GLOSSARY OF EMBRYOLOGICAL TERMS 305 



Theca externa- — outermost of coverings of ovarian follicle, rather loose con- 

 nective tissue with abundant blood supply. Continuous with perito- 

 neum. 



Theca interna — layer of connective tissue consisting of closely packed 

 fibers, possibly some of smooth muscle, immediately external to egg. 

 Consists of cyst wall. 



Thymus — derivatives of first pair of branchial pouches of frog embryo 

 which separate from pouches (12 mm.) and migrate to a position 

 posterior to auditory capsules near surface of the head. Endocrine 

 functions. 



Thyroid (Body or Gland) — originates as an endodermal thickening in floor 

 of pharynx between second pair of visceral arches; evaginates to form a 

 vesicle temporarily connected with gut by a duct; separates from gut; 

 becomes divided; and migrates to position near hyoglossus muscle. 

 Somewhat similar history in all vertebrate embryos. Endocrine func- 

 tion. 



Tissue Culture — in vitro culturing of isolated tissues; excision of tissues or 

 organs and their maintenance in an artificial medium, generally consist- 

 ing in part of embryonic extracts or blood plasma. 



Tongue — solid mesodermal mass, covered with endoderm, derived by cell 

 proliferation from floor of pharynx beginning in the 9 mm. frog tadpole. 



Tonsils — lymphatic structures derived from endoderm and mesoderm of 

 second pair of visceral pouches. 



Torus Transversus — thickening in median ventro-anterior wall of lamina 

 terminalis of telencephalon, just exterior to optic recess, representing 

 rudiment of anterior commissure. 



Totipotency — related to theory that isolated blastomere is capable of 

 producing a complete embryo. 



Trachea — that portion of respiratory tract between larynx and lung buds, 

 lined with endoderm, probably derived from posterior portion of origi- 

 nal laryngotracheal groove. 



Tracheal Groove — Syn., laryngotracheal groove. 



Transplant — an embryonic area (cell, tissue, or organ) removed to a dif- 

 ferent environment. 



Transverse — a plane (or sections) which divides antero-posterior axis at 

 right angles, separating more anterior from more posterior. (Syn., 

 cross section, but this synonym is not generally satisfactory.) 



