DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 159 



than the 'intermediate cell mass,' within the embryo, and the islands 

 on the caudal and ventral yolk surfaces which have been formed as 

 described by the early wandering cells that migrate away from the 

 caudal region of the embryo. All embryos whose history for lack of 

 circulation throughout their existence is actually known show the 

 blood pattern most consistently, there being of course a certain 

 amount of variation in the extent and position of the yolk-islands 

 but not enough in any case to confuse the problem. 



These observations may readily be made by any observer, 

 but can only be made in a reliable fashion with the high power 

 microscope and strong condensers so that the light may be 

 sufficiently regulated to observe the most transparent cells. 

 The movements and differentiation of these cells should be 

 carefully followed through every step in a number of cases, 

 in order to fully appreciate the significance of their position and 

 behavior. 



The cells may be seen even with an ordinary binocular micro- 

 scope to some extent, but the arrangements for light regulation 

 and the magnification are insufficient for determining the im- 

 portant details. After the red blood cells have formed, they 

 are readily located even with a low power yet such an examination 

 could only determine their places of origin provided the embryo 

 has been carefully watched with a high power magnification to 

 make certain that it has had no blood flow. 



The condition of the yolk-sac mesenchyme must be fully under- 

 stood and must always be considered in interpreting the origin 

 of blood-islands and clots. For example, clots seen at the ven- 

 ous end of the heart or on the extreme anterior surface of the 

 yolk must be most cautiously considered, remembering the 

 scarcity or even absence of the wandering mesenchyme in these 

 regions. Clots in such places probably always result from a 

 partial circulation of short duration and there is abundant evi- 

 dence to support such a view. 



Although the future red blood cells migrate upon the yolk in 

 their early mesenchymal stages, after they once group them- 

 selves and differentiate into erythrocytes their powers of wander- 

 ing become very much limited if they exist at all. I have never 



