LIST OF ILLUSTEATIONri. XV 



t'IG. PAGE 



49. The blastula stage in the development of the frog's egg . . . lOS 



50. The frog's egg at the close of segmentation . . . . 103 



51. Median sagittal section of a frog embryo, showing the spreading of 



the epiblast, and the commencing formation of the mesenteron. 104 

 .'^2. Sagittal section of a frog embryo during the formation of the 



mesenteron .... .... 10& 



53. Horizontal section of a, frog embryo during the formation of the 



mesenteron ... .... . . 107 



54. Sagittal section of a frog embryo just before the disappearance of 



the segmentation cavity . 108 



55. Sagittal section of a frog embryo after the disappearance of the 



segmentation cavity and completion of the mesenteron . . 109 

 5li. A transverse section through the middle of a frog embryo at about 



the stage represented in Fig. 55 ... . . . 110 



37. A frog embryo at the time of appearance of the neural folds : seen 



from the dorsal surface 113 



5H. Stages in the early development of the frog embryo, seen obliquely 



from the hinder end. (From a series of wax models by Dr. F. 



Ziegler of Freiburg i/B) ... . . 114 



59. Transverse section through a frog embryo, showing the neural folds 



shortly before they meet each other to complete the neural 

 tube ... 115 



60. Sagittal section of a frog embryo, shortly before closure of the 



blastopore ...... .... 116 



CI. Sagittal section of a frog embryo, shortly after closure of the blasto- 

 pore and formation of the anus 117 



62. The brain of the adult frog : dorsal surface 119 



G3. The brain of the adult frog : ventral surface 119 



64. Sagittal section of the head end of a tadpole, just before the opening 



of the mouth . . 120 



G5. Sagittal section through the head and body of a tadpole of 12 mm. 



length,. at the time of appearance of the hind limbs . . 121 



G6. Diagrammatic horizontal section of a 12 mm. tadpole, at the time of 



appearance of the hind limbs . .... 135 



67. Transverse section through the head of a tadpole of 65 mm. length, 



about the time of hatching . 137 



68. Transverse section across the posterior part of the head of an adult 



frog, showing the position and relations of the auditory organs, 

 Eustachian tube, and byoid apparatus . ... 144 



69. Sagittq.1 section through a tadpole at the time of hatching . . 140 



70. Transverse section across the middle of the length of a frog embryo 



3| mm. in length 147 



71. Horizontal §ectiop of the head and body of a 12 mm. tadpole . . 155 



72. Side view of a tadpole at the time of hatching l.o7 



78. Yentral view of q. tadpole at the time of hatching . . . 157 



74. Horizontal section of a tadpole at the time of hatching . . 158 



75. Transverse section through the head of a 12 mm. tadpole . . . 162 



76. Diagrammatic figure of a 12 mm. tadpole, about the-time-of appear- 



ance of the hind limbs ... . . . I66 



77. Diagragnmatic figure of the head and anterior part of the body of a 



