473 



terpretation may be given to each of the constituent parts, riveting 

 the expression of the root in the memory, and converting the solu- 

 tion of a problem into a condensed enunciation of various theorems. 

 The author exhibits the application of these principles to equations 

 of various degrees, beginning with quadratic and cubic, and pro- 

 ceeding to those involving higher powers. 



6< On the first changes in the Ova of the Mammifera, in conse- 

 quence of Impregnation ; and of the mode of origin of the Chorion." 

 By Thomas Wharton Jones, Esq. Communicated by Richard Owen, 

 Esq., F.R.S. 



The author having, in a former paper, described the structure 

 of the unimpregnated ovum of mammiferous animals, now proceeds 

 to investigate the changes which the ovum undergoes in consequence 

 of impregnation, in the rabbit, the first perceptible difference is the 

 addition of a thick gelatinous matter surrounding the parts of which 

 the ovum was composed in its original state, and apparently derived 

 from the ovaries. In the progress of development the vitellary 

 membrane gives way, as happens in the ova of the newt, and of 

 many of the oviparous animals. The gelatinous envelope acquired 

 in the ovary, and which is more especially circumscribed and defined 

 after impregnation, constitutes the only covering of the vascular 

 blastoderma, after the giving way of the vitellary membrane, and 

 afterwards forms the chorion, which in rodent animals, at a further 

 stage of development, presents itself under the form of a thin and 

 transparent membrane, very similar to the vitellary membrane of a 

 bird's egg, and situated immediately outside the non-vascular and 

 reflected layer of the umbilical or erythroid vesicle. The author 

 draws similar conclusions with regard to the developement of the 

 human ovum. 



The second part of the paper relates to the changes taking place 

 in thevitellus, the inferences concerning which are deduced chiefly 

 from observations of the developement of the ova of batrachian 

 reptiles. The author concludes that the disappearance of the ger- 

 minal vesicle is prior to impregnation. In the newt, the vesicle, 

 at first imbedded in the substance of the yelk, gradually approaches 

 the surface, until its situation is immediately underneath thevitellary 

 membrane : its coat, havingnow become very soft, gives wav, allowing 

 the contained fluid to be effused on the surrounding surface of the 

 yelk ; and the small depression in which the vesicle was lodged 

 now forms the cicatricula. The effused fluid gives a degree of con- 

 sistence to the matter composing the surface of the yelk, and thus 

 promotes the formation of the blastoderma. In the frog, the sur- 

 face of the yelk becomes every day more and more broken up, and 

 the resulting crystalline forms described by Prevost and Dumas be- 

 come smaller and smaller, until the surface of the black blasto- 

 derma appears under a magnifying glass like shagreen. The bla- 

 stoderma, consisting of an aggregation of clear globules, different 

 from these of the rest of the yelk, is now fully formed, and has ex- 

 tended itself so as to close in the white spot. The change which 

 takes place in the yelk of the bird's egg appears to be limited to 

 the neighbourhood of the cicatricula. 



