274 GLOSSARY OF EMBRYOLOGICAL TERMS 



Dorsal Root Ganglion — aggregation of neuroblasts which are derived 

 from neural crests and which send their processes into dorsal horns 

 of spinal cord. 



Dorsal Thickening — roof of mesencephalon which gives rise to optic 

 lobes. 



Duct. See ducts under specific names. 



Ductus Arteriosus — See Ductus Botalli. 



Ductus Botalli — dorsal portion of sixth pair of aortic arches which nor- 

 mally becomes occluded after birth, remainder of arch giving rise to 

 pulmonary arteries. (Syn:, ductus arteriosus.) 



Ductus Cuvieri — union of all somatic veins which empty directly into 

 heart, specifically the vein which unites common cardinals and sinus 

 venosus. Sometimes regarded as synonymous with common cardinal. 



Ductus Endolymphaticus — dorsal portion of original otic vesicle which 

 has lost all connections with epidermis, and which is partially con- 

 stricted from region which will form semi-circular canals. 



Duodenum — portion of embryonic gut associated with outgrowths of pan- 

 creas and liver (bile) ducts. 



Dyads — aggregations of chromosomes consisting of two rather than four 

 (tetrad) parts, term used to describe condition during maturation 

 process. 



Ecdysis — process of molting a cuticular layer, shedding of epithelium. 



Ectoblast — See Epiblast. 



Ectoderm — outermost layer of didermic gastrula. (Syn., epiblast.) 



Ectoplasm — external layer of protoplasm of egg cell; layer immediately 

 beneath cell membrane. (Syn., egg cortex.) 



Edema — condition in which tissues hold an excess of water, common in 

 parthenogenetic tadpoles. (Older spelling: oedema.) 



Egg, Alecithal — eggs with little or no yolk. Literally means "without yolk." 



Egg, Cleidoic — eggs, such as those of reptiles, birds, and oviparous mam- 

 mals, which are covered by a protective shell. 



Egg, Ectolecithal — egg having yolk around formative protoplasm. Op- 

 posed to centrolecithal. 



Egg Envelope — material enveloping egg but not necessarily a part of the 

 egg, such as vitelline membrane, chorion, jelly, albumen. 



