THE LYMPHATIC SYSTEM. 



channels and spaces have been considered in connection with their 

 respective systems. 



The development of the lymphatic system in all its parts 

 involves the mesoderm alone. Very early in the life-history of the 

 embryo, shortly after the appearance of the three blastodermic layers, 

 the mesoderm undergoes cleavage into two leaves, the separation 

 affecting the mesodermic layer on either side as far as the lateral 

 margin of the uncleft axial band. The resulting sheets of meso- 

 dermic tissue become the parietal layer (somatopleuric) and vis- 

 ceral layer (splanchnopleuric) ; the former clings to the ectoderm 

 to become the future wall of the body-cavity, while the latter adheres 

 to the entoderm to form the wall of the digestive tube. 



The space included between these leaves is the primitive body- 

 cavity, or coelom, and the mesodermic tissue forming its imme- 

 diate wall becomes dif- 



ferentiated into a special FIG. 160. 



lining the mesothe- 

 lium whose elements 

 are the ancestors of the 

 later endothelium. 



The fully-formed se- 

 rous membranes, 

 represented by the peri- 

 toneum, the pleura, the 

 pericardium, and the 

 tunica vaginalis, are all 

 derived as constric- 

 tions from the common 

 pleuro - peritoneal sac, 

 or body - cavity, first 

 formed, the subdivision 

 of which into the above- 



mentioned special serous compartments occurs secondarily and at a 

 much later period. 



Bearing in mind the origin of the primary lining of the serous 

 membranes, the claims of endothelium to near kinship with con- 

 nective tissue must be admitted ; likewise, the reasons for regarding 

 endothelium as distinct in nature from epithelium will be appreciated. 

 Inasmuch as the epithelium of the genito-urinary tract is derived 

 indirectly from the mesoderm, it is related genetically to the endo- 

 thelium of the abdominal serous membrane. 



In the course of the differentiation and growth of the fibrous con- 

 nective tissue, clefts appear within the ground-substance between the 

 bundles of young tissue, which become the lymph-spaces of the 



Transverse section of ten-day rabbit embryo, showing the 

 cleavage of the mesoderm and the formation of the primary 

 body-cavity : E, ectoderm ; M, M, the letters occupy the 

 body-cavity and have the parietal (/) and visceral (v) layers 

 of the cleft mesoderm respectively above and below them ; the 

 immediate lining of the cavity constitutes the mesothelium ; 

 En, entoderm ; N, neural canal ; c, notochord ; s, s, cavities 

 within the somites really parts of the body-cavity ; a, one of 

 the paired primitive aorta;. 



