288 GLOSSARY OF EMBRYOLOGICAL TERMS 



Mesendoderm — newly formed layer of (urodele) gastmla before there has 

 been any separation of endoderm and mesoderm. (Syn., mesentoderm, 

 mesentoblast, ento-mesoblast.) 



Mesentery — sheet of (mesodermal) tissue generally supporting organ sys- 

 tems (e.g., mesorchium, mesocardium). 



Mesial — median, medial, middle. 



Mesoblast, Gastral — See Gastral Mesoderm. 



Mesoblast, Peristomial — involuted, ventral lip mesoderm, continuous with 

 gastral mesoderm from dorsal lip. 



Mesocardium — mesentery of heart; may be dorsal, ventral, or lateral. (See 

 under Lateral Mesocardium.) 



Mesoderm — the third primary germ layer developed in point of time, may 

 be derived from endoderm in some forms and from ectoderm in 

 others. (See other terms such as Mesoblast, Mesenchyme, Lateral 

 Plate Mesoderm, Epimere, Mesomere, Hypomere, Gastral Meso- 

 derm, Peristomial Mesoderm, Axial Mesoderm, etc.) 



Mesomere — cell of intermediate size where there are conspicuous size dif- 

 ferences in an early embryo; also refers to intermediate cell mass: 

 intermediate mesoderm. 



Mesonephric Duct — duct which grows posteriorly from mesonephros to 

 cloaca and functions also as vas deferens in male. (Syn., Wolffian 

 duct.) 



Mesonephric Tubules — primary, secondary, and sometimes tertiary tubules 

 developing in Wolffian body, functioning in adult amphibia. 



Mesonephros — Wolffian body, or intermediate kidney, functional as kidney 

 in adult fish and amphibian. 



Mesorchium — mesentery (mesodermal) which surrounds and supports testis 

 to body wall. 



Mesothelium — epithelial layers or membranes of mesodermal origin. 



Mesovarium — mesentery (mesodermal) which suspends ovary from dorsal 

 body wall. 



Metamerism — serial segmentation, as seen in nervous, muscular, and circu- 

 latory systems. 



Metamorphosis — end of larval period of amphibia when growth is sus- 

 pended temporarily. There is autolysis and resorption of old tissues 

 and organs such as gills, and development of new structures such 

 as eyelids and limbs; changes in structure correlated with changes in 



