GLOSSARY OF EMBRYOLOGICAL TERMS 301 



plate giving rise to mucous glands (oral suckers) of amphibia and to 

 parts of olfactory organs, lens of eye, and possibly to part of inner 

 ear. 



Septum — partition. 



Serial Sections — thin (often of microscopic dimensions) sections of em- 

 bryos which are mounted on slides in order of their removal from the 

 embryo, so that a study in sequence will provide an understanding 

 of all organ systems from one region of embryo to the other. 



Sertoli Cell — derivative of sexual cords of testis, found within seminiferous 

 tubule and functionally similar to follicle cell in ovary in that it is the 

 nutritive, supporting, or nurse cell of the maturing spermatozoa. The 

 heads of adult spermatozoa may be seen embedded in the cytoplasm of 

 Sertoli cells. 



Sex Cell Cord — division of sex cell ridge or gonad primordium, not to be 

 confused with sexual (rete) cords. 



Sex Determination — See Determination of Sex. 



Sexual Cords — derivatives of germinal epithelium from which they become 

 separated and give rise to bulk of gonads of both sexes. 



Sexual Cords of the Ovary — sex cords of the originally indifferent gonad 

 primordium which form only cords of ovary, the functional follicles 

 coming from germinal epithelium. 



Sexual Cords of the Testis — sex cords of the originally indifferent gonad 

 primordium which give rise to seminiferous tubules of testis, forming 

 a rather solid mesenchymatous reticulum when cavities begin to appear 

 lined with spermatogonia (from primordial germ cells) and Sertoli cells, 

 the whole constituting seminiferous tubules. 



Sheath, Myelin — myelin covering of axons in so-called white matter of 

 spinal cord. 



Sinus Venosus — point of fusion of vitelline veins of amphibian embryo 

 bilaterally symmetrical and related to ducts of Cuvieri and ductus 

 venosus. 



Skeletogenous Sheath — sclerotomal cells which first form a continuous layer 

 around both notochord and nerve cord. 



Skin — See Dermis and Epidermis. (Syn., integument.) 



Somatic — relating to body in contrast to germinal cells; or relating to 

 outer body in contrast to inner splanchnic mesoderm. 



Somatoblast — blastomeres with specific germ layer predisposition, i.e., ecto- 

 dermal somatoblasts. 



