THE HYPOMERE (LATERAL PLATE MESODERM) 249 



Posterior cardinals — ventro-lateral to dorsal aorta, dorsal to nephro- 

 genic tissue. Carry blood to common cardinals. 

 Subcardinals — smaller veins, ventral to mesonephric tissue, and bring- 

 ing blood from caudal vein. Become renal portals. 

 Common cardinals — ducts of Cuvier which receive blood from anterior 

 and posterior cardinal vessels, and pronephric sinus, and convey it to 

 sinus venosus at its postero-lateral margin. 

 Post-caval vein — posterior vena cava or inferior vena cava. This vessel 

 has grown posteriorly from hepatic vein and incorporates mesoneph- 

 ric level of right posterior cardinal vein. It therefore passes through 

 liver into sinus venosus ventral to entrance of right common cardinal 

 vein (see diagram). 

 Hepatic system — derivatives of original vitelline veins. 



Hepatic vein — fused vitelline veins carrying blood from liver sinuses to 

 the sinus venosus. Short and difficult to distinguish from post-caval 

 vein to which it gives rise. 

 Portal vein — common vessel receiving blood from pancreas, stomach, 

 intestine, etc., representing original vitelline veins. 

 Lymphatic system — not well developed at this stage. 

 Chondrocranium — cartilage centers may be found, within which bone will de- 

 velop in formation of the embryonic skull. 

 Neurocranluni — skeleton around brain and primary sense organs. 

 Anterior to posterior. 

 *Labial (supra-ostral) cartilages — paired, short, within upper lip and 



lateral to mouth. 

 *Cornua trabecularum (trabecular horns) — paired cartilage bars which 



accompany olfactory tubes. 

 Ethmoid (internasal) plate — fused trabeculae within median line, anterior 

 to ascending process of pterygoquadrate. 

 *Pterygoquadrate (palatoquadrate) — derived from maxillary portion of 

 first visceral arch and joins posterior end of trabecula by its posterior 

 ascending process. At its anterior end joins anterior ascending process. 

 Basicranial fontanelle — skeletal cavity for pituitary gland. 

 Trabeculae — paired cartilage bars on either side of basicranial fonta- 

 nelle, converging slightly anteriorly. 

 Basilar plate — single, fused cartilage mass immediately anterior to noto- 



chord. Joins parachordals and trabeculae. 

 Parachordals — lateral to notochord, at its anterior end. 

 Splanchnocranium — derived from the visceral arches. Anterior to posterior. 

 Mental (infra-rostral) cartilages — paired, but meet in mid-line of lower lip. 

 *Meckelian cartilages — laterally projecting flanges to which muscles will be 



attached joining pterygoquadrate. 

 *Ceratohyals — large cartilages arising within hyomandibular arches, posterior 



to Meckelian cartilages. 

 *May be ectodermal. 



