author now states that the chorion is 
formed of cells, w'hich gradually collect around the thick transparent 
membrane, and coalesce ; and that the cells in question are not those 
of the “disc” brought with the ovum from the ovary. The cells 
which give origin to the chorion are intended to be more particularly 
described in a future paper. 
I’he existing view, namely, that a nucleus, when it leaves the 
membrane of its cell, simply disappears by liquefaction, is inappli- 
cable to any nucleus observed in the course of these investigations. 
Phil Mag. S. 3. Vol. 16. No. 105. June 1840. 2N 
