284 GLOSSARY OF EMBRYOLOGICAL TERMS 



splanchnic mesoderm. Source of origin of excretory system. (Syn., 

 nephrotome or middle plate.) 



Internal Gills — filamentous outgrowths on posterior side of first three pairs 

 of branchial arches and a single row on anterior side of fourth pair 

 of branchial arches of frog tadpole, which have a respiratory function 

 concurrent with and following absorption of external gills. 



Internal Limiting Membrane — membrane which develops on innermost 

 surface of inner wall of optic cup during fourth day of chick develop- 

 ment. 



Intersex — individual without typical sexual differentiation. 



Interstitial Cells — specialized cells between seminiferous tubules of testis 

 which produce hormones. 



Interstitial Tissue of Testis — cell aggregations between seminiferous tubules 

 of testis which elaborate a male sex hormone. 



Invagination — folding or inpushing of a layer of cells into a preformed 

 cavity, as in one of the processes of gastrulation. Opposed to involu- 

 tion. 



Involution — rolling inward or turning in of cells over a rim, as in gastrula- 

 tion of chick embryo. 



Iris — ^narrow zone bounding pupil of eye in which two layers of optic cup 

 become blended so that pigment from outer layer invades material of 

 inner layer, giving eye a specific color by variable reflection. 



Isogamy — similar gametes, without differentiations into spermatozoa and 

 ova. 



Isolation Culture — removal of a part of an organism and its maintenance 

 in a suitable medium in living condition. 



Isthmus of the Brain — depression in dorsal wall of embryonic brain which 

 partially separates mesencephalon from metencephalon. 



Isthmus of the Oviduct — short, tubular, posterior end of oviduct (e.g., 

 chick) in which fluid albumen and shell membranes are applied to egg. 



Iter — See Aqueduct of Sylvius. 



Jacobson's Organ — ventro-medial evaginations from olfactory pits (am- 

 phibia and reptilia) which later become glandular and sensitive olfac- 

 tory epithelia. 



Jelly — mucin covering of amphibian egg derived from oviduct and applied 

 outside vitelline membrane. 



