FROM LAYING TO FORMATION OF FIRST SOMITE 89 



it is split off from the germ-wall (His and others). One thing 

 is perfectly clear, viz., that the mesoderm of the opaque area 

 arises in continuity with the primitive streak mesoderm; the 

 second view would therefore be better expressed, as Riickert 

 states it, that the primitive streak mesoderm grows in the region 

 of the area opaca at the expense of elements of the germinal wall. 



If the cells of the primitive streak mesoblast be compared | 

 with the cells of the forming blood-islands a sharp contrast is \ 

 observed; the mesoblast cells of the area pellucida are devoid of 

 yolk-granules; young blood-islands on the other hand contain 

 yolk-granules of precisely the same character as those of the 

 germ-wall (Fig. 41), which must have been derived from the latter. 

 If the origin of the blood-islands be carefully traced, they are \ 

 found to be rooted in the protoplasm of the germ-w'all; and prior i 

 to the appearance of the blood-islands proper, protoplasm and 

 nuclei of the germ-wall aggregate superficially in a manner that 

 appears to foreshadow the blood-islands. Therefore, either the 

 blood-islands are derived from the cells of the germ-wall, or 

 cells of the mesoderm growing over the germ-wall burrow into \ 

 the latter, engulf yolk-spheres, and reappear in masses as blood- 

 islands. We shall not attempt to decide between these pos- 

 sibilities. 



Another question concerns the origin of the layer of ccelomic 

 mesoblast that overlies the blood-islands: is it derived from the 

 primitive streak mesoblast, or is it split off from the blood-islands? 

 When the latter first appear, in the periphery of the vascular 

 area at least, there is no ccelomic mesoblast above them. It 

 appears later, at first not as a coherent layer, but as scattered 

 cells that rapidly unite to form a layer. In many places the 

 microscopical appearances indicate strongly that the cells are 

 split off from the surface of the blood-islands; but, as they are 

 usually not far from the edge of the advancing co'lomic meso- 

 blast, it may be that they are derived from the latter. Riickert 

 states, however, that, in the case of some isolated blood-islands 

 behind the embryo, a layer of mesoblast is formed over them 

 Avhile they are still isolated. This would render the derivation 

 from the blood-islands probable in such cases. It is possible, 

 therefore, that the ccelomic mesoblast grows partly, at least, at 

 the expense of the superficial cells of blood-islands. 



As rapidly as they are formed the various blood-islands con- 



