DEVELOPMENT OF WAXDERIXG MESEXCHYMAL CELLS 141 



puscles often strike against the cell processes and cause them to 

 wave back and forth as the current flows past. 



The cells of the vessel wall thus maintain much of their indi- 

 viduality and may actually separate themselves or loosen away 

 from the small growing tip of a vessel. The tips of the vascular 

 sprouts probably break up or disassociate in this manner to 

 include small groups of corpuscles which may be seen to enter 

 the vessels from the yolk surface. 



A most instructive specimen for a study of the cellular elements 

 of the vascular endothelium is one in which the circulation has 

 just begun. The vessels in such an embryo are still growing in 

 length and sprouting off branches rather actively. Figure 28 

 illustrates such a vessel with its incipient branches. Corpuscles 

 are passing through the vessel in the plasma current; one of these, 

 X, is seen harbored behind an endothelial cell at the base of the 

 outgrowth to the right. This corpuscle remained in position 

 for more than one hour, being protected from the current by the 

 projecting endotheUal process. Such a condition is frequently 

 seen and conveys some idea of the actual irregularity of the 

 vascular wall. 



The cells constituting the walls of the outgrowths from the 

 vessel are changeable in shape and doubtless move their positions 

 to some extent. The cells at the tip of the growing sprout may 

 be seen to send out processes as if they were actually creeping 

 along. The behavior of these vessel walls is strikingly similar 

 to that which Clark ('09 and '12) has so clearly described for the 

 growing lymphatics in the tail of the tadpole. 



Figure 29 shows a similar vessel with a projecting bud. The 

 cells of this bud are seen to exhibit a most indefinite arrangement ; 

 their processes project across the space and join the cells of the 

 opposite side and none are completely elongated or flattened as 

 are the cells of the main vessel wall. The tip cells might still 

 be described as stellate mesenchjTiial cells. The appearance 

 shown in figure 30 is much the same. The walls of these early 

 vascular buds are extremely loose membranous arrangements 

 with irregularities in their surfaces and openings and spaces 

 between the cellular components. 



