xiv Mst ok ILLUSTRATIONS. 



WVi, '' AI ' K 



25. Amphioxus embryo at the time of hatching; bisected vertically. 



(AfterHatschek) B9 



26. Amphiozof embryo at the time of hatching; bisected horizontally. 



(AfterHatschek) 59 



27. Amphioxus embryo shortly after hatching ; seen in optical section 



from the right side. (AfterHatschek) 02 



28 and 29. Transverse sections of Amphioxus embryos shortly after 

 hatching ; showing stages in the formation of the notochord and 

 mesoblastic somites. (After Hatschek) 64 



30. Amphioxus embryo with nine pairs of mesoblastic somites, seen in 



optical section from the right side. (After Hatschek) . . 66 



31. Amphioxus embryo with nine pairs of mesoblastic somites, seen in 



horizontal section. (After Hatschek) 66 



32. Transverse section through the middle of an Amphioxus embryo with 



nine pairs of mesoblastic somites. (After Hatschek) . . 67 



33. Amphioxus embryo with fourteen pairs of somites ; seen in optical 



section from the right side. (After Hatschek) .... 69 



34. Amphioxus larva at the commencement of the second or ' larval ' 



period of development. (After Hatschek) . . . . .74 



35. Young Amphioxus during the ' adolescent ' period. (After Kowa- 



levsky) 74 



36. The anterior end of an Amphioxus larva with four primary gill-slits, 



seen from the left side. (After Lankester and Willey) . . 76 



37. The anterior end of an Amphioxus larva with fourteen primary gill- 



slits, seen from the right side. (After Willey) .... 76 



38. The anterior end of an Amphioxus larva with thirteen primary and 



eight secondary gill-slits, seen from the right side. (After 

 Willey) 77 



39. The anterior end of an Amphioxus larva with twelve primary gill- 



slits, of which the first and twelfth are disappearing, and eight 

 secondary gill-slits ; seen from the ventral surface. (After 

 Willey) * 78 



40. Diagrammatic transverse section across an Amphioxus larva with 



eleven or twelve primary gill-slits, but no secondary ones. 

 (Slightly modified from Lankester and Willey) .... 82 



41. Diagrammatic transverse section through an advanced Amphioxus 



larva with fully formed atrial cavity. (Slightly modified from 

 Lankester and Willey, and from Boveri) 83 



42. Diagrammatic transverse section across the intestinal region of an 



Amphioxus larva with five primary gill- slits. (After Hatschek) 85 



43. Diagrammatic transverse section across a young Amphioxus imme- 



diately after the completion of the larval period. (After 

 Hatschek) 86 



THE FKOG 



44. Various stages in the development of the frog. (From Brehm's 



'Thierleben') 93 



45. Stages in the maturation of the egg of the frog. (After 0. Schultze) 9« 

 46-48. Segmentation of the frog's egg 101 



