546 ADAM SEDGWICK. 



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

 D (fig. 29, Pt. I). Reduced \. 



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

 growth. 



b. Through the posterior part. 



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 thickening (»/e.). The parietal thickening always 

 appears to be free in front; it is attached 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 J. 



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

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

 leg. 

 A few sections behind Fig. 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. 



Figs. 19, «, 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 (Z.). In this and the previous section the mesoderm 

 cells next the stomoda;al ectoderm have proliferated to form the com- 

 mencing pharyngeal and lingual musculature. 



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

 dorsally. 



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

 ridge (/. r.) and the mesodermal thickening with its cavity {b. lai.) 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 ot 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 J. 



