XVII 



PROTOCHOEDATA 



627 



stigmata, increase in a remarkable manner at the time of meta- 

 morphosis (Fig. 453, C). On each side, each of the slits, at its ventral 

 end, curves inwards towards its fellow of the same side, so that each 

 takes on the form of a U. Then by a downgrowth from the dorsal 



per 



per 



FIG. 456. Transverse sections through the hinder part of the pharynx of a series of 

 larvae and just-fixed young of the simple Ascidian dona infcsti/iuHx, in order to 

 illustrate the development of the heart pericardium and epicardium. (Alter Kuhu.) 



A, through free-swimming tadpole the pericardium arises as a thickening of the vent nil pharyugeal 

 wall. B, through tree-swimming tadpole a later stage ; the pericardium forms a sac opening into the 

 pharynx. C, a still later stage (still free-swimming) the pericardial vesicles cut otf from the pharynx. 

 I), through a.just-lixed tadpole the heart arises as a fold of the dorsal wall of the pericardium. E, 

 through an older specimen than that shown in I) the heart has become hollow and contains lilooil 

 cells. F, through a still older specimen the epical-dial tubes arise as independent ex-aginations of tin- 

 pharynx above the pericardium, ep.l, left epicardial tube ; ep.r, right epicardial lulic ; //. heart ; h.f. 

 fold which gives rise to the heart ; pli, pharynx ; jn'r, pericardium ; .< stomach. 



wall, each U becomes divided into two slits, and so two new slits are 

 intercalated between the first two, which for this reason we numbered 

 No. 1 and No. 4 when we described them above. This process 

 reminds us of the division of the primary slits in A /////// m/: //.s by the 



