GLOSSARY 445 



medulla. The marrow or core of an organ. 



medulla oblongata. The posterior brain division which contains the fourth 

 ventricle. 



Meibomian glands. Sebaceous glands of the eye lids. 



Meissner's corpuscles. Tactile corpuscles with thick capsule and brush of nerve 

 terminations. 



melanophore. A pigment cell containing melanin. 



membrane bones. Bones which are not preformed in cartilage. 



meninges. The three membranes which enclose the brain and spinal cord. 



menopause. The time in life when menstruation normally ends. 



menstruation. The monthly How of women. 



mentum. The chin. 



Merkel's corpuscles. Tactile corpuscles of the tongue and mouth. 



mesencephalon. The third or mid-brain vesicle. 



mesenchyme. The embryonic connective tissue. 



mesentery. The peritoneal membrane which attaches the intestine to the body wall. 



mesethmoid. The median cribriform portion of the ethmoid bone. 



mesocardium. The membrane which connects the embryonic heart with the body wall. 



mesoderm. The middle germ layer. 



mesogaster. The peritoneal membrane which attaches the stomach to the body wall. 



mesoglea. The gelatinous middle tissue of sponges and coelenterates. 



mesomere (nephrotome). That portion of the trunk mesoderm which connects each 

 somite fepimere) with the ventral mesoderm or hypomere. 



mesomerism. The segmentation of the epimeric or dorsal mesoderm into somites. 



mesonephros. The middle kidney or Wolffian body, the functional kidney of anamnia. 



mesopterygium. The middle basal cartilage of the elasmobranch pectoral fin. 



mesorchium. The peritoneal membrane which attaches the testis to the dorsal body 

 wall. 



mesorectum. The mesentery of the rectum. 



mesothelium. The cellular layer which encloses the coelom. 



mesovarium. The peritoneal membrane which connects the ovary to the body wall. 



Mesozoic. The middle life era of geologic time. 



metabolism. The chemical cycle of matter in living organisms. 



metacarpus. The group of five bones between the wrist and the fingers. 



metacone. The postero-external cusp of the upper molar teeth of mammals. 



metaconid. The postero-internal cusp of the lower molar teeth of mammals. 



metamere. One of the serial divisions of the body of a segmented animal. 



metamorphosis. A striking change of form during development, as seen in the trans- 

 formation of the tadpole into a frog. 



metanephros. The definitive kidney of the amniotes. 



metaotic. Posterior to the ear vesicle. 



metapleural folds. Paired folds on the ventral side of the body of Amphioxus. 



metapterygium. The posterior basal cartilage of the pectoral fin of elasmobranchs and 

 some ganoids. 



metastemum. The posterior or xiphoid process of the breast bone. 



metatarsus. The five bones between the ankle and the toes. 



metazoa. Many-celled animals. 



metencephalon. The fourth brain division which forms cerebellum and pons. 



midbrain. The third or mesencephalon division of the brain. 



mitral valve. The left atrioventricular valve of the heart. 



moa. An extinct running bird of New Zealand. 



molar teeth. The three grinding teeth of each half jaw. 



