. Rathke, Growth of the Eggs of the Crawfish. 248 
during their continuance in the ovarium and oviducts. The ovum first 
appears in the shape of a small, almost perfectly transparent, vesicle, rather 
lenticular than spherival, consisting of an extremely fine membrane, and 
apparently filled with aclear watery fluid. This vesicle afterwards becomes 
surrounded by a second, and seemingly a still finer one, the proper 
membrane of the vitellus. The first traces of the vitellus itself consist of 
a fluid, interposed between the two vesicles, in the first instance as trans- 
parent as that of the inner coat, but gradually becoming whitish, opake, 
thick, and viscid, and simultaneously exhibiting a number of extremely 
small, snow-white, scattered granules. During this process, the outer 
envelope gradually enlarges, and from lenticular becomes spherical, but 
the inner remains nearly of the same size, and instead of occupying the 
central point of the other, as at the commencement, becomes excentric, 
and places itself almost in contact with the paries on one side, while it is 
at a considerable distance from the other. The ovum thus formed re- 
mains within the parietes of the ovary for somewhat more than half-a- 
year, during which time the constantly increasing fluid of the outer vesicle, 
or in other words, the vitellus, becomes more and more viscid, changes 
in colour successively to Isabella-yellow, orange, and brown, and is at last 
almost entirely converted into a mass of very small granules of various 
sizes, intimately adhering to each other by means of the small quantity 
of viscid fluid that remains. 
But the last and most important change that takes place within the 
ovary, consists in the evanescence of the internal vesicle, and the produc- 
tion of the embryo. The authour has never been able to ascertain what 
becomes of the former; he has remarked it in mature ova in the month of 
November, but has failed to detect itin the ensuing March. He suspects 
therefore, as was previously conjectured by Von Baer with respect to the 
corresponding part in the ova of the higher animals, that the embryo is 
formed from the evolution of its contents. The latter, when it first be- 
comes visible, appears like a light whitish cloud of indeterminate form, 
spread over a small portion of the vitellus, having some thickness in the 
middle, but becoming gradually thinner towards its edges. Up to this 
period of its developement the egg remains enclosed within the walls of 
ee» the ovary, in which it forms for itself a cavity, and to which it is attached 
by means of the mucous coating that surrounds it. As it increases in size, 
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