Contents xi 



CHAPTER XXIII 



PAGE 



THE LIFE-CYCLE OF SOME HYMENOPTEROUS INSECTS. BEES . . .351 



Other Hymenoptera 356 



EXPERIMENTAL STUDY OF THE DETERMINATION 



OF SEX 



CHAPTER XXIV 



INTRODUCTION: THE DIFFERENT KINDS OF SEXUAL INDIVIDUALS . . 363 



Dioecious Species, composed of Unisexual Individuals , 365 



Monrecious Species, composed of Bisexual or Hermaphrodite Individuals 368 



Parthenogenetic Species 372 



CHAPTER XXV 



EXTERNAL FACTORS OF SEX DETERMINATION 376 



Influence of Food 376 



Supposed Influence of Nourishment in determining Sex in Man and 



other Mammals . . . . . . . . . . 3^4 



Geddes's and Thomson's Theory of Sex ...... 3^7 



CHAPTER XXVI 



INTERNAL FACTORS OF SEX DETERMINATION 391 



Age of Parents . . 39 1 



Condition of the Germ-cells 393 



Vigor of the Parents . 393 



Effects of Inbreeding .......... 394 



Size of the Egg ' 394 



Ratio of Nucleus to Cytoplasm 39^ 



The Extrusion of the Polar Bodies, and the Analogous Process in the 



Sperm-cells 397 



The Formation of Male and Female Producing Spermatozoa . . 401 



CHAPTER XXVII 



THE INTERNAL FACTORS OF SEX DETERMINATION {Continued) . . 407 



The Origin of Gynandromorphs 47 



The Sex of Multiple Embryos 4 ID 



The Sex of Human Twins and Double Monsters . . . . .4" 

 Two Recent Theories of Sex Determination based on the Assumption 



of Male and Female Eggs 4'3 



The Reduction Process in Parthenogenetic Eggs . . . . 4 1 S 



Influence of the Cytoplasm 4 1 ? 



