Xll 



List of Illustrations. 



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

 three species of Quadruinana where it normally occurs, 

 and in Man, where it does not normally occur . 



26. Antlers of stag, showing successive addition of branches 



in successive years 



27. Fission of a Protozoon 



28. Hydra viiidi's, partly in section .... 



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



of a worm . ' . 



30. Ovarian ovum of a Mammal .... 



31. Amoeboid movements of young egg-cells 



32. Human ovum, mature and greatly magnified 



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



a star-fish ........ 



34. Fertilization of the ovum of an echinoderm . 



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



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



mander) ........ 



37. Study of successive changes taking plnce in the nucleus 



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



38. Formation and conjugation of the pronuclei in Ascaris 



megalocephala 132 



39. Segmentation of ovum 



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



tation, drawn in perspective .... 



41. Formation of the gastiula of y4«/////^,a« 



42. Gastrulation ....... 



43. Gastrula of a Chalk Sponge 



44. PropJiyseina primordiale, an extant gastrsea-form , 



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



46. The same in transverse section through the ovaries 



47. Amphioxus lanceolatus 



48. Balanoglossus ....... 



49. A large Sea-lamprey {Petroinyzon 7nariniis) . 



50. h.A\A\.?>\i2:tV {^Carcharias 7tielaiwpterus< 

 51 Diagram of heart and gill-arches of a fish 



52. One gill-arch, with branchial fringe attached 



53. Diagiam of heart and gill-arches in a lizard . 



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



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



56. The same, modified for a mammal 



FAGB 



9.S 



100 

 107 

 III 



113 

 121 

 122 



i?3 



I2f, 

 126 



127 



129 



135 

 137 



13S 



139 



140 



M3 

 144 



14.^ 



14S 

 1 48 



149 



= 50 



150 

 150 



151 

 16' 



