1 88 EMBRYOLOGICAL TYPES 



cava, which runs down in the mesentery from the hinder 

 region of the body. 



As the liver develops, the vitelline veins (which are really 

 that part of the subintestinal vessel which is behind the heart) 

 undergo some modification. The hinder portion connects 

 the intestine behind with the liver in front, forming the hepatic 

 portal vein. The anterior portion connects the liver with the 

 heart, and gives rise to the hepatic veins. The formation of 

 the renal portal veins will be described in connexion with the 

 kidneys. 



The blood itself, or rather its red corpuscles, arise from 

 structures known as blood-islands. These are formed from 

 the layer of mesoderm which was split off from the floor of the 

 original archenteric wall just beneath the mass of yolk-cells, 

 behind the rudiment of the liver ; they are regions of rapid cell- 

 proliferation. From here, the corpuscles enter the blood- 

 vessels (which were previously empty) through the vitelline 

 veins. 



The Kidneys. — The kidneys are formed from the meso- 

 dermal tissue situated at the junction between the myoccels and 

 the splanchnoccel and which is known as the nephrotome or 

 intermediate cell-mass. On each side a thickening appears 

 in the region of the 2nd to 4th segments of the trunk of the 

 embryo. This thickening extends back on each side as a rod 

 of cells, between the outer layer of the splanchnocoel and the 

 skin, to the cloaca. The thickening hollows out and a cavity 

 appears which connects with the splanchnoccel by three 

 openings surrounded with cilia. These are the ccelomic 

 funnels. The rod of cells also becomes hollow and opens into 

 the cloaca behind and connects with the cavity in front into 

 which the ccelomic funnels open. There is now therefore a 

 direct communication on each side between the ccelom and the 

 cloaca. The three ccelomostomes and the little tubes or 

 tubules into which they lead, together form the pronephros, 

 which is the first and most anterior portion of the kidney to 

 develop. The tube connecting it with the cloaca is the 

 pronephric duct. 



The tubules elongate and coil about, and as the posterior 



