GLOSSARY OF EMBRYOLOGICAL TERMS 271 



Cornea — transparent head ectoderm plus underlying mesenchyme form a 

 layer directly over eye of vertebrates, known as cornea. 



Corticin — sex-differentiating substances which spread in some amphibia 

 by blood stream and in other forms by diffusion and act as a hor- 

 mone. (See Medullarin.) 



Cranial — relative to head; "craniad" means toward head. (Syn., rostral, 

 cephalad.) 



Cranial Flexure — bending of forebrain forward with angle of bend occur- 

 ring transversely at level of midbrain. (See Cephalic Flexure.) 



Crescent, Gray — crescentic area between original animal and vegetal pole 

 regions on surface of frog's egg, gray in color because of migration 

 of pigment away from area and toward sperm entrance point (Roux, 

 1888); region of presumptive chorda-mesoderm, future blastopore, 

 and anus. 



Crest, Neural — paired cell masses derived from ectoderm cells along edge 

 of former neural plate, and wedged into space between dorso-lateral 

 wall of closed neural tube and integument. Gives rise to spinal gan- 

 glia, sympathetic ganglia, and chromatophores. 



Crest Segment — original neural crest becomes divided into segments from 

 which develop spinal and possibly cranial ganglia. 



Cross-Fertilization — union of gametes produced by different individuals 

 which, if they are of different species, may produce hybrids. 



Crossing Over — mutual exchange of portions of allelomorphic pairs of 

 chromosomes during process of synapsis in maturation. 



Cyclopia — failure of eyes to separate; median fusion of eyes which may 

 be due to suppression of rostral block of tissue which ordinarily 

 separates eyes; exaggeration of vegetativization tendencies. 



Cyst — tubular portions of testis within which aggregations of germ cells 

 mature, often (e.g., Rhomaleum) containing cells all in same stage 

 of maturation. 



Cystic Duct — narrow, proximal portion of embryonic bile duct leading 

 from gallbladder to common bile duct. 



Cytasters — asters arising apart from nucleus in cytoplasm. 



Cyte — suffix meaning cell (e.g., osteocyte for bone cell, oocyte for egg 

 cell). (See specific definitions.) 



Cytology — study of cells. 



