1885.] 



The Development of Peripatns Capensis. 



357 



The spiral soon straightens ont, and the embryo becomes bent 

 donble, the ventral surface of the hind part of the body being applied 

 to the ventral surface of the front portion, the tail end of the embryo 

 being curled round the front end of the head. The bend occurs at 

 the level of the 8th somite. This stage has been figured by Moseley 

 (" Phil Trans.," vol. 164, Plate 75, fig. 4). 



The embryo gains this form at the beginning of October, and 

 retains it until birth. The most important internal changes take 

 place between the stage called E and this stage. 



The spiral stage is characterised by the presence of the full number 

 of somites, the disappearance of the primitive streak, and the shift- 

 ing of the anus to the hind end of the body. The appendages also 

 begin to appear in this stage. They arise as hollow processes of the 

 body wall, containing prolongations of the somites. The first pair to 

 appear are the antennae, into which the praeoral somites are prolonged. 

 The remainder appear from before backwards in regular order, viz., 

 jaws, oral papillae, legs, 1, 2, . . . 17, and the rudimentary anal papillae 

 which are the appendages of the last, i.e., the 21st somite. The end 

 of the spiral stage is also characterised by the appearance of the 

 buccal fold or fold which encloses the jaws and buccal cavity, and so 

 constitutes the tumid lips of the adult. This is a fold of the side 

 walls of the body immediately outside the jaws, and extending from 

 the praeoral lobe of its side to just behind the jaw. This is most 

 marked in front, which fact led Moseley to describe it as a backward 

 process of the praeoral lobe. 



I now pass to the internal changes, by which the various organs 

 are established. 



The Ectodeem. 



The ectoderm, excepting at the points where it gives rise to the 

 nervous system, is always one-layered. During stage E the cells on 

 the median portions of the dorsal and ventral surfaces are extremely 

 flat ; those on the sides are columnar, and especially so over the ventral 

 parts of the sides of the body. Here indeed the ectoderm becomes 

 during the spiral stage more than one cell thick, and gives rise to a 

 thickening extending along the whole length of the body, passing- 

 forward on each side of the mouth on to the praeoral lobes. 



Nervous System. — The entire central nervous system develops 

 from these continuous ventro-lateral thickenings of the ectoderm. 

 The ventral nerve cords and oesophageal commissures arise from the 

 deeper rounded elements of the thickenings as two ventro-lateral 

 cords, which in front of the mouth are still continuous with the ecto- 

 derm. 



The nervous ectodermal thickenings in front of the mouth are 

 enormously developed, even during the spiral stage, and the nervous 



