302 GLOSSARY OF EMBRYOLOGICAL TERMS 



Somatopleure — layer of somatic mesoderm and closely associated ectoderm, 

 extension of which (from body wall) gives rise to both amnion and 

 chorion. 



Somite — blocks of paraxial mesoblast, metamerically separated by trans- 

 verse clefts, derived from enterocoelic or gastral mesoderm and 

 giving rise to dermatome, myotome, and sclerotome. 



Spawning — act of expelling eggs from uteri of anamniota (e.g., amphibia). 



Sperm — germ cell characteristically produced by the male. (Syn., sperma- 

 tozoon, sperm cell, male gamete, spermatosome.) 



Spermatid — products of the second maturation division in spermatogenesis, 

 the spermatids having certain cytological characteristics and being in- 

 variably haploid; cells which go through a metamorphosis into func- 

 tionally mature spermatozoa. 



Spermatocyte — stages in spermatogenesis between the time the primordial 

 germ cell (spermatogonium) begins to grow, without division, until 

 after the division which results in spermatids. (See Primary Spermato- 

 cyte, Secondary Spermatocyte.) 



Spermatogenesis — entire process which results in maturation of sperma- 

 tozoon. 



Spermatogonium — primordial germ cell of male gonad, indistinguishable 

 from somatic cells, both of which are diploid; stage prior to maturation 

 when the presumptive spermatozoon undergoes rapid multiplication by 

 mitosis. 



Spermatophore — sperm-bearing bundle, such as that which is shed by male 

 urodele, the bundles later to be picked up by cloacal lips of female. 



Spermatosphere — See Idiosome. 



Spermatozoon — functionally mature male gamete. (Syn., sperm.) 



Spina Bifida — split tail, generally involving spine, in developing embryo 

 caused by a variety of environmental conditions, most of which act 

 through interference with normal gastrulation and neurulation. 



Spinal Cord — that portion of central nervous system, excluding brain, 

 which is derived from epithelial and neural ectoderm of original 

 blastula, consisting of ependyma, glia, neuroblasts and their derivatives, 

 and connecting cells. 



Spindle — group of fibers between centrosomes during mitosis, to which 

 chromosomes are attached and by means of which (mantle fiber por- 

 tion) chromosomes are drawn to their respective poles. 



