GLOSSARY OF EMBRYOLOGICAL TERMS 287 



integral part of the organism, through chemical changes. (Syn., the 

 scientific attitude.) (See Vitalism.) 



Meckel's Cartilage — core of lower jaw derived from ventral part of car- 

 tilaginous mandibular arch. 



Median plane — "middle" plane, as of an embryo. May be median sagittal 

 or median frontal. 



Medulla Oblongata — that portion of adult brain derived from rhom- 

 bencephalon. 



Medullarin — sex differentiating substance spread in some amphibia by 

 blood stream as a hormone and in other forms by diffusion. (See 

 Corticin.) 



Medullary — See terms under Neural, such as canal, fold, groove, plate, 

 tube. 



Medullary Cords — that portion of suprarenal glands derived from sym- 

 pathetic nervous system; central cords. Also that portion of embryonic 

 gonad presumably derived from pre-migratory germ cells upon reach- 

 ing genital ridge. 



Meiosis — process of nuclear division found in maturation of germ cells, 

 involving a separation of members of pairs of chromosomes. (Syn., 

 reductional division.) 



Melanophore — well with black or brown pigment (melanin), derived from 

 neural crests and migrating throughout body. 



Membrane Bone — bone developed in regions occupied by connective tissue, 

 not cartilage. 



Membrane, Vitelline — See Vitelline Membrane. 



Membranes — See Egg Membranes. 



Meroblastic Cleavage or Ova — See under Cleavage or Egg. 



Mesencephalon — section of primary brain between posterior level of 

 prosencephalon and an imaginary line drawn from tuberculum pos- 

 terius to a point just posterior to dorsal thickening. Gives rise to optic 

 lobes, crura cerebri, and aqueduct of Sylvius. (Syn., midbrain.) 



Mesenchyme — form of embryonic mesoderm or mesoblast in which 

 migrating cells unite secondarily to form a syncitium or network hav- 

 ing nuclei in thickened nodes between intercellular spaces filled with 

 fluid; often derived from mesothelium. 



