538 G. 0. BOURNE. 



nerve-fibres communicating with the ventral nerve-chain. As 

 the seta-sac grows in size the dorsal cirrhus is pushed further 

 and further away from the ventral cord, so that the nerve 

 connecting the two becomes longer. 



Before the development of the cirrhi has progressed very 

 far the segmentation has begun, not as a simple process of 

 division of mesoblastic plates into somites, but by the differ- 

 entiation of the already existing foundations of various organs. 

 The tissues composing the muscular and neural plates are at 

 first dense and homogeneous throughout, but now they become 

 divided into masses of denser tissue, separated by transverse 

 tracts of loose tissue, which mark the boundaries of the seg- 

 ments. Each segment of the neural plate contains the two 

 halves of a rudiment of a ventral ganglion, two pairs of cirrhi, 

 and a pair of seta-sacs, in addition to which are several masses 

 of undifferentiated cells. The more anterior segments are 

 further advanced than those posteriorly situated. The seg- 

 mentation involves only the lateral moieties of the neural 

 plates, it does not extend to the middle line. The ventral 

 ganglion-chain is not, in fact, divided up into segments at any 

 period of its development ; and, although at a later stage the 

 ganglia correspond exactly with the body segments, this does 

 not alter the fact that the foundations of the organ, as well as 

 the fully-developed chain of ganglia and connectives, are at all 

 times continuous throughout the length of the body. A para- 

 podial nerve system is developed, not directly from the ventral 

 nerve-cord, but in connection with it ; and the latter may be 

 said to determine its formation. 



The visceral nerve system of the adult is developed directly 

 from the anterior ganglion pair of the ventral nerve- chain, by 

 the differentiation and separation of its dorsal moiety. The 

 cells thus separated spread over the pharynx, and in the earlier 

 stages of development do not extend over more than the 

 pharyngeal, and therefore ectodermal portion of the alimentary 

 canal. 



The muscular system, like the nervous system, is deve- 

 loped as separate larval and adult elements. The two arise 



