IN AMPHIBIA AND FISHES. 73 



some of the yolk-substance. The larger corpuscules may, in some cases, 

 be segments containing each an embryonic cell.* 



It must be mentioned, however, that Bischoff has never been able to 

 detect any nuclei in the transparent corpuscules or vesicles contained in the 

 segments of the yolk of the frog's ovum, and, therefore, does not regard 

 them as true cells. In other of the Amphibia, as Aly tes obstetricans, and 

 also in fishes, the process of cleaving affects only a part of the yolk, as 

 in the second type described in the invertebrate animals. The external 

 characters of the process as witnessed by Rusconi in the tench (Cyprinus 

 tinea), are described at page 1510, of Miiller's Physiology. The more 

 intimate nature of the changes have been investigated by Vogt, in 

 Alytesf and Coregonus palseaj (one of the salmon tribe). In Alytes, the 

 cleaving affects only one half of the ovum, and, according to Vogt, only the 

 surface of the yolk. For, at that stage of the process, when the surface is 

 mulberry-like, the different segments, though rounded and defined towards 

 the exterior, are towards the interior uninterruptedly continuous with the 

 general substance of the yolk, just as is the case with the earlier segments 

 of the ovum of Sepia. The segments contain (for the most part), each, a 

 transparent round vesicle ; and after the process is completed, and previous 

 to the appearance of the embryo, the whole yolk is composed of cells, in the 

 centre of each of which, a similar transparent vesicle, as a nucleus, is 



* Reichert's later account (Miiller's Archiv. 1841, p. 523) of the process of cleaving in 

 the frog's ovum does not accord with the facts observed by other anatomists. The opinion 

 which he advances is, that the smaller corpuscules which he finds composing the ovum at the 

 end of the process, and regards as cells, all exist completely formed before impregnation 

 takes place. Not, however, that he regards these cells as existing in a free state, but every 

 two or three included within larger cells, and these again in still larger, and so on : the cells 

 of each set enclosing within them smaller cells, and being themselves enclosed within larger, 

 and all being contained within two large cells, which, in their turn, are held together by an 

 investing membrane forming one large cell. The process of cleaving thus consists, according 

 to Reichert, simply in the liberation, first of the two large cells, and then of each successive set 

 of enclosed cells, by the solution of the including cells, till at length the smallest nucleated 

 cells, destined to form the embryonic structures, are set at liberty. The facts really observed 

 on which this hypothesis is based, seem to have been very few and very inconclusive. In a 

 still more recent account (Muller's Archiv. 1846, p. 196), of the process of cleaving in the 

 ovum of Strongylus Auricularis, Reichert modifies this opinion, and admits that the process 

 really consists in the formation of successive crops of new cells. Even the largest segments 

 of the ovum, he (like Dr. Sharpey) regards as true cells, and he states that in the process of 

 duplication the nucleus of each such cell first breaks up into a number of oil-like particles 

 which mix with the granular contents of the cell, that then this mixed mass gradually 

 divides into two equal portions, each of which becomes invested with a distinct membrane ; 

 and that, subsequently, a clear vesicular body or nucleus forms in the centre of each. When 

 fully formed these two cells are liberated by the solution of the parent cell-wall, and then 

 undergo a similar process of division ; the nucleus of each invariably disappearing before 

 the division commences. 



t Entwickelungs-gesch. der Geburtshelferkrote. 



J Histoire Natur. des Poissons d'eau douce, by M. Agassiz. Tome i. 1842. 



