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 . . 9=, 



26. Antlers of stag, showing successive addition of branches 



in successive years 100 



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 



3 1 . Amoeboid movements of young egg-cells . . .122 



32. Human ovum, mature and greatly magnified . . 173 

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



a star-fish . . . . . , . . . I2 ; ' ; 



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



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



36. Karyolunesis 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 . . . . . 1 35 



41. Formation of the gastrnla of Amfhioxus . . .137 



42. Gastrulation , , 138 



43. Gartrula of a Chalk Sponge 139 



44. Prophysema pri m ordiale, 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 lanccolatus 145 



48. Balanoglossus 148 



49. A large Sea-lamprey (j'elromyzon 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 . uu 



