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 Rlickert 

 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-wall; and prior 

 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. Patterson (1909) has shown by an experimental study 

 that in any region in which primitive streak mesoblast is pre- 

 vented from reaching the germ-wall, blood-islands fail to develop. 

 The second alternative is therefore probably right in principle. 



Another question concerns the origin of the layer of coelomic 

 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 coelomic 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 ap- 

 pearances indicate strongly that the cells are spHt off from the sur- 

 face of the blood-islands; but, as they are usually not far from the 

 edge of the advancing coelomic mesoblast, it may be that they are 

 derived from the latter. Ruckert states, however, that, in the case 

 of some isolated blood-islands behind the embryo, a layer of meso- 

 blast is formed over them while they are still isolated. This would 

 render the derivation from the blood-islands probable in such cases. 

 It is possible, therefore, that the coelomic mesoblast grows partly, 

 at least, at the expense of the superficial cells of blood-islands. 



