ON THE DTPLOCHORDA. 295 



(PL 18, fig. 2, and PI. 20, fig. 20). It consists of a mass of 

 ganglion-cells and fibres in which the inner ends ot" the over- 

 lying epiblast take part. 



The raesoblastic posterior wall of the pre-oral lobe can be 

 seen passing downwards from its front border (figs. 1, 20, 23, 

 &c., mes.), and the anterior mesoblastic wall of the collar 

 cavities is in like manner to be observed passing from its 

 posterior border inwards to the oesophagus (PI. 18, fig. 2). The 

 space lying immediately below the ganglion is therefore a large 

 hseraocoelic cavity (fig. 2, s.n.s.). The position of this ganglion 

 is exactly similar to that of the central nerve-ganglion of 

 Cephalodiscus, as described by Mcintosh, and the homology 

 of the two cannot be doubted. 



In early stages this ganglion lies just under the epiblast, 

 which is continued forwards over the hood (PL 18, fig. 1) ; but as 

 development proceeds a depression of the epiblast immediately 

 in front of the ganglion takes place. The ganglion having a 

 convex anterior border, the opening of this epiblastic pit is at 

 first crescentic in shape, but afterwards it becomes only 

 elongated from side to side. The depression becomes deeper, 

 and eventually forms a long sac-like diverticulum lying under 

 the nerve-ganglion (PL 18, fig. 2, PL 20, fig. 20, &c., n.p.). 



The ganglion is, as already stated, a proliferation of the 

 inner cells of the epiblast, and in almost every direction there 

 radiate nerve-tracts formed by the inner ends of the epiblastic 

 cells. This is especially the case in the anterior direction 

 over the pre-oral hood. Here there are three main nerves, 

 which run parallel to each other, one median and the other two 

 on either side of it (PL 18, fig. 3, a. n.) ; and on either side of 

 these again are a great number of smaller trunks, the course 

 of which is indicated in fig. 2. They are seen to run forwards 

 and outwards, and then to bend backwards and take a course 

 to the posterior corners of the hood. All these are involved 

 in the invaginated pit here referred to, and they appear in 

 surface view to radiate from its front border. They are con- 

 tinued backwards, however, along the lower wall of the pit, 

 and then round its base and up to the ganglion. The pit is 



