GLOSSARY OF EMBRYOLOGICAL TERMS 279 



Gasserian Ganglion— fifth cranial or trigeminal ganglion, derived from 

 midbrain. 



Gastraea Theory — theory of Haeckel that since all higher forms have gas- 

 trula stages there may have existed a common ancestor built on the 

 plan of a permanent gastrula, as are the recent Coeloenterata. 



Gastral Mesoderm — mesoderm derived from dorso-lateral bands (entero- 

 coelic) in Amphioxus or from dorsal lip in frog. Opposed to peristo- 

 mial mesoderm. 



Gastrocoel — cavity formed during process of gastrulation. (Syn., archen- 

 teron.) 



Gastrula — didermic embryo, possessing a newly formed cavity, gastrocoel 

 or archenteron. The two layers are ectoderm and endoderm. 



Gastrular Cleavage — separation of ectoderm and endoderm, during gas- 

 trulation, by a slit-like crevice, actually compressed blastocoel. 



Gastrulation — dynamic process involving cell movements which change 

 embryo from a monodermic to either a di- or tridermic form. Gen- 

 erally involves inward movement of cells to form enteric endoderm. 

 Description includes epiboly, concrescence, confluence, involution, 

 invagination, extension, and convergence. 



Genital — refers to reproductive organs or processes, or both. 



Genital Ducts — any ducts which convey gametes from their point of origin 

 to region of insemination (e.g., collecting tubules, vas deferens, vas 

 efTerens, seminal vesicle, oviduct, uterus, etc.). 



Genital Ridge — initial elevation or thickening for development of external 

 genitalia. 



Germ — egg throughout its development, or at any stage. 



Germ Cell — cell capable of sharing in reproductive process, in contrast 

 with a somatic cell (e.g., sperm or egg cell). (Syn., gamete.) 



Germ Layer — more or less artificial spatial and histogenic distinction of 

 cell groups beginning in gastrula stage, consisting of ectodermal, 

 endodermal, and mesodermal layers. No permanent and clear-cut dis- 

 tinction, as shown by transplantation experiments. 



Germ Plasm — hereditary material, generally referring specifically to the 

 genotype. Opposed to somatoplasm. 



Germ Ring — ring of cells showing accelerated mitotic activity, generally 

 a synonym for lips of blastopore. The rapidly advancing cells in epib- 

 oly. 



