80 CHARLES R. STOCKARD 



cavity with which they are continuous. The pericardial cavity 

 in the molluscs is true coelom and not a part of the primarj^ body 

 cavity and contains no blood. In almost all of the molluscs 

 the pericardium is in communication with the nephridia and the 

 nephric duct usually leads from the pericardium to the outer body- 

 wall. The pericardial cavity in contrast to the primarj^ bodj- 

 cavity is designated as secondary body cavity or true coelom. 



The final step in the phylogeny of the blood vascular system 

 is characterized by an important expansion of the secondary 

 body cavity or coelom as is the case in the echinoderms, annelids 

 and vertebrates. As a result of the expansion of the secondarj^ 

 body cavity, the primary cavity is reduced merely to a system 

 of channels or vessels and small interstitial lacunae. In the 

 vertebrates, therefore, according to Ziegler, the blood and 

 h-mph vascular system represents the persistent part of the pri- 

 marN^ body cavity. Ziegler considers the blood vascular system 

 and lymph vascular system to have had a common origin. The 

 blood vessel endothelium is closely similar in all respects to the 

 lymphatic endothehum. He thus agrees with Biitschli ('82) 

 that in all metazoa the blood vascular system has its origin from 

 the blastocoel. 



Fehx ('97) holds that his studies on the Salmoniden will not 

 fit into Ziegler's scheme. He claims that the origin of the stamm- 

 vene in the cranial portion is the same as that of the primary 

 mesenephros in the caudal region, and is also of the same origin 

 as that of the primary nephric duct. Cells of the splanchnic 

 as well as cells of the parietal layer of the mesoderm enter into 

 the structural material of the stammvene. The cavity of the 

 venenstrang is the same as the cavity between the lamellae of 

 the secondary lateral plate, that is, true coelomic cavity. The 

 three structures referred to are all portions of the same base, 

 the lateral plate mesoderm, the primary seitenplatte. Felix 

 states, as there is httle doubt that the cavity of the primarj- 

 nephric duct is homologous with the secondary body cavity, so 

 there is httle doubt that the cavity of the venenstrang is also. 

 The development of the aorta shows similar relations. It arises, 

 according to Felix, from the sclerotomes which come from the 



