ENTOZOA. 



77 



enlarged, b}^ the multiplication of the opake granules of the yolk 

 around the essential vesicle : then the delicate, smooth, and polished 

 membrana vitelli is acquired. Towards the fundus. Dr. Bagge 

 describes the germinal vesicle as being obscured by the aggregation 

 of the vitelline granules around it; and he thinks it probable that the 

 vesicle bursts and pours its contents over those granules. At all 

 events it ceases to be visible as a clear central cell.* The ovum is 

 now apparently occupied by the opake and minute vitelline granules, 

 which become aggregated or condensed, so as to leave a clear narrow 

 interspace between the vitelline mass and the smooth outer membrane 

 {^jig. 34.). In the centre of this mass, however. Dr. Bagge detects 



a clear cell (33), but much more minute than the primitive ger- 

 minal vesicle. This clear cell becomes lengthened, and its rounded 

 extremities mutually recede ; while the middle part becomes at- 

 tenuated (yfig- 35.), and finally breaks ; whereby two pellucid cells are 

 developed, which recede towards the opposite poles of the ^^^ 

 {fig> 36.) ; and this process immediately precedes the first division of 

 the yolk into two parts {^fig. 37-), each of which has the pellucid cell 

 for its centre. This preliminary division of the clear central cell to the 

 spontaneous fission of the yolk is closely analogous to that division of 

 the central cell in the polygastrian animalcule, preparatory to the 

 spontaneous division of its body into two individuals, which Ehrenberg 

 has described. Dr. Bagge next traces the changes of the pellucid 

 central cell of each primary division of the yolk, and describes it as 

 undergoing the same change of form and division {fig. 38.) which he 

 had observed in the primitive cell : and these changes are followed 

 by the spontaneous fission of each primary division of the yolk, 

 whereby the quadripartite character of the ovum is produced {Jig.^9.)y 



* Dr. M. Barry's description of the true processes which, at this stage, obscure 

 the germinal vesicle will be found in the Philos. Transactions, 1839, p. 307. 



