THE ECTODERM. 53 



At tlie beginning of Stage c, a number of smaller, more 

 rounded nuclei appear in the ectoderm within the outer layer 

 of oval nuclei (Plate VI, fig. 6 c). These nuclei are, in 

 the next stage (Plate VI, fig. ^, v.n.), found along the inner 

 i. e. ventral portion of the ectodermic thickening just men- 

 tioned as overlying the somites. They constitute the rudi- 

 ments of the central nervous system. 



The nuclei of the outer, i. e. latero-dorsal portions of the 

 original ectodermal thickening (Plate VI, fig. 9), remain in 

 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 Ch. 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, in 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 proctodseum and stomodaeum. 



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 (Plate VI, 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 



