THE HYPOMERE (LATERAL PLATE MESODERM) 247 



Summary of Embryonic Development of the 11 mm. Frog Tadpole: 

 Mesodermal Derivatives 



Mesenchyme — loose, primitive mesoderm found scattered throughout tadpole. 

 There are condensations in head region where cartilage-forming cen- 

 ters are developing, later to give rise to neurocranium except dura and 

 pia mater, which come from neural crest. Connective and blood vas- 

 cular tissues are also to be derived from mesenchyme. Around meso- 

 dermal notochord are sclerotomal (mesenchyme) cells which will 

 form axial skeleton. 

 Epimere — most dorsal mesodermal masses appearing as metameric somites, 

 the most anterior of which are being transformed. About 12 somites in 

 trunk and 32 in tail remain. 



Dermatome — distinct band of mesenchymatous cells lying dorso-laterally 

 just beneath ectoderm, to form dermis (cutis). 



Myotome — central portion of somite which is being organized into muscle 

 bundles, divided in tail region into dorsal and ventral bundles. 



Sclerotome — thin layer of mesenchyme cells surrounding nerve cord and 

 notochord from which will develop axial skeleton. 

 Mesomere — intermediate mesodermal mass from which urogenital system is 

 derived. 



Pronephros — primary, embryonic kidney made up of a few coiled tubules 

 and ciliated nephrostomes. 



Pronephric ducts — lateral to dorsal aorta and dorsal to posterior cardinals, 

 these ducts lead from pronephros posteriorly to join each other just 

 before they fuse with cloaca. 



Mesonephros — large mass of nephrogenic tubules, without nephrostomes, 

 forming permanent frog kidney. 



Gonads — cell masses hanging into coelom from dorsal peritoneum between 

 mesonephros and gut, in vicinity of lung buds. 

 Hypomere — lateral plate mesoderm, ventral to mesomere, and consisting of 

 somatic and splanchnic layers with intermediate coelomic cavity. 



Pericardial cavity — surrounds heart, not yet separated from true peritoneal 

 cavity. 



Peritoneal cavity — body cavity, surrounding viscera. 



Mesenteries — double-layered dorsal mesentery supports viscera. A remnant 

 of ventral mesentery (gastrohepatic omentum) is found between stom- 

 ach and liver. 



Spleen — accumulation of cells lateral to mesenteric artery in dorsal mesentery. 

 Circulatory System — differentiated as heart, arteries, veins, lymphatics, and 

 contained corpuscles. 



Heart — already a three-chambered structure as in the adult. 



Sinus venosus — junction of post-caval and common cardinals, leading to 



right atrium. 

 Atria — thin-walled, paired heart chambers, at least partially separated from 



