Rathke, Growth of the Eggs of the Crawfish. 243 



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 spherical, consisting of an extremely fine membrane, and 

 apparently filled with a clear 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 j)laces 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 it in 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 

 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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