DEVELOPMENT OF THE OAPE SPECIES OF PEEIPATUS. 477 



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 give rise to the 

 internally placed rounded nuclei (PI. XXXIV, 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 (PI. XXXVI, fig. 28, com.), and rapidly in- 

 creasing in number, remain in connection with the ectoderm 

 until Stage e, when they become detached (PI. XXX V, fig. 

 22 a, PI. XXXVI, 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 separa> 

 tion between the cerebral rudiments and the ventral cords ? 



There can be no doubt whatever that in Peripatus 

 capensis there is no such break at any stage of develop- 

 ment. A study of transverse sections of many different 

 embryos from Stages c — e conclusively demonstrates that the 

 ventral nerve-cords, which are developments of the inner 

 portions only of the lateral thickenings, are continuous at 

 all stages of their existence, both before and after their 

 separation from the ectoderm, across the boundary between the 



