Plate VI. 



Fig. 58. Section of yolk-plug stage. Dorsal lip above, longitudinal section. 

 Xl3. 



Fig. 59. Late blastopore, early medullary plate stage, longitudinal section 

 through the blastopore. Shows cavity of the archenteron surrounded by small 

 cells wath small yolk granules, x ^'^^ 



Fig. 60. Section through the blastopore of stage shown in Fig. 59, showing 

 the dorsal lip above of two parts, also ventral lip of small cells, x 47. 



Fig. 61. Section through later medullary plate stage such as shown in 

 Fig. 33, PI. III. In this section the beginning central nervous system, the 

 notochord, mesoderm and small slit of the archenteron may be seen. Near 

 the center of the yolk cells is what seems to be a secondary segmentation 

 cavity, x 13. 



Fig. 62. Dorsal portion of Fig. 61, enlarged. Showing nervous system, 

 notochord, two sheets of mesoderm on each side and cavity of archenteron. 

 X47. 



Fig. 63. Caudal end of medullary plate stage showing the formation of 

 notochord from archenteron. x 47. 



Fig. 64. Section of early embryo of the stage shown in Fig. 48, PI. IV. 

 X 13. 



