DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 139 



Wenckebach's description cannot be fully agreed with in all 

 detail. He thinks, for instance, that cells forming part of the 

 vessel wall are brought by the blood stream. These cells have 

 small protoplasmic processes but they are not in any way to be 

 confounded mth the ''definitiven Blutkorperchen. " Such cells 

 have never been observed in the Fundulus embryos and if they 

 exist, which is very improbable, their part in vascular formation 

 is extremely insignificant. 



Wenckebach observed the three primary vessels on the yolk 

 to bud and give off sprouts forming other vessels. The wander- 

 ing cells also formed small separate tubes which later became 

 connected to form a portion of the vascular net. By these 

 methods the complex vascular net of the yolk-sac was finally 

 formed. This agrees closely with what may actually be seen to 

 occur in the embrj^os of Fundulus. 



Considerable variation occurs in the position of vessels and 

 a number of actual abnormalities are found in which the bilateral 

 arrangement is completely disturbed. These abnormalities are 

 frequently very instructive for a thorough understanding of the 

 origin and development of the yolk-vessels. Occasionally, a 

 group of cells will form a completely isolated endothelial space 

 which may fail to associate itself with a vessel. Figure 27 shows 

 such an isolated space in a yolk-sac of 90 hours old; solid endo- 

 thelial tips project from the space, yet it is completely isolated 

 so far as can be determined with the highest power, and at the 

 same time every part of it is clearly and distinctly seen. 



When the early yolk vessels are studied under high magni- 

 fication, the individual cells may be clearly observed and they 

 are strikingly the same as before they became associated to form 

 the vessel. The cells are not closely arranged but distinct inter- 

 vals and spaces exist between them and filamentous processes 

 often project far into the lumen and may actually at times fuse 

 with a similar process from a cell on the opposite side of the wall. 

 These filaments thus stretch across the vessel and may even 

 persist after the blood has begun to flow. They are well seen by 

 focussing so as to get an optical section through the cavity. Cor- 



