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



Fig. 17, a — d. — A series of sections through the region of tlie third somite 

 of an embryo of same age as the last (fig. 29, Pt. I). Reduced ^. 



a. Through the anterior part of the somite, in front of the attachment 

 of the parietal thiclceuing (nie.). The parietal thickening always 

 appears to be free in front; it is attaclied behind. 



b. Point of attachment of parietal thickening to somatic mesoderm. 



c. Two sections further back. 



d. Six sections further back through the region of the appendage 

 {or. p.). 



Fig. ]8, a — c. — Three sections through the seventh somite of the same 

 embryo as that from which fig. 17 was taken. Reduced §. 



a and b show the leg-ridge, which in fig. 18 c— a section through the 

 hinder part of the somite — is enlarged to form the developing fourth 

 leg. 

 A few sections behind Pig. 18 c the cavity of the somite extends into the 

 appendage. The anterior less developed part of the mesodermal 

 thickening lies immediately within the leg-ridge, while the posterior 

 larger part occupies the appendage itself. 



Pigs. 19, a, b ; 20 ; 21, a — c are from sections through young embryos of 

 Stage E. Reduced \. 



a. Through the head and first somite. The optic pit is closed. 



b. Through the mouth and first somite. This section shows the 

 developing lip (i.). In this and the previous section the mesoderm 

 cells next the stomodseal ectoderm have proliferated to form the com- 

 mencing pharyngeal and lingual musculature. 



Fig. 20. Through the second somite, with the third somite overlapping 



dorsally. 

 Fig. 21 a. Through the anterior part of the third somite. The lirab- 



ridge [1. r.) and the mesodermal thickening with its cavity (b. lat.) are 



well shown. 



b. Ten sections further back, through the anterior part of the appen- 

 dage (oral papilla). The mesodermal thickening is much larger. 



c. Through the centre of the appendage. The somite is nearly divided 

 into two parts by the septum {sep.). The portion in the appen- 

 dage sends down a diverticulum, which lies against the outer border 

 of the nerve-cord and reaches the ectoderm. 



Figs. 22— 25 are through a late embryo of Stage e. Reduced 5. 



a. Through the pre-oral region, at the level of the cerebral commissure. 

 The commencing cerebral groove {c. ff.) is shown. 



b. Through the mouth and hind end of first somite, showing the 

 rudimentary nephridium {s. 0. 1). One half of the section only is 

 represented. (Drawn with Zeiss's D, oc. 2). 



