xii List of Illustrations. 



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

 three species of Quadmmana where it normally occurs, 

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



26. Antlers of stag, showing successive addition of brandies 



in successive years loo 



27. Fission of a Protozoon ..,.,.. 107 



28. Hydra viridis, partly in section 1 1 1 



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 iar 



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) 129 



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 135 



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



tation, drawn in perspective . . . . . 135 



41. Formation of the gastrula of Amphioxui ... 137 



42. Gastrulation 138 



43. Gastrula of a Chalk Sponge 139 



44. Prophysema primordiaky an extant gasti sea- form . ,140 



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



46. The same in transverse section through the ovaries 144 



47. Amphioxus lanceolattu 145 



48. Balanoglossus I4 8 



49. A large Sea-lamprey (Petromyzon 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 . . . . IRI 



