xii List of I Illustrations. 



Ml 



FIG. 15. Perforation of the humerus (supra-condyloid foramen) in 

 three species of Qnadrumana 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 

 37. 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 12: 



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 Amphioxm . . 137 



42. Gastrnlation 138 



43. Gastrula of a Chalk Sponge 139 



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



45. Ideal prim'tive vertebrate, seen from the left side . . 143 



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



47. Amphioxus lanceolatitt 145 



48. Balatioglossus .148 



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 . . . .I'll 



