LIST OF ILLUSTRATIONS. xv 



49. The blastula stage in the development of the frog's egg . . . 103 



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 

 ~i2. Sagittal section of a frog embryo during the formation of the 



mesenteron 106 



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 



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



the stage represented in Fig. 55 110 



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



from the dorsal surface 113 



58. 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 



61. 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 



6:3. 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 



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



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



66. 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 6i 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 hyoid apparatus 144 



69. Sagittal section through a tadpole at the time of hatching . . 146 



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



3\ mm. in length . . . 147 



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



72. Side view of a tadpole at the time of hatching 157 



73. Ventral view of a 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 . 166 



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



