XVII PEOTOCHOEDATA 573 



The front end of the gut, as we may term the remnant of the 

 archenteron, now became bent towards the ventral surface, and here 

 came into contact with the ectoderm ; this ventral prolongation con- 

 stitutes the rudiment of the larval oesophagus. At the same time 

 the hinder part of the gut exhibited the constriction separating a 

 globular stomach from a narrow intestine. 



At this stage, reached one and a half days after the blastula stage 

 had been attained, the embryo burst the egg-membrane and began its 

 career as a free-swimming larva which was uniformly cihated all over. 

 On the next day both mouth and anus broke through ; there was a 

 very shallow wide stomodaeum similar to but much shallower than 

 the stomodaeum of the Echinoderm larva, but almost aU the 



Fig. 417. — Still later stage.? in the development of Balanoglossus clavigenis. 

 (After Heider. ) 



A, formation of mesenchyme. B, retreat of anterior coelom from apical plate, and formation of 

 apical string, ap, apical plate ; ap,s, apical string ; coe^, anterior coelom ; int, intestine ; mes, mesen- 

 chyme cells; muse, muscular iibrils — outgi-owths of mesenchyme cells; oc, eye-spot ;' oes, oesophagus; 

 St, stomach ; w.p, water-pore. 



oesophagus was of endodermal origin. Oesophagus, stomach, and 

 intestine were now sharply marked off from one another, and the 

 whole interior of the ahmentary canal was cihated. The anterior 

 prolongation of the coelomic sac became sohd and so formed the 

 apical string (ap.s, Fig. 417, B), which connects the apical plate 

 with the proboscis-coelom and with the oesophagus, and on the next 

 day its cells developed contractile fibrils, and the string was thus con- 

 verted into a muscular strand. From this string were also given off 

 the first mesenchyme cells which wander into the blastocoele. These 

 consequently originate at a later stage of development in this larva 

 than in any Echinoderm larva studied. Where the posterior aspect 

 of the vesicle touched the oesophagus, pseudopodium-like strings 

 grew out from its cells which were converted into circular muscles 

 (muse, Fig. 417, A), and we may remind our readers that the circular 

 muscles of the oesophagus of the Echinopluteus larva are formed in 

 a precisely similar way. 



