EXPLANATION OF PLATES VI, VII, VllI, AND IX. 



beliind the anus. It is marked by a slight pit. A section to one side of the 

 middle line of this embryo shows a considerable mass of nuclei in connection 

 with it. The anterior end of the body has been drawn back in such a way 

 that no part of the alimentary canal projects in front of the mouth. The 

 anterior wall of the stomodseum is therefore now inclined dorsalwards and 

 slightly backwards. Zeiss's A, oc. 2. 



Fig. 30. — Longitudinal vertical section through the hind end of an embryo of 

 Stage E. The anus is now practically terminal, and the primitive streak 

 aborted. A rudiment of the latter still indeed exists, but there are no lateral 

 masses of nuclei. The rudiment of the proctodseum is present (also in the 

 last figure). Zeiss's A, oc. 2. 



Fig. 31. — Longitudinal vertical section through the anterior end of an 

 embryo of Stage E. Zeiss's C, oc. 2. Reduced i- The anterior ectodermic 

 ■wall of the body has grown forward in the middle line, and separated from the 

 anterior wall of the alimentary canal (cf. fig. 34, Pt. I). The anterior wall 

 of the stomodeeum has now become its dorsal wall, and is directed backwards ; 

 and an anterior pouch of the alimentary canal lies dorsal to it. The ventral 

 wall of the stomodseum has began to be formed. 



Fig. 32. — Longitudinal horizontal section through the anterior end of an 

 embryo of Stage d. Zeiss's A, oc. 2. 



Figs. 33 — 42 are transverse sections of an embryo of Stage F. 



Fig. 33. Through the first somite, brain and cerebral grooves. The 

 section passes in front of the region where the two halves of the brain 

 are connected, and the eye (e.) is just included in the section on the 

 right side. Reduced i. 



Fig. 34. The section is taken at the junction of somites 1 and 2, and 

 passes through the posterior part of the brain, tlie anterior part of the 

 permanent buccal cavity, and the anterior wall of the pharynx (pA.) 

 The posterior part of the cerebral grooves {c. g.) are seen opening into 

 the buccal cavity, the roof {T.) of which becomes the so-called tongue 

 of the adult. The jaw (/.) is visible on the right side. Reduced 4. 



Fig. 35. Through the mouth {m.) ; the opening which leads from the buccal 

 cavity into the pharynx. In consequence of the contraction of the 

 ectoderm, the second somite {s. 2) is hardly visible, and the median 

 part of the space b. h. is obliterated. Reduced 4. 



Fig. 36. Behind the mouth, through the oral papillae {or. p.). The slime- 

 gland (sl.g.) is cut through just behind its opening, and the anterior 

 part of the ventral ccelom of the third somite (internal vesicular por- 

 tion, /. s. 3.) is shown. Reduced J. 



Fig. 37. Immediately behind the junction of the pharynx and mesenteron, 

 through the external opening of the salivary gland (ventral division of 

 somite 3) into the hinder part of the buccal cavity {buc. cav.). 



