GLOSSARY 609 



spermatid (sptir' ma tKd). A male sex cell after the second maturation division in 

 spermatogenesis, before its metamorphosis into a sperm cell. 



spermatocyte (spur' ma t6 sit). A male sex cell during the period of maturation in 

 spermatogenesis. 



spermatogenesis (spur ma t6 j6n' e sis). The production of sperm cells from pri- 

 mordial germ cells. 



spermatogonium (spur ma to go' nl iim). A male sex cell during the periods of 

 multiplication and growth in spermatogenesis. 



spermatophore (spur' ma to for). A mass of sperm cells, usually inclosed in a mem- 

 brane or capsule. 



sphincter (sfink' ter). A muscle surrounding an opening which, by its contraction, it 

 closes. 



spicule (spik' ul). A minute limy or siliceous crystal-like object embedded in the 

 tissues of an animal which serves to stiffen or support the body or part. 



spinal cord. Of vertebrates the central nervous sj^stem exclusive of the brain. 



spinal column. The bony column, or vertebral column, which incloses the spinal cord. 



spindle, mitotic. The portion of a cell in mitosis lying between the centrosomes, 

 having the shape of a spindle and appearing in a microscopical preparation as if 

 containing a framework of fibers. 



spiracle (splr'ak'l). In insects, one of the openings into the tracheal tubes; in 

 sharks, a modified and usually non-functional gill sUt opening internally into the 

 cavity of the pharynx and externally upon the surface of the head ; in amphibians, 

 the external opening of a chamber on the inner wall of wliich are the external 

 openings of the gill slits. 



spiral cleavage. Cleavage in which the blastomeres are spirally arranged. 



spireme (spl' rem). A thread of chromatin appearing early in the prophase of mitosis 

 which later breaks transversely into chromosomes. 



splanchno (splank' no). G.; viscera; adj., splanchnic (spla?ik' nik), pertaining to the 

 viscera. 



spongin (spun' jtn). The horny substance which forms the fibers of fibrous sponges. 



spontaneous generation. Same as abiogenesis. 



spore (spor). A minute reproductive cell produced by a protozoan, either sexually or 

 asexually, and usually contained in a shell, though in some cases motile. 



sporocyst (spo' ro sist). A stage in the development of the liver fluke following the 

 encystment of the miracidium. 



sporulation (spor 60 la' shtin). The production of spores by a protozoan. 



squam (skwam). L.; a scale; adj., squamous (skwa'mils). 



stato (staf 6). G.; standing, fixed. 



statoblast (staf 6 blSst). An asexual winter bud of a bryozoan, which is inclosed 

 in a chitinous shell. 



statocyst (staf 6 sist). An organ of equilibrium and orientation in many inverte- 

 brates. 



steapsin (ste ap' sin). The enzyme in the pancreatic secretion which changes fats to 

 fatty acids and glycerin. 



stigma (stig' ma). An eyespot in protozoans; also the same as a spiracle in an insect. 



stimulus (stim' u liis). Any condition either external or internal to the body which 

 causes a response in a living organism. 



stom (stom). G.; mouth. 



stomach (stum' tlk). An enlarged portion of the alimentary canal in which the food 

 is accumulated and in which it also may be reduced to fine particles and partly 

 digested. 



stomodeum (sto mo de' tim). An invagination of the surface of an embryo which 

 meets the anterior end of the archenteron and forms the mouth cavitv. 



