226 BULLETIN OF THE 



fold, after separating from the layer which forms the lateral sheath of the 

 myotome, passes inward, and is there lost in a loose mass of tissue (Fig. 

 18), resulting from the disassociation of certain cells of the somite in that 

 region. lutersegmeutal regions also present appearances which are con- 

 fused by the occurrence of cells belonging to the partition between two 

 successive somites. The points which I especially wish to emphasize in 

 this description are (1) the origin of the capsule from the somatic layer 

 of segmented mesoderm, and (2) the fact that the layer from which the 

 capsule is developed is also in other regions converted into mesenchy- 

 matic tissue. 



lu the younger specimens of this stage a horizontal fold of the splanch- 

 uopleure is to be noticed, forming a slight ridge directly across the body 

 cavity from the pronephros. It first appears in front of the second ne- 

 phrostome, and develops from this point backwards. It is the fundament 

 of the glomus or pronephric glomerulus.-' In the earliest trace of this 

 organ that I have been able to find (Plate I. Fig. 8) there were already 

 a few small mesenchymatic cells {vischy.') located in the angle of the 

 fold. The source of these cells I have been unable to determine with 

 certainty. The nuclei of all the cells in the fold itself lie very close to 

 the body cavity, and it does not seem probable that those small cells 

 could be produced by delamination from the splanchnopleure without an 

 actual migration of the nuclei of the somatopleural cells to the basal, or 

 entodermal, surface of that layer. I have never seen signs of such migra- 

 tion, and I therefore do not believe that it occurs. Furthermore, the 

 folded portion of the somatopleure does not at once become thinner than 

 the neighboring portions of that layer. In older stages, such a thinning 

 takes place, but it seems to be due to a superficial extension of the layer, 

 rather than to delamination. The position of the nuclei of the large 

 entodermal cells in this neighborhood is equally unfavorable for the 

 formation of these small cells by delamination. The only remaining 

 explanation is that the latter have migrated into their present position 

 from relatively remote parts. Other loose cells may be found between 

 entoderm and splanchnopleure, and the question here raised is only a 

 part of the larger problem as to the source of all such cells, including 

 those which bound the yolk veins. The fate of the cells which I have 

 found in the fundament of the glomus, I shall consider in treating of a 

 later stage. I may, however, here anticipate to the extent of stating 

 that they are connective-tissue elements. 



1 The former term seems to me preferable, and will be employed in the follow- 

 ing pages. The exact relations of the glomus to the mesonephric glomeruli will be 

 explained in the general discussion. 



