556 
DR. MARTIN BARRY’S RESEARCHES IN EMBRYOLOGY. 
by no means to be limited to the period in question. Thus it is very common to 
meet with several varieties of epithelium-cells in the oviduct (including those which 
carry cilia) filled with cells ; but the whole embryo at a subsequent period is com- 
posed of cells filled with the foundations of other cells. 
404. In the Second Series of these researches, it was shown that the mulberry-like 
object above mentioned, is found to contain a cell larger than the rest, elliptical in 
form, and having in its centre a thick-walled hollow sphere, which is the nucleus of 
this cell. It was farther shown that this nucleus is the rudimental embryo. From 
what has been just stated, it appears, that the same process, by which a nucleus in 
one instance transforms itself into the embryo, is in operation in another instance, 
where the product does not extend beyond the interior of a minute and transitory 
cell. Making allowance, indeed, for a difference in form and size, the description 
given of the one might be applied to the other. It was shown in the Second Series, 
that in the production of the embryo out of a nucleus, layer after layer of cells comes 
into view in the interior, while layers previously formed are pushed further out; each 
of the layers being so distinctly circumscribed as to appear almost membranous at 
its surface. The same membranous appearance presents itself at the surface of the 
several layers of a nucleus in many situations. Farther, in the formation of the em- 
bryo, a pellucid centre is the point around which new layers of cells continually come 
into view ; a centre corresponding to that giving origin to similar appearances in 
every nucleus described in the present memoir. It was 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. 
405. The process above described as giving origin to the new being in the mammi- 
ferous ovum, is no doubt universal. There appears to be 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 cicatricula, 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 such being the case, it is probable that the whole substance of 
the cicatricula in the laid egg lias its origin within the germinal vesicle, in the same 
manner as in the ovum of Mammalia. 
406. There is no fixed relation between the degree of development of ova, and their 
size, locality, or age. The variation with regard to size is referable chiefly to a dif- 
ference in the quantity of fluid imbibed indifferent 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. Tiie 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 
