THE CEREBRAL GANGLIA. 59 



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

 they meet and coalesce in the middle line (Plate VIII, 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. 29) the ventral organs. The ventral organs, 

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

 which they are much more conspicuous in some species than 

 in others. The ventral organs of the jaws and oral papillae 

 undergo special changes which have already been quite correctly 

 described by Kennel, and which I shall have occasion only to 

 refer to hereafter. 



It now only remains for me to describe the changes which 

 take place in the pre-oral portions of the lateral thickenings. 

 I have already stated that these are from the first continuous 

 with one another in front and with the postoral portions behind 

 the mouth, that the internal rounded nuclei appear sooner in 

 them than elsewhere, and in greater numbers. In fact, nearly 

 the whole of the pre-oral lateral thickenings gives rise to the 

 internally placed rounded nuclei (Plate VI, fig. 16 a) which 

 will form ultimately the cerebral ganglia. These rounded 

 nuclei extend, though only in a thin layer, even across the 

 middle line in front of the mouth, in which position they first 

 appear in Stage c (Plate VIII, fig. 28, com.), and rapidly in- 

 creasing in number, remain in connection with the ectoderm 

 until Stage e, when they become detached (Plate VII, fig. 

 22 a, Plate VIII, fig. 31, com.) though the cerebral rudi- 

 ments themselves are still in connection with the ectoderm. 

 It thus appears that the two cerebral ganglia and 

 their connecting commissure are developed as a 

 single structure from the ectoderm. 



The ventral cords, beginning at the level of the jaws, are, as we 

 have seen, continuous developments from the lateral thicken- 

 ings ; and the question arises : Does this continuity extend in 

 front of the jaws to the preoral region, or is there a sharp 



