of the Fishery Board for Scotland. 



39 



cellular parablast has been covered. The disc during these changes does 

 not thin out equally in all directions. In the axis of the future embryo 

 a longitudinal band of cells remains, which is several rows deep, and 

 these gradually thin off towards the periphery on both sides, at which 

 there is again a slight thickening. The thickening of cells in the 

 embryonal axis is the ' keel,' and is composed almost solely of cells 

 derived from the segmented disc. Later it increases considerably in 

 thickness, particularly in the anterior region of the embryo. A diagram- 

 matic view of the manner in which the disc extends, as seen in a plane at 

 right angles to the axis of the embryo, is shown in figure 19. Figures 10 

 to 13 show optical sections of the living egg in the direction of the 

 embryonal axis, and at a plane showing the greateat thickness of the 

 keel. 



With the extension of the disc over the yolk the germinal layers are 

 diff'erentiated. The process can be seen in operation in the living 

 egg, but it is only by a study of sections that the nature of the process can 

 be understood. I have reviewed the literature on this subject at length 

 in my paper communicated to the Eoyal Society of Edinburgh, and will 

 content myself here by simply stating the modus operandi as I have 

 observed it in the herring. 



I shall speak of the segmented disc as archiblast, although it must be 

 remembered that in the herring this also contains a number of cells 

 derived from the parablast. This is, however, not a constant feature in 

 the development of teleosteans. In retaining the name I have been 

 guided more by the fate of the cells contained within the germinal disc 

 than by a knowledge of their mode of origin. It may be that when 

 the whole subject has been more thoroughly investigated a change of 

 nomenclature will be necessary. 



When the segmentation cavity is formed the central portion of the 

 disc is raised up from the parablast, but at the periphery the two layers 

 remain in contact over a distance covered by several rows of cells. When 

 the so-called invagination process takes place a new layer is formed, 

 which may be called the primitive hypoblast. The process is not really 

 one of invagination ; but, seeing that the new layer commences at the 

 periphery, and extends inwards, the term pseudo-invagination may be 

 applied. The process in the herring is identical with that which I have 

 already described for Trachinus and Motella, and for which I suggested 

 the term ' segregation ' (5). I am not prepared to state that the new layer 

 contains no elements of archiblastic origin, but these are, I think, with- 

 out doubt chiefly derived from the parablast. This layer forms a poly- 

 nucleated mass underlying the archiblast. At the periphery, where the 

 two layers are in contact, cells are formed around the nuclei in the para- 

 blast, and these attach themselves to the cells of the archiblast. In this 

 way a thickening is produced around the circumference of the archiblast, 

 which is the embryonal rim. In sections of this stage the distribution 

 of the parablast is very instructive. In the region of the thickening the 

 parablast is very thin, and the nuclei, which were previously situated in 

 it, have all been used up. Immediately in front of the thickening the 

 parablast forms a comparatively thick layer, forming the floor of the 

 segmentation cavity, and contains a large number of nuclei. 0 itside the 

 thickened rim the parablast is also thick, and charged with nuclei. It is 

 evident, therefore, that the nuclei of the parablast and a large part of its 

 protoplasm have been used up, in that portion on which the archiblast 

 rests, to form the thickened rim. Next the new layer extends inwards in 

 the axis of the embryo, the nuclei and protoplasm of the parablast being 

 utilised for the purpose. As the new layer grows inwards, the segmenta- 



