224 



CONKLIN. 



EXPLANATION OF PLATE VIII. 



Fig. 83. Section of egg in four-cell stage, showing inclination of spindles in the 

 formation of the first quartette. 



Fig. 84. Section of egg similar to the one shown in Fig. 15. Formation of 

 second quartette. Position of spindles not usually so nearly in the same plane. 



Fig. 85. Section of egg similar to the one shown in Fig. 25. Extension of 

 ectoblast over the yolk cells. 



Fig. 86. Section of egg similar to Fig. 41 or 42. Further extension of ecto- 

 blast. Migration of nuclei and protoplasmic areas of the yolk cells in advance of 

 ectoblast. Mesoblast cells seen in section. 



Fig. 87. Transverse section of egg similar to Fig. 52. 



Fig. 88. Vertical longitudinal section of embryo similar to Fig. 65 or 68. As 

 the section is taken to one side of the mid line the opening from the stomodaeum 

 into the mesenteron is not shown. Small yolk cells, derivatives of the fourth 

 quartette, form the floor of the mesenteron. 



Fig. 89. Median horizontal section through embryo of same stage as preceding, 

 showing mesoblast and enteroblast cells at the posterior end. 



Fig. 90. Median transverse section of same stage as preceding, showing open- 

 ing from stomodaeum into mesenteron. Here and elsewhere the smaller yolk 

 cells form the floor and sides of the mesenteron. 



Fig. 91. Vertical longitudinal section of stage shown in Fig. 74. 



Fig. 92. Vertical longitudinal section of stage shown in Fig. 75, showing in- 

 vagination of shell gland and formation of intestine. 



Fig. 93. Oblique longitudinal section of stage shown in Fig. 78. At the pos- 

 terior end the section lies to the right of the mid line, and hence passes through 

 the opening from the intestine into the mesenteron. The shell gland has evagin- 

 ated, and the intestine has been carried to the ventral side. By the growth of the 

 foot the mouth has been carried far forward. The mesenteron contains a coagu- 

 lum derived from the albuminous fluid in which the embryos are immersed in the 

 capsules. 



Fig. 94. Plorizontal section of the same stage as the preceding taken in the 

 direction of a line connecting the points CC. and Int. of Fig. 93. The formation 

 of the cerebral ganglia and the cerebro-pedal connectives is shown. 



