974 



Sense receptors, types of, 840 

 Septa ovarii, 796 

 Septula, of testis, 793 

 Septula compartments, 13 

 Septulum, 793 



Septum primum, of mammalian heart, 757 

 Septum transversum, primary, 627, 864, 

 866 

 secondary, 867 



formation of in amphibians, reptiles, 

 and birds, 867 

 Serial homology, 534 

 Serosa (chorion), 908, 910 

 Serous cavity, 6, 13 

 Serpentes, xviii 



Serranus atrarius (sea bass), early develop- 

 ment of, 323 

 ectoderm of, 559 

 Sertoli cell, 793 



as nurse cell for developing sperm, 147 

 conditions of in fowl, 139 

 Sex cells, definitive, structure of, 126 

 Sex characteristics, secondary, effects of 



male sex hormone on, 27 

 Sex chromosomes, 887 

 Sex cords, 792 



Sex-determining mechanisms, chromoso- 

 mal, 887 

 possible influence of sex field on, 889 

 Sex differentiation, factors involved in, 

 summary of, 893 

 influence of endocrine (hormone) sub- 

 stances on. 886, 891 

 Sex features, general, in animal kingdom, 



886 

 Sex field, cortical, 889 

 medullary, 889 



possible influence of on sex determina- 

 tion, 889 

 Sex gland differentiation, 790 

 Sex hormone, androgenic, 20, 21 

 gynogenic, 85 



male, biological effects of, 27, 28, 29 

 production of, and cells of Leydig, 23, 



24, 25 

 sources of, 26 

 Sex reversal, in axolotl, 890 

 Sexual cycle, 92 



female, length of, 92, 94 



non-ovulatory (anovulatory), 96 

 male, as influenced by external environ- 

 mental factors, 43, 47 

 Seyle, H., 108, 879 

 Shaft, of bone, 691 



of hair, 582 

 Shaft (rachis; scape), of feather, 571, 576 

 Shapiro, H., 219 



Shark, origin of eye muscles in, 716 

 skeletal muscular development in, 708 

 "tongue" of, 603 

 Shaver, J. R., 219, 704 

 Sheath, inner hair (epithelial), of hair fol- 

 licle, 581 

 outer, of hair follicle, 581 

 Sheath cuticle, inner, of hair root, 583 

 Sheldon, E. F., 665 

 Shell membranes, 906 

 Shettles, L. B., 198 

 Shreiner, K. E., 664 

 Shumway, W., 297 

 Simpson, M. E., 41 

 Sinu-atrial (auricular) valves, of chick 



heart, 756 

 Sinus, urogenital, 18 

 Sinus node, 754, 167 

 Sinus septum (Eustachian valve), of chick 



heart, 756 

 Sinus venosus, 728, 747, 750, 757, 758 

 Sinusoids, of liver, 627 

 Siredon {Amby stoma) mexicanum, sex re- 

 versal in, 890 

 Siren lacertina, xvii 

 Sirenia, xx 



Skeletal portion, of visceral arch, 619 

 Skeletal system, definition of, 653, 654 

 Skeletal tissue, adult, contribution of em- 

 bryonic mesenchyme to, 655 

 development of, 663 

 Skeletogenous septum, horizontal, 706 

 Skeleton, appendicular, 668 

 axial, 668 



development of, 674 

 of tail, 688 

 basic embryonic, origin and significance 



of, 655 

 dermal, 568 



external (exoskeleton), 668 

 primitive or "ghost," 655 

 visceral, 668, 669 

 muscles of, 717 

 Skin, accessory structures associated with, 

 development of, 579 

 color of, factors concerned with, 590 

 component parts of, origin of, 557 

 development of, in fishes, 561, 565 

 general functions of, 557 

 mammalian, characteristics of, 578 



development of, 579 

 origin of dermal component of, 559 

 vertebrate, basic structures of, 557 

 coloration and pigmentation of, 590 

 definition of, 556 



