148 CHARLES R. STOCKARD 



This confined local origin of the round cells and the period at 

 which they wander out, along with their general appearance, 

 lead one to believe that these cells are actually derived from the 

 same general mass or group of cells w^hich goes to form the 

 intermediate cell mass or red blood anlage within the embryo. 

 All of these slowly wandering circular cells finally differentiate 

 into red blood corpuscles, as described below, just as cells of the 

 intermediate cell mass will finally do. 



In this connection a most instructive defect is frequently 

 found among embryos developing in the stronger solutions of 

 alcohol. jMany such embryos are of the common short type 

 with their tails spHt, cauda-bifida, figure 4 of the previous paper. 

 This defect is due to the fact that the germ-ring descends over 

 the yolk in a slow or arrested fashion and may never succeed 

 in fully enclosing it. The caudal end of the embryo is thus 

 divided and the two tail moieties remain spread apart laterally 

 along the line of the germ-ring. This condition renders the 

 caudal portion incapable of including all of its usual median tis- 

 sues and such cells extend past the angle of the spht and lie 

 between the two parts of the bifid tail. The interesting thing is 

 that the cells constituting the blood-forming intermediate cell 

 mass lie in just this position. 



Figure 33, a diagram, illustrates the embryonic body with a 

 bifid caudal end. The great lake of blood corpuscles is situated 

 beyond the angle of the split tail. Such an abnormality liber- 

 ates the future blood forming mass from the body of the embryo 

 and the mass spreads posteriorly over the yolk surface, yet not 

 in a diffuse manner since it maintains its densely packed cellular 

 structure. We might consider that here the evolutionary events 

 are reversed. The blood anlage in the primitive fishes, the se- 

 lachians, is spread over the yolk in the area opaca. In the nor- 

 mal teleost this primary yolk-sac blood anlage has been included 

 within the embryonic body and localized in the intermediate 

 cell mass. While in the abnormality here considered the inter- 

 mediate cell mass is again outspread upon the yolk somewhat 

 suggestive of the old ancestral selachian arrangement. This 

 abnormality, in other words, may give some notion of the actual 



