144 WILLIAM BATE SON. 



forms a slight inwardly projecting prominence in transverse 

 section. 



2. The Middle and Posterior Body Cavities. 



A section through the tract behind the autei'ior mesoblast, 

 and in front of the middle body cavities, is shown in fig. 29. 



The form of the body is elliptical in section, and the epiblast 

 does not differ from that previously described. No rudiment 

 of the nervous system is as yet found as far back as this. The 

 hypoblast is here closely applied to the epiblast throughout its 

 circuit, no space being left between them. The cells of the 

 hypoblast are here from two to three deep, and have the con- 

 stitution described already. No other structures are present 

 in this region ; this part of the body consists, therefore, merely 

 of a tube, with two walls placed in apposition. This simple, 

 two-walled condition only extends for a very short distance, 

 and transverse sections taken immediately behind that shown 

 in fig. 29 exhibit a narrow split-like cavity in the wall of the 

 archenteron on either side of the body. This pair of cavities 

 is bounded on the inner sides by the cells forming the wall 

 of the gut, and the external boundary is made up of a single 

 layer of cells continuous dorsally and ventrally with the hypo- 

 blast. These two cavities are the middle pair of body cavities, 

 and their walls constitute median mesoblastic tracts. Imme- 

 diately behind the point at which they first appear, their 

 cavities may be seen to be connected with that of the archen- 

 teron by means of two small pores rather below the middle 

 horizontal line (fig 30, for. 2). This connection is only visible 

 in very few of the larvae, and may possibly be due to the action 

 of reagents. Since, however, the middle mesoblastic tracts in 

 Tornaria are said to be archenteric diverticula (Spengel (7), 

 &c.), it seems more likely that the rarity of their occurrence is 

 due to the shortness of the time for which they are present. 



An examination of the succeeding sections shows that these 

 foramina are placed almost at the anterior end of the second 

 body cavities, and that their principal extension is therefore 

 posterior to their openings. A section through these cavities 



