xiv Illustrations 



FIGURE 



47. Surface View (dorsal) of an Embryo of Thirty- 



three Hours. (After Duval) . . . 140 



48. Transverse Section of a Chick Embryo with 



Seven Segments, to Show the Beginning of 



the Formation of the Heart. (After Minot) 142 



49. Ventral View of the Anterior Region of the 



Embryo Shown in Fig. 47. (After Duval) 146 



50. Sagittal Section of the Embryo Shown in Fig. 



47. (After Duval) . . . . . 150 

 5oA. Diagrammatic Representation of Fig. 50. 



(After Foster and Balfour) . . 152 



5 1 . Sagittal Section of an Embryo Chick of Eighty- 



two Hours. (After Duval) . . . 153 



52. Transverse Section of a Chick Embryo with 



about Twenty-eight Segments. (After Minot) 156 



53. Transverse Section through the Heart Region 



of an Embryo of Thirty-three Hours. (After 

 Duval) 158 



54. Transverse Section through the Dorsal Region 



of an Embryo of Forty-six Hours. (After 

 Duval) ....... 159 



55. Transverse Section through a Chick Embryo 



to Show a Slightly Later Stage in the De- 

 velopment of the Heart than is Shown in 

 Fig. 48. (After Minot) . . . .161 



56. Diagram of the Circulation of a Chick Embryo 



at the End of the Third Day of Incubation, 



as Seen from Below. (After Minot) . . 166 



57. Surface View of an Embryo of Fifty-two Hours. 



(After Duval) 175 



