476 ADAM SEDGWICK. 



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 

 (PL XXXVI, 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 (PI. XXXV, 

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

 ferred to (p. 472), is seen first, and is always most conspicuous 

 in the region of the seventh (PI. XXXV, 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 (PI. XXXV, fig. 23 a — c). 



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



of extremely thin ectoderm (PI. JiXXIV, figs. 9—13). The 



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



Stage e), so that they approach one another until, in Stage f, 



they meet and coalesce in the middle line (PI. XXXVI, figs. 38 — 



41) . They retain, however, always a trace of their paired origin. 



In late embryos of Stage f, the nerve-cords separate from them, 



and they become segmented in such a manner that they persist 



only between the appendages, the intervening portions having 



disappeared. In this condition they have been called by 



Kennel (No. 13) the ventral organs. The ventral organs, 



as Kennel has already described, persist into the adult, in 



