290 GLOSSARY OF EMBRYOLOGICAL TERMS 



Muscle Plate — See Myotome. 



Myeloblasts — muscle-forming (embryonic) cells. 



Myoblasts— formative cells within myotome or muscle plate which will 

 give rise to true striated muscles of adult. 



Myocardium — muscular part of heart arising from splanchnic mesoblast. 



Myocoel — cavity within which ovaries of Amphioxus develop; temporary 

 cavities within myotomes which may have been connected with coelom. 



Myotome — thickened primordium of muscle found in each somite, (Syn., 

 muscle plate.) 



Nares, External — external openings of tubes which are connected with 

 olfactory vesicles. 



Nares, Internal — openings of tubular organ from olfactory placodes into 

 anterior part of pharynx of 12 mm. frog tadpole. (Syn., choanae.) 



Nasal Choanae — openings of olfactory chambers into mouth. 



Nasal Pit — See Olfactory Pit. 



Nebenkern — cytological structure near nucleus of early spermatid. 



Neoteny — condition of many urodeles and of experimentally produced 

 (thyroidless) anuran embryos in which larval period is extended or 

 retained, i.e., larvae fail to go through normal metamorphosis. Sexual 

 maturity in larval stage (e.g., axolotl, Necturus). 



Nephrocoel — cavity, found in nephrotome or intermediate cell mass, which 

 temporarily joins myocoel and coelom. 



Nephrogenic Cord — continuous band of intermediate mesoderm (meso- 

 mere) without apparent segmentation, prior to budding off of meso- 

 nephric tubules. 



Nephrogenic Tissue — intermediate cell mass, mesomere, or nephrotome 

 which will give rise to excretory system. 



Nephrostome — funnel-shaped opening of kidney tubules into coelom; outer 

 tubules of amphibian mesonephric kidney acquire ciliated neph- 

 rostomal openings from coelom and shift their connections to renal 

 portal sinus. 



Nephrotome — intermediate cell mass. 



Nephrotomic Plate — intermediate mesoderm, mesomere. 



Nerve, Abducens — sixth (VI) cranial nerve arising from basal plate of 

 rhombencephalon which controls external rectus muscles of eye. 



