58" THE CHANGES FROM STAGE A TO F. 



seen from an inspection of transverse sections that they are 

 special developments of the posterior portion of lateral ridges 

 of ectoderm which extend on each side for the whole length of 

 a somite (Plate VII, figs. 18 a, b, 21 a, I. r.). Immediately 

 within these ridges there is a thickening of the somatic 

 mesoderm (m. t.) which I shall speak of again later, and 

 which is continuous with the thickened mesoderm of the 

 appendage itself. The postoral appendages, therefore, may be 

 described as special developments of longitudinal ridges^ which, 

 however, are not continuous up the whole length of the body, 

 but are interrupted at the lines of segmental division. These 

 limb-ridges in Stage e become separated from the appendages 

 and continuous with one another dorsal to the appendages 

 (Plate VIII, fig, 36). They now, therefore, form one con- 

 tinuous ridge on each side of the body, placed just dorsal to 

 the insertion of the appendages. They eventually disappear. 



In Stage e the portions of the lateral thickenings dorsal to 

 the line of insertion of the appendages, and the dorsal ecto- 

 derm itself undergo a peculiar modification. The ectoderm 

 here becomes thicker, and this increase in thickness is due, 

 mainly, to an increase in the protoplasmic layer, elsewhere of 

 extreme tenuity, on the outer side of the nuclei (Plate VII, 

 fig. 23 a — d). This modification, which has already been re- 

 ferred to (p. 54), is seen first, and is always most conspicuous 

 in the region of the seventh (Plate VII, fig. 25) to about the 

 tenth somite, where the nuclei become numerous and arranged 

 in several layers. 



There is only one other portion of the postoral ectoderm 

 which needs consideration, viz. the inner portion of the original 

 lateral thickening — the parts which give rise to the nerve-cords. 



These are at first perfectly continuous from end to end of 

 the body. They consist, in Stage e, when they first become 

 well marked, of a number of oval nuclei with intermixed 

 round nuclei in the deeper parts (Plate VII, fig. 23 a — e) 

 In Stage d they were wide apart, being separated by an area 

 of extremely thin ectoderm (Plate VI, figs. 9 — 13). The 

 latter, however, soon becomes of less extent [vide sections of 



