282 GLOSSARY. 



Medulla. The ordinary marrow of bones. Medulla spinalis, the 

 spinal cord. 



Medulla oblongata. The upper enlarged portion of the spinal 

 cord where it joins the brain. 



Megalopa. The latest larval stage in the development of the 

 higher Crustaceans. 



Membrana nictitans. The third eyelid of birds, a duplicature of 

 the conjunctiva; it is found also in sharks, frogs, in some rep- 

 tiles, and in many mammals ; in man and monkeys it is repre- 

 sented by the "plica semilunaris." 



Menisci. Cartilaginous rings between the vertebrae of some birds. 



Mentum. The part to which the lower lip of many Arthropods 

 is attached. 



Meroblastic. When, as in birds, the germ and yolk of the egg 

 are separate. 



Meropodite. The long fourth joint in the leg of a Crustacean. 



Mesenteries. Vertical partitions in the somatic cavity of the 

 Actinozoa. 



Mesentery. The membranous connection between the small in- 

 testines and the posterior wall of the abdomen. 



Mesoblast. A cellular layer between the epiblast and the hypo- 

 blast ; in the adult the mesoderm. 



Mesoderm. The part between the ectoderm and the endoderm. 



Mesomyodic. In birds, when certain muscles are attached to the 

 middle of the bronchial semirings. 



Mesothorax. The intermediate segment of the thorax of insects ; 

 the mesosternum corresponds to it beneath. 



Mesozoic or Secondary period in geology ; it includes the Trias, 

 Oolitic, and Cretaceous formations. 



Metabolic. Complete metamorphosis. 



Metacarpus. The assemblage of bones between the wrist and the 

 fingers. 



Metagenesis. "When the produced zooid is dissimilar to the 

 producing zooid " ( Greene). " Changes of form which the repre- 

 sentative of a species undergoes in passing by a series of suc- 

 cessively generated individuals from the egg to the perfect 

 state" (Owen). See also Alternation of generations. 



Metameres. Coalesced segments which have lost their primitive 

 distinctness. See Antimeres. 



