THE ANATOMY AND DEVELOPMENT OF PERIPATUS NOVAE-BRITANNIAE. 51 



Fig. 55. Section passing immediately in front of preceding showing the minute lumen 

 of the stomodoeum enclosed within the thickness of the ectoderm at the base of the cranial 

 groove. Only ectoderm is shown in the figure. 



Fig. 56. Section passing slightly anterior to preceding showing the solid anterior end 

 of the stomodoeum lying below the cranial groove. The somites on the right are 1, 2 and 3 ; 

 those on the left, 1 and 2. 



Fig. 57. Section through anterior region of embryo in front of the stomodoeum. The 

 embryo proper is as usual restricted to the ventral (upper in the figure) moiety of the trophic 

 vesicle. As before, the embryonic ectoderm merges into the trophic ectoderm at the level 

 of the outer and dorsal margins of the somites. The figure gives a good impression of the 

 relatively enormous capacity of the trophic vesicle. Somites as in preceding. 



Figs. 58 — 65. Stage VIII. Sections through the embryo represented in Figs. 30 and 31. 



Fig. 58. Through middle of procephalic lobes with first pair of somites, in front of 

 stomodoeum. The outline of the trophic vesicle is rendered diagrammatically. At the outer 

 angles made by the procephalic lobes with the trophic vesicle, the embryonic ectoderm passes 

 into the trophic ectoderm. 



Fig. 59. Section somewhat posterior to preceding passing through the stomodoeum which 

 in this stage has a well-defined lumen and runs straight from behind forwards as a caecal 

 tube below the cranial groove. The section cuts the lobes which give rise to the manducatory 

 appendages, tangentially. The shallow groove on the outer side of the procephalic lobe (only 

 shown on the right of the figure owing to slight obliquity) occupies the position of the optic 

 vesicle and is no doubt to be interpreted as the optic groove. 



Fig. 60. Section through the posterior end of the procephalic lobes showing the stomo- 

 doeum fused with the ectoderm ; also passing through the second pair of somites. 



Fig. 61. Section immediately behind the procephalic lobes (on the left one of them is 

 cut tangentially) through the external opening of the stomodoeum. On the left is somite 2 ; 

 on the right somites 2 and 3. In somite 2 on the left there is a short canalicular portion 

 of the somite bounded by the somatic mesoderm and opening into the cavity of the somite. 

 This presents, in the preparations, the appearance of being a vestigial segmental organ. 



Fig. 62. Section through the third pair of somites. On the right a small portion of 

 the 4th somite is involved in the section. The segmental tubules of the third pair of 

 somites have commenced to form and one of them is shown to the right of the figure, as 

 a small tube enclosed in the somatic mesoderm, and shut off from the main cavity of the 

 somite. In front of this point the tubule opens freely into the latter ; behind it ends blindly. 



The trophic ectoderm is indicated by the dotted lines. 



Fig. 63. Threefold section of embryo due to the double flexure — abdominal and caudal. 

 Trophic ectoderm is indicated by dotted lines. The lower section is through the caudal region, 

 that on the left through the abdominal region and that on the right through the anterior 

 region at the level of the fifth somite. 



The trophic ectoderm accompanies the abdominal flexure but not the caudal flexure. It 

 enables the dorsal side of the embryo to be distinguished. 



