(19) 

 aiid I suspect that ti;e"meso dejrm cells" obaorvoa by "^oettiii^er 

 (5), Metschnikoff (1£) aiirj E. Schulti (21) in the rouna blas- 

 tulac were of tr.c same cr.aracter, 



yrom my ';;orK oi. PhonDnis architecta I am able to coufirm 

 the view of Mastermaii (16), Ikecla (9) aiia Loij(_^ci. aiiif s (12),tl:al 

 the mesoderm arises from the eiiaodenr!. Caldwell (oa) also 

 holds that the mesoderm arises from the eudoaerm assumiai; that 

 the "posterior pit" (nephridial diverticulum) vAiich he coiisiders to 

 be one point of orii:iri of the mesoderm, is of endodermal or- 

 igin, Roule (20) aerives most of the meaoacrm from the endo- 

 derm but also cotiSidors tne "banaelettes mcsoblastiques" v/liich 

 Schultz (21) first pointea out to be the same as the posterior 

 diverticulum of Caldv/ell, as giviii;^ rise to mescaerm. 



As is seen on referring to "Pig. 15 the flattened part of 

 the wall '■ the blastula has become more than one cell tl^ick. 

 In fact, active coll division has taken place. Yet most of 

 these cells are acstined to become the wall of the arcnenteron 

 and only a fov>r are to t:,ivG rise to mesoderrrj. Careful exam- 

 ination of many sections fails to sho^v that nesoderm cells ever 

 have thej r o ""igin frojn tixO aorsal surface of the arc.'.ej.teron. 

 In thi< respect the development of Plioronis arciiitecta seems 

 to agrf^e -"ith th^at of Phoronis Kowalevskii as aescribea by Cald- 

 well (3a) and Longchamps (12) a.nd that of Phoronis Euskii .vhich 



