ON THE DIPLOCHORDA. 393 



hood a slight ridge may be traced, and on either side this 

 ridge passes downwards till it is lost on the surface of the 

 tentacles. It does not appear to be ciliated, though this may 

 be due to the inadequate preservation. Lastly, there appears 

 at the hind end posterior to the two tentacles a single 

 median protuberance. This is the anal papilla, and its pre- 

 sence gives the embryo the appearance of bearing at this 

 stage three tentacles. 



Figs. 40 to 45 are selected from a series of transverse 

 sections through this stage. In fig. 40 may be seen the 

 large stomod^iim which is forming the oesophagus, and is 

 spreading out laterally to form the atrial grooves. Sur- 

 rounding the oesophagus and part of the stomach is the 

 spacious pre-oral coelom. The stomach still consists of long 

 granular cells with more or less irregular nuclei. In fig. 41 

 the atrial grooves are still wider, a mere tip of the pre-oral 

 coelom is cut in the hood, and there is no mesoblast present 

 in the body. 



Lower down, however (fig. 42), the two collar elements 

 {msc.) appear in a ventro-lateral position, and these may be 

 traced through figs. 43, 44, and 45 as gradually expanding out, 

 and containing a collar cavity within them. The epiblast in 

 figs. 44 and 45 can be seen to grow out into the two tentacular 

 rudiments with great numbers of elongated cells. In fig. 45 

 the collar mesoblast can be seen to grow out into these 

 rudiments. 



In figs. 44 and 45 the trunk mesoblast [mtc.) can be recog- 

 nised as lying dorsally to the stomach and more or less united 

 into one mass. The anus does not appear to open at this 

 stage, nor do any of the coelomic cavities open to the exte- 

 rior. 



Figs. 17, 18, and 19 illustrate the exterual appearance of 

 two later stages. In fig. 17 the first pair of tentacles has 

 increased in size, and the rudiments of the second have 

 grown out on either side posterior to the first. 



The ridge round the hood can still be noticed, and it now 

 passes down to the second tentacle on each side, The annl 



