EXPLANATION OP PLATES VI, VII, VIII, AND IX. 



c. Between the mouth and anus. Wide separation of the somites. Very 



thin ventral ectoderm. 

 «?. "Through the hind end of the body, in the region of the curvature. 



The embryo is cut in tvpo places, through the anus and through the 



grovping point (primitive streak and groove). 



Figs. 7 — 12 are from a young embryo of Stage d (6g. 28, Pt. I). The 

 embryos of this age are always much narrower, both dorso-ventrally and 

 laterally, than those older or younger. Reduced ^. 



rig. 7. Section through the roots of the budding antennae. 

 Fig. 8. Through the anterior part of the mouth. 



Fig. 9. Through the posterior part of the third somite, in the region of 

 the outgrowth of the oral papilla {or. p.). The sheets of cells extend- 

 ing from the dorsal and ventral ends of the somites are present {d. s. 

 and V.S.). The endoderm and ectoderm have separated from one 

 another, excepting along the dorsal middle line. The cavity so formed 

 is marked 6. h. and b. be. 

 Fig. 10. Through the anterior part of the fourth somite (in front of the 

 region of the future leg), to show the anterior part of the thickening 

 of the somatic mesoderm (rne.). 

 Fig. 11. Through the region of the future leg (posterior part of the 

 somite), showing the position of the thickening on the ventral side of 

 the outgrowth. 

 Fig. 12. Through the fifth somite. The changes which have produced 

 the parietal mass of cells from the somatic mesoderm have not yet 

 occurred here. The somite is partly collapsed dorsally and ventrally. 

 Fig. .13. — Through the third somite of an embryo of Stai,e d, slightly older 

 than the last. Reduced |. On the left hand side the section passes through 

 the posterior part of the somite, and shows the developing oral papilla and 

 septum tending to divide the cavity of the somite into a part within the 

 appendage and a part within the body. The first trace of the third system of 

 body cavity [b. lat.) is visible. 



FiG.],14. — Section through an embryo of Stage d, through the pre-oral 

 somite, brain, and eye. The latter (e.) has the form of an open pit. Re- 

 duced \. 



Fig. 15. — Section through a slightly older embryo, showing a more ad- 

 vanced stage in the brain and eye. Reduced ^. 



Fig. 16, a, b. — Two sections through the mouth of a late embryo of Stage 

 D (6g. 29, Pt. 1). Reduced \. 



a. Through the anterior part in the region of the stomodseal in- 

 growth. 



b. Through the posterior part 



