xii List of Illustrations. 



PAGE 



FIG. 25. Perforation of the humerus (supra-condyloid foramen) in 

 three species of Quadrumana where it normally occurs, 

 and in Man, where it does not normally occur . . 95 



26. Antlers of stag, showing successive addition of branches 



in successive years 100 



27. Fission of a Protozoiin 107 



28. Hydra viridis, partly in section in 



29. Successive stages in the division of the ovum, or egg-cell, 



of a worm . . . . . . . . .113 



30. Ovarian ovum of a Mammal 121 



31. Amoeboid movements of young egg-cells . . .122 



32. Human ovum, mature and greatly magnified . .123 



33. Stages in the formation of the polar bodies in the ovum of 



a star-fish 125 



34. Fertilization of the ovum of an echinoderm . . .126 



35. Fertilization of the ovum of a star-fish . . . .127 



36. Karyokinesis of a typical tissue-cell (epithelium of Sala- 



mander)' . . . . . . . . . i 29 



37. Study of successive changes taking place in the nucleus 



of 'an epithelium-cell, preparatory to division of the cell 131 



38. Formation and conjugation of the pronuclei in Ascaris 



megalocephala 132, 133 



39. Segmentation of ovum . . . . . . 1 35 



40. The contents of an ovum in an advanced stage of segmen- 



tation, drawn in perspective 135 



41. Formation of the gastrula of Amphioxus . . 137 



42. Gastrulation 138 



43. Gastrula of a Chalk Sponge ...... 139 



44. Prophysema primordiale, an extant gastrsea-form . .140 



45. Ideal primitive vertebrate, seen from the left side . . 143 



46. The same in transverse section through the ovaries . 144 



47. Amphioxus lanceolattts . . . . . . .145 



48. Balanoglossus 148 



49. A large Sea-lamprey (Fetromyzon marinus} . . .148 



50. Adult Shark (Carcharias melanopterus] . , -149 



51. Diagram of heart and gill-arches of a fish . . . 150 



52. One gill-arch, with branchial fringe attached . .150 



53. Diagram of heart and gill-arches in a lizard . . .150 



54. Ideal diagram of primitive gill- or aortic-arches . -151 



55. The same, modified for a bird . ... -151 



56. The same, modified for a mammal . . . .151 



