DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 173 



in the specimens without a circulation of the blood that the 

 vessels arise and increase in size and persist for a long time with- 

 out ever experiencing any effect of the blood current upon their 

 walls. In many embryos the circulation after having begun may 

 stop for a time and then later be reestablished, the vessels having 

 persisted in a normal condition. Thoma's so-called laws of 

 vessel formation are, therefore, rudely violated by the develop- 

 ment of the vascular system in these embryos. 



The vessels arising from independent mesenchymal cells in the 

 space of the blastocoel in the teleost yolk-sac entirely overthrow 

 any notion that vessels arise ontogenetically as portions of the 

 coelomic epithelium. The vascular lumen is originally continu- 

 ous with the primary body cavity, the segmentation cavity, and 

 never with the secondary body cavity or coelomic cavity. 



4. The fourth class of cells wander out from the embryonic 

 body somewhat later than the three former types. These are 

 small globular cells with short pseudopod-like processes. They 

 move very slowly, but finally collect into groups on the posterior 

 and ventral regions of the yolk-sphere. 



The round cells wander away only from the caudal region of 

 the embryo and probably are derived from the so-called inter- 

 mediate cell mass which is the anlage of the red blood corpuscles 

 in the fish embryo. 



The groups of round cells are slow in their differentiation but 

 just before the circulation of the blood begins, they are seen to 

 be circular erythroblasts. The observer may follow the dis- 

 appearance of the islands of cells one by one as they are enclosed 

 by the vessels and swept into the circulating stream. About the 

 fifth day these circular erythroblasts become flattened ellipsoidal 

 erythrocytes filled with haemoglobin, the typical red blood cor- 

 puscle. The complete change from wandering more or less 

 spherical mesenchymal cells into typical haemoglobin bearing 

 corpuscles may be followed in the living yolk-sac. 



In several instances the entire body proper of the embryo failed 

 to develop or else degenerated very early, yet the yolk-sac 

 formed or persisted with numerous blood islands fully differ- 

 entiated. 



