DEVELOPMENT OF THE OAPE SPECIES OF PEUIPATUS. 471 



a single layer, and the ectoderm containing them becomes 

 pushed out by processes of the mesoblastic somites. These 

 processes constitute the rudiments of the appendages. The 

 latter are^ as I have already mentioned in Part I, formed from 

 before backwards, the first to appear being the appendages of the 

 pre-oral somites — the later antennae — which exactly resemble 

 in their development the other paired appendages of the body. 

 After this general account I will now describe, m greater 

 detail, the development of the ectoderm up to Stage e, under 

 the following heads : 



1. The dorsal and ventral ectoderm which intervenes between 

 the lateral thickenings. 



2. The lateral thickenings. 



3. The ectoderm of the proctodaeum and stomodseum. 



It will be convenient to defer a description of the ectoderm 

 of the primitive streak to the section of this paper on the 

 history of the mesoderm. 



4. The slime-glands. 



1. Dorsal and Ventral Ectoderm.' — During Stages a and b the 

 ectoderm on the dorsal and ventral surfaces is composed of 

 what may be called cubical cells with oval nuclei, but it must 

 be remembered that these cells are not isolated from one 

 another or from the endoderm. During Stage c, the nuclei of 

 the dorsal ectoderm become spherical (PI. XXXIV, fig. 6 c), as 

 do also the nuclei of the ventral ectoderm on each side of the 

 mouth (fig. 6 b). But the greater part of the ventral ectoderm, 

 namely, that which intervenes between the mouth and anus 

 (PI. XXXIV, fig. 6 c), and that which is placed on each side 

 of the anus, becomes reduced to an extremely thin layer 

 (PL XXXIV, fig. 6 c, d), the character of which will be 

 obvious from an inspection of the figures. In Stages d and e, 

 the ventral ectoderm retains the characters already described, 

 but its width becomes less, the lateral thickenings having 

 somewhat approached one another on the ventral surface (figs, 

 9, 25). The dorsal ectoderm, on the other hand, becomes in 

 Stage D somewhat reduced in thickness (figs. 9 — 13), though it 

 never becomes as thin as the ventral portion. Subsequently, in 



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