OVUM: 



[97 



disappearance. He attributed their origin, 

 therefore, to this source ; and regarded it as 

 probable that these cells, which may perhaps 

 be descendants of the original macula? of the 

 germinal vesicle (but this is a point which he 

 leaves undetermined), constitute afterwards 

 the nuclei round which the tabular and granu- 

 lar substance of the yolk group themselves ; 

 and thus probably form, subsequent to seg- 

 mentation, the nuclei or foundation of the 

 cells which are the seat of true embryonic 

 development. 



Newport's description of these changes 

 differs somewhat from that of Cramer, but is 

 not altogether at variance with the view now 

 suggested as to the nature of the process 

 with which they are connected. He states *, 

 that towards the period of maturity he found 

 the germinal vesicle filled with secondary 

 cells, and that each of these contained other 

 or tertiary cells within them, and in the in- 

 terior of these last were granules which he 

 called quaternary. " In the midst of these 

 numerous cells, and in the centre of the ger- 

 minal vesicle, I was able to distinguish," says 

 he, " in some specimens, one or two cells of 

 larger size than the rest, and which I regarded 

 as the remains of the germinal spot or cen- 

 tral nucleus." He further states -f , that these 

 internal cells were, he conceived, afterwards 

 thrown loose by the solution of the parent 

 cells. 



As to the mode of disappearance of the 

 germinal vesicle, Von Baer had stated { that, 

 it gradually rises from its deeper situation, 

 towards the surface of the yolk, and that, 

 finally bursting or being dissolved there, its 

 contents are allowed to flow over the sur- 

 face of the yolk. This process he also 

 described in several other animals as pro- 

 ceeding in a similar manner; and he sup- 

 posed that the germinal substance from the 

 vesicle was thus diffused over that part of 

 the ovum which is most closely related to the 

 subsequent changes of development. He re- 

 garded the canal of the yolk as the remains of 

 a passage through which the vesicle had been 

 carried to the surface. Newport, on the other 

 hand, is quite confident that no such passage 

 of the vesicle to the surface occurs in the ova 

 of Batrachia, and that the vesicle most pro- 

 bably dissolves or disappears in its situation 

 below the germinal part of the yolk. From 

 the facts he has pointed out, Newport in- 

 fers that the germinal vesicle is burst or de- 

 stroyed by the development of the progeny 

 of cells within it, and that the cells thus set 

 free are mingled with the rest of the yolk. 

 It belongs rather to the history of the changes 

 which the ovum undergoes after fecundation, 

 than to our present subject, to trace the re- 

 lation between the cell progeny of the germinal 

 vesicle now described, and the cells of em- 

 bryonic formation afterwards developed ; but 

 it may be proper here merely to mention that 



* Loc. cit. p. 176. 

 f Loc. cit. p. 177. 

 j Epistola de Ovi, &c., Fig. xxv. 

 Supp. 



from the concurrent testimony of several ob- 

 servers, it seems probable that the origin of 

 the blastodermic cells is closely connected 

 with a combination of the vesicles or cells 

 from the germinal vesicle with the other solid 

 elements of the yolk-substance. To this 

 process of cell formation the change of seg- 

 mentation seems, in the Batrachia, as in all 

 other animals, to be the necessary prelude. 



It may be proper here also to state, in 

 conclusion, that Newport has shown that 

 the process of segmentation begins by a fis- 

 sure which passes in a determinate direction 

 through the canal of the yolk. 



Although the statement of Prevost and Du- 

 mas as to the existence of an aperture in the 

 membranes of the ovum, through which they 

 supposed the spermatozoa might be introduced 

 in fecundation, has not yet been confirmed 

 by subsequent observers, but has on the 

 contrary, met with an explicit denial from 

 Von Baer, Newport, and Remak, after a 

 very careful examination by these authors ; 

 and although it would appear, from New- 

 port's statement, that the spermatozoa 

 penetrate the vitelline membrane of the 

 frog's egg over a considerable portion of 

 its surface, yet the discoveries which have 

 in the last few years been made as to the ex- 

 istence of the micropyle in fishes and some 

 other animals, are of so unexpected a kind, 

 that we must not regard this point as altoge- 

 ther settled. Dr. Ransom, indeed, in some 

 observations communicated to me, has stated 

 his belief that a micropyle may still be dis- 

 covered in the membrane of the Batrachian 

 ovum. The statement of Prevost and Du- 

 mas on this subject is so precise that it 

 deserves to be recorded in their own 

 words. " On remarque ensuite qu'il existe 

 an centre de 1'hemisphere brim une tache 

 circulaire tres reguliere, jaune, et marquee 

 d'un point opaque dans son milieu. Celuici 

 provient d'un petit trou dont les deux mem- 

 branes sont percees, ce qui met a decouvert 

 la bouillie bruneque renferme 1'ovule. Pour 

 s'en assurer il suffit de vider I'oauf et d'exa- 

 miner a la loupe les membranes transparentes 

 qui sont restees intactes dans toutes leurs 

 parties, sauf 1'endroit qu'on a pique pour 

 evacuer la pulpe qu'elles contenaient." * 



The observations of Von Baer, Rusconi, 

 Newport, and myself have shown that with 

 certain differences in the form and structure 

 of the external membranes, the colour of the 

 yolk-substance, &c., previously referred to, the 

 structure of the ovum, and the phenomena of 

 change at the time of its discharge, are essen- 

 tially the same in the Newts as in the common 

 Frog. 



A few observations which I have made on 

 the Menobranchus lateralis and Siredon 

 mexicanum, show that the Perennibran- 

 chiate Amphibia agree very closely with the 



* See Deuxieme Me'moire sur la Ge'ne'ratiou : 

 Developpement de 1'ceuf des Batraciens, &c., par 

 MM. Prevost et Dumas, in Amial. des Scien. Nat. 

 torn. ii. 1824, p. 104. 



