DR. MARTIN BARRY’S RESEARCHES IN EMBRYOLOGY. 
537 
spot having resolved itself into cells, layers of the foundations of other cells come 
into view around a point which had been the centre of the spot; and that these, en- 
larging, are succeeded by layer after layer of the foundations of yet other cells. The 
central point, it will be recollected, lies immediately under the orifice in the thick 
transparent membrane f, apparently for the purpose of receiving the fecundating 
element of the seminal fluid. It must be the point of fecundation. Here we saw a 
minute cavity, with dark walls, and containing a pellucid fluid. This point, as already 
stated, subsequently passes from the periphery of the germinal vesicle to its centre. 
In the latter situation, two incipient cells come into view (Plate XXIV. fig. 187. bs), 
having essentially the same form and general appearance as the cells of other parts of 
the ovum described in this memoir ; but in general attaining a more considerable size 
(Plate XXV. figs. 198, 199. bs.), and being objects of much more importance. These 
two cells constitute the foundation of the new being, — that is, the germ. 
347- In the ovum, Plate XXIV. fig. 191. bs., they measured in length 5 V", and con- 
tained in a transparent fluid, a nucleus having a central situation. The nucleus of 
one of them is represented more highly magnified in fig. 192. Its external part con- 
sisted of the foundations of new cells. Its interior presented a cavity, apparently 
globular in form and filled with fluid, which was colourless and remarkably pellucid. 
The nucleus in this instance measured in diameter Tiro '" ; from which it will be seen 
that the cavity in its interior could not have exceeded about one-third of this size, 
namely, two'"; as the proportions were carefully observed in the drawing-f-. 
348. The nucleus in each of the twin cells, which together constitute the germ, 
undergoes essentially the same changes as those presented by the germinal spot (par. 
327- 328.) ; but seems to pass sooner than the centre of the altered spot to the interior 
of its cell;}:. The nucleus having increased in size, dark objects — the foundations of 
new cells — come into view in its interior (Plate XXV. fig. 199. bs.) ; and these, enlar- 
ging, present a set having a still more central situation (fig. 200 .). The pellucid centre 
of the nucleus eventually increases considerably in size (Plate XXVI. fig. 206.). 
349 . The two cells (bs) which constitute the germ distend, until they nearly fill 
the germinal vesicle (c). This takes place at the expense of the surrounding cells, 
with which it will be recollected that the germinal vesicle had filled (par. 326.). These 
surrounding cells, having successively enlarged, disappear by liquefaction ; the outer 
layer of them being apparently the first to undergo this change. The inner layers § 
are at first pushed forth by the two distending cells (Plate XXIV. fig. 193. bs), but 
f This remark is applicable to every other figure ; so that where the size of the whole object is given (see 
the Table of Measurements, par. 429.), that of its parts may he inferred. 
+ It will be obvious why the central portion of the altered germinal spot continues up to a certain period at 
the surface of its cell. 
§ These layers of cells within the germinal vesicle, and around the germ, are sometimes seen to be circum- 
scribed by a proper membrane (fig. 193.); so that here, within the germinal vesicle, we find apparently the 
same process in operation as that above described (par. 339.) as forming proper membranes (e) under the thick 
transparent membrane/. 
3 z 
MDCCCXL. 
