FORMATION OF THE GERMINAL LAYERS IN TELEOSTEI. 229 



tion may be traced from the basal portion of the parablast, in which no nucleus 

 is to be found, to the completely separated cells, which are already included in 

 the morula. The first trace of cell formation is seen in a number of somewhat 

 deeper stained patches of protoplasm, the colour getting more intense towards 

 a centre, and gradually fading away from that point. A little later the cell 

 contour becomes visible as a less deeply stained outline. In the next stage 

 the nucleus appears as a less deeply stained portion in the centre of the cell. 

 Its outline is almost circular, and a delicate more deeply stained reticulum 

 may be observed in its interior. The deepest staining is noiv around the peri- 

 phery of the nucleus, and the colour gradually becomes less intense towards 

 the cell wall. Later the more deeply stained granules in the cell plasma 

 arrange themselves in the form of a reticulum, which is ultimately connected 

 with that of the nucleus. By this time the cells are indistinguishable from 

 those already forming part of the morula. In some of the cells more trans- 

 parent vacuole-like structures are found in the cell plasma*, both in the 

 parablast cells and in those of the morula, but are much more numerous in 

 the latter. Possibly these structures may be connected with the nourishment 

 of the cell, but this is not clear. 



During the next few hours nearly the whole of the subgerminal parablast 

 has been used up in budding off cells to join in the morula, which has in con- 

 sequence increased very much in thickness, and presents a sharply curved 

 upper surface. The subgerminal parablast in eggs forty-five hours after fer- 

 tilisation consists of only a very thin film, as will be seen from fig. 18. The 

 peripheral parablast, however, consists of a comparatively thick wedge-shaped 

 mass, stretching from the base of the morula to the equator of the egg, and 

 contains a considerable number of rows'of nuclei. This portion of the parablast 

 has been gradually increasing in importance while the changes which I have 

 just described have been taking place in the subgerminal parablast, but has not 

 taken part in these changes. It is this peripheral portion of the parablast 

 which has usually received attention from various investigators, and in which 

 the development of nuclei has been so often observed in the living egg. The 

 nuclei appear in concentric rings around the base of the morula, the first ring 

 being formed in the thickest portion of the layer, i.e., the part immediately 

 adjoining the morula. A faint cell outline can usually be observed in the proto- 

 plasm around each nucleus, but after a time this appears to become obli- 

 terated, though not so early as in Trachinus. In the living egg a single row of 

 nuclei can be observed at a stage corresponding to fig. 12. At the stage 

 shown in figure 14 there were three or four rows of cells in the peripheral 

 parablast. In the living egg, of which fig. 18 is a section, the peripheral 

 parablast has become cellular almost to the equator. The cells are arranged 

 somewhat irregularly. In the portions more densely crowded with nuclei, the 



