Researches in Emhryologij : 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 vi'hite spot, or cica- 

 tricula, on the yelk of the bird's laid e^^, 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 diffei-ence 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 moi-e 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 

 vdth 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, which 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. 



The 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. ii N 



