Anatomy and Development of Posterior Lymph Hearts of Turtles. 87 



anastomose freely with each other and communicate, by means of two 

 or three openings, with the vein rmming along their mesial borders. 



(4) At the beginning of the fourth week, the mesenchymal tissue 

 condenses about the veno-lymphatic channels to form definite walls. 

 These cardiac walls are then invaded by muscle cells from the adjacent 

 muscle plates. 



(5) The final stage in the development of the posterior lymph 

 hearts is reached by the dilation and confluence of these veno-lymphatic 

 sinuses, from before backward, forming a pair of sac-like organs, each 

 with a single central cavity. 



