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



Fig. 23, a — e. A series through the third somite. 



«. The aaterior part of the mesodermal thickening and its cavity (5'.) is 



much enlarged. The section passes through the hind end of the 



jaw (•/.) and the lip (Z.). 



b. A few sections further back. A tube {a. v.) ending blindly in 

 front, and opening behind into the limb portion of the somite, is 

 present. 



c. Nine sections behind fig. 23 6, through the point of junction 

 of the portion of the somite in the body {s. 3), the portion in the 

 appendage (/. s. 3), and the anterior diverticulum («. v. of Fig. 23 I/). 



d. Through the centre of the appendage, seven sections behind the 

 last. The anterior end of the fourth somite (s. 4) is visible, and 

 rudiment of the slime-gland as an ectodermal ingrowth at the apex the 

 of the oral papilla {si. g.) is present. 



e. Nine sections behind the last. The external opening of the third 

 somite covered over by the lip (i.), which has grown back to this 

 point, and the mesodermal thickening and its cavity {b. lat.) of the 

 wall of the fourth somite are present. 



Fig. 24. Between the oral papilla and first leg, through the fourth somite, 



twelve sections behind fig. 23 e. 

 Fig. 25. Through the fourth leg. The eighth somite overlaps dorsally. 

 The leg portion of the seventh somite opens to the exterior (o. s. 7). 

 The great ectodermal thickening, which is so conspicuous in embryos 

 of this stage, is cut through at d. 

 Fig. 26. — Transverse section through the anus and twentieth somite of an 

 embryo of Stage d. The rudiment of the proctodaeum with its special Hning 

 {pr.) is present. The germinal nuclei (ffen.) are present, both in the eudoderm 

 and splanchnic mesoderm. 



Fig. 27. — Transverse section through an embryo of Stage e, at the region 

 of the seventeenth somite. The germinal nuclei are present in large numbers. 

 The coelom has not yet become divided into body and leg portions (see right 

 hand side of section). 



Fig. 28. — Longitudinal vertical section through an embryo of Stage c. 

 The section passes through mouth and anus. The hind end of the body 

 is bent round and projects forward, bearing the primitive streak on its ventral 

 surface. The alimentary canal reaches the anterior end of the body, and the 

 transverse commissure {com.) connecting the two halves of the cerebral 

 ganglion is visible in front of the mouth. The modified endoderm (sL) or 

 ingrown ectoderm — whichever view of its nature be taken — of the anterior 

 (future dorsal) wall of the stomodseum is present. Zeiss's A, oc. 2. 



Fig. 29. — Longitudinal vertical section through an embryo of Stage d. 

 The hind end of the body has grown and become spirally coiled. The primi- 

 tive streak is still present — but in a rudimentary form — on the ventral surface 



