Plate IV. 



Fig. 54.— Sagittal (non-median) section through gastrula. x 700. 



Fig. 55. — Frontal section through an embryo at the stage of Fig. 35. 



x 490. 

 Figs. 56, 57. — Two sections from a series of sagittal sections of an embryo 



a little more advanced. Fig. 57 is median sagittal. In this embryo 



the formation of the mantle has been greatly retarded, x 700. 

 Fig. 58. — Median sagittal section through an embryo at the stage of Fig. 



42. Here the arm-apparatus is seen as a fold of the dorsal mantle. 



x 59G. 

 Fig. 59. — Portion of the ectoblast of the last section, showing the remain- 

 ing yolk in the cells, x 912. 

 Figs. GO, 61. — Two consecutive sagittal sections through an embryo of the 



stage (if Fig. 44 ; Fig. Gl being nearly median, jr. furrow between 



the two posterior mantle-lobes, x 700. 

 Figs. 62, 63. — Two frontal sections through an embryo like Fig. 44. Of 



these Fig. 63 is further dorsal than Fig. 62. x 700. 

 Fig. 64. — Nearly median sagittal section through an embryo of a stage 



similar to Fig. 46. jr. furrow between the posterior mantle-lobes. 



x 700. 

 Fig. 65. — Frontal section through an embryo of the stage of Fig. 46. The 



lateral diverticula of the stomach are seen, x 490. 

 Fig. 6(5, 67. — Two consecutive sections of a series of transverse sections 



through an embryo of nearly the same stage as Fig. 46. x 490. 

 Fig. 6S. — Transverse section a little posterior to Fig. 67 from a series 



through an embryo of the stage of Fig. 50. x 532. 

 Fig. 69. — Portion of the arm-apparatus in transverse section through an 



embryo of the stage of Fig. 52. x 532. 

 Fig. 70. — Median sagittal section through an embryo of the stage of Fig. 



52. x 532. 



Fig. 71. — Sagittal section through a larva of the 3 p. c. stage, x 490. 

 Fig. 72. — Transverse section (slightly oblique) through a larva of the 3 p. c. 

 stase, x 490. 



