(25) 



enteroii has almost rf^achcd the posterior end, a fev; mesoderm 

 cells are seen linin^ ti:e ventral ectoderm in t::e [.osterior re- 

 gion. (Fig.i;0(d) ) These cells, nowever, do not have their 

 ori,Gin -^rom the wall o'' tue posterior part of the enteron nor 

 from the ventral ec to derm v/hicn Caldwell (5a) v.oula call the 

 "primitive streak". The cells formin,, the ventral ectoderm are 

 very re^riQarly arran^red into a lawyer one cell thick ana all the 

 nuclei 3,re in a resting state. The mesoderm cells have eitner 

 micrrated from the colls of itl;e lateral cord whicii are j. ro- 



lontrations o"" ti'ie sac , formin^^ the linini, of t ne preoral lobe 

 (■^ig,cO(e) ) or from the rej^ion of the blastopore vi'nero some mes- 

 oderm cells are still arisinfT. In ^^eneral, my int-'rp retation 

 of the facts bearinj^ on the orit-;in of tne mesoderm in tj.e post- 

 erior region of the larva agrees v,lth that of long champs (12) 

 for Phoronis Kov/alevskii . 



When the larva reaches the stage sho'.vn in "'''ig.21 where the 

 blastopore is transverse and the archenteron -f^uses with tne post- 

 erior ectoderm, ti;e mesoaerm cells are found to be no re numer- 

 ous in the posterior re.;ion ijrid in nearly all cases, they are 

 applied to th.e ventral surface o -^ the blastocoel. Ac tnis time 

 the proliferation o -^ mesoderm c ells from the endoderm has ceased 

 in the anterior region arid there is no indication o"!^ any meso- 

 derm cells being given off from most a -<' the posterior region. 



