vi OEIGIN OF DOESAL AOETA IN POLYPTEEUS 365 



dorsal aorta of Polypterus which requires further investigation 

 both in that genus and in any other Vertebrates in which it may 

 be found to occur. In Polypterus in the stage immediately pre- 

 ceding that in which the aortic cells collect together the position 

 of the future aorta is distinctly marked out by the arrangement 

 of the delicate reticulum that is visible connecting up the various 

 organ-rudiments of the larva. This reticulum is usually regarded 

 as an artifact caused by the action of the fixing and preserving 

 solutions upon the albuminous substances contained in the fluids of 

 the embryonic body but the fact that 

 it becomes arranged in this peculiar 

 fashion to foreshadow the future 

 aorta at once raises the question 

 whether it is hot really a reticulum 

 of living substance. 



The aortic cavity in Polypterus 

 has been seen to originate by the 

 fusion of intracellular vacuoles. The 

 cavity is filled with clear fluid and 

 this condition persists even after 

 the main channels of the vascular 

 system are laid down. The blood is 

 at first simply fluid or plasma with- 

 out corpuscles. This plasmatic con- 

 dition may persist even after circula- 

 tion has commenced and the heart 

 propels through the vessels simply 

 the clear cell-less fluid. Here we find 

 repeated in ontogeny an extremely 

 archaic condition of the circulation. 

 The plasma becomes peopled with 

 corpuscles comparatively suddenly. 



The portions of vessel wall lying 

 external to the lining endothelium 

 appear to arise from mesenchyme 

 cells. 



Source of the Corpuscles. — The blood corpuscles are to 

 be looked on, broadly speaking, as mesenchyme cells which 

 have lost their connexion with their neighbours and float free 

 in the plasma. Their precise sources in ontogeny appear to be 

 various : — 



(1) They can frequently be seen in process of being budded off by 

 the wall of the embryonic blood-vessel into its cavity. 



(2) In other cases the vessel with its contents is seen to arise as a 

 solid mass of cells, those at the periphery becoming the wall 

 (endothelium) of the vessel rudiment while those more deeply placed 

 round themselves off, becoming separated by chinks containing 

 fluid, and develop into corpuscles. This may be regarded as a 



Fig. 175. — Portion of transverse section 

 through Polypterus of stage 25 

 showing the relations of the aortic 

 rudiment [A) to the sclerotome (scl). 



ent, enteric cavity ; my, myotome ; 

 N. notocliord. 



