GLOSSARY OF EMBRYOLOGICAL TERMS 303 



Spinous Process — prolongation of neural processes fused dorsally to neural 

 canal; becomes dorsal spine of vertebra. 



Spiracle — short funnel between body wall and operculum on left side of 

 head of frog tadpole, the only exit for water passing through gill 

 chambers to exterior. 



Spireme — continuous chromatin thread characteristic of so-called resting 

 cell nucleus. Existence questioned by current cytologists. 



Splanchnic — refers to viscera, opposed to somatic or body. 



Splanchnic Mesoderm — visceral mesoderm, or that nearest embryonic axis 

 in lateral plate. 



Splanchnocoel — that portion of enterocoel (of Amphioxus) which lies 

 between somatic and splanchnic mesoderm within body. (Syn., coelom.) 



Splanchnocranium — that portion of skull which is preformed in cartilage 

 and which arises from the first three pairs of visceral arches. Opposed 

 to neurocranium. 



Splanchnopleure — layer of endoderm and inner (splanchnic) mesoderm 

 within which develop the numerous blood vessels of area vasculosa and 

 later yolk sac septa; layers within the body which give rise to lining 

 and to musculature of alimentary canal. 



Spongioblasts- — cells of mantle layer of developing spinal cord destined 

 to form merely supporting tissue. 



Stereoblastula — solid blastula as found in Crepidula. 



Stomodeum — ectodermal invagination (pit) which fuses with pharyngeal 

 endoderm to form oral plate, which later ruptures to form margins of 

 mouth cavity. Stomodeal portion of mouth lining is therefore ecto- 

 dermal. 



Stroma — mesodermally derived, medullary, supporting tissues of an organ. 



Sub-Germinal Cavity — See Blastocoel, Segmentation Cavity. 



Sub-Notochordal Rod or Bar — hypochordal rod of amphibian embryo, 

 found dorsal to midgut. Transitory. 



Sucker — adhesive, connecting organ of oral region (larval stage). 



Sustentacular Cell — cell which provides nourishment for another, such as 

 Sertoli or follicle cells of gonads. 



Sylvius, Aqueduct of — See Aqueduct of Sylvius. 



Sympathetic System — originating either from mesenchymal element arising 

 in situ or, more probably, from ectodermal elements emanating from 



