GLOSSARY. 373 



Reproduction. See Ovum. 



Sac. A hollow cavity, e.g. air sac, dart sac, scrota! sac. 



Segmentation. (1) The process by which an ovum is cleaved into a number 

 of cells or blastomeres. (2) The division of the body into a number 

 of divisions or segments. Seymental organs, a term applied to the 

 excretory organs or aephridia of the earthworm. Segmental tube, the 

 primitive excretory duct of the vertebrate. 



Sella turcica. A depression in the basisphenoid bone of the skull, some- 

 what resembling, in the human subject, a Turkish saddle. In the 

 rabbit there is in this region a hole through the bone, the canalis 

 craniopharyngeus. 



Septum. A division or partition, e.g. Septum nasi, between the nasal 

 chambers ; septum auricularum between the auricles ; septum lucidum 

 between the hemispheres of the brain. 



Serous membrane. A membrane that secretes a thin serous fluid (e.g. 

 peritoneum), in distinction from a mucous membrane, which secretes a 

 thicker mucous fluid (e.g. that lining the nasal chamber). 



Skeleton. The supporting framework of the body, consisting of (a) con- 

 nective tissue, (b) cartilage or gristle, (c) bone. In addition to this 

 endoskekton (or instead of it in invertebrates) there may be an 

 exoslceleton. 



Spermary. A term used for the male gonad or testis. 



Spinal cord. The central nervous system of the vertebrate posterior to 

 the brain. The spinal nerves are the nerves given off therefrom. 

 The spinal column is the vertebral column which forms the backbone, 

 and the arches of which protect the spinal cord. This is sometimes 

 called the spine. The neural spine is a median dorsal process of a 

 vertebra. 



Splanchnic. Equivalent to visceral. The terms splanchnic and somatic 

 are the Greek-derived words answering to the Latin derivatives 

 visceral and parietal. 



Spleen. A red organ attached to the mesentery. Splenial, a bone in the 

 lower jaw. 



Squame. A plate-like portion of the antenna of the crayfish, also called 

 the scaphocerite. Squamosal, a bone of the vertebrate skull. 



Sternum. (1) The vertebrate breast-bone. (2) The ventral part of the 

 crayfish's, or insect's exoskeleton. 



Stomodseum. That part of the alimentary canal which arises in develop- 

 ment as an anterior epiblastic invagination, giving rise in.the crayfish, 

 e.g. to the fore-gut ; the mid-gut being derived from the mesenteron, 

 and the hind-gut from the proctodseum. 



Stroma. The connective tissue framework of an organ such as the 

 ovary. 



Sympathetic nerve system. A part of the general nerve system of the 



