72 CHARLES E. STOCKARD 



ment of vessels and blood upon the yolk. There are here two 

 opposed or different views. The first derives the yolk vessels 

 and blood directly from the yolk syncytium or periblast. The 

 second derives blood cells exclusively from the intermediate cell 

 mass in the embryo, but admits that cells may secondarily come 

 to lie on the yolk by being pushed out from the intermediate 

 cell mass with which, however, they maintain a definite conti- 

 nuity. The vascular endothelial cells are also derived from the 

 embryo as mesoblastic wandering cells, but these are not to 

 be compared directly with blood cells since their parent cells 

 have a separate place of origin. Most of the earlier workers 

 thought that the blood in the Teleosts arose on the yolk-sac, as 

 it does in other meroblastic embryos. The more recent workers 

 have gone to the other extreme and deny the presence of blood 

 islands upon the yolk-sac as separate from the intermediate cell 

 mass. 



As mentioned in describing the heart formation, numerous 

 investigators have recorded wandering mesenchymal cells upon 

 the yolk-sac, but from a study of the literature no clear con- 

 ception can be formed as to the origin of blood cells or the vas- 

 cular endothelial cells upon the yolk from these wandering cells. 

 Some authors claim that the majority of wandering cells become 

 pigment cells, while the remainder form the yolk vessels. In 

 Fundulus the pigment cells very soon present a different appear- 

 ance from the mesenchymal cells which are' to form the vascular 

 endothelium. Both types of cells may be readily seen wandering 

 over the yolk between the ectoderm and periblast. Before the 

 yolk vessels are completely formed, the circulation of a cell free 

 plasma has begun. The extent of the spaces in which this cir- 

 culation takes place is very variable. The arrangement of the 

 veins of the yolk circulation is also extremely different in the 

 different groups of Teleosts. One must agree with Hochstetter 

 ('93) in stating that the yolk circulation in different forms is so 

 different from the start that it is not possible clearly to summar- 

 ize the condition in order to give satisfactory comparisons with 

 the same vessels in Selachians, and Amphibians. 



