ASTERIA.S PALLIDA. 



257 



-I'Mse, the mesentei-}' is complete. In Fig. 22, PJ. XXI, I have reprodac- 

 ■ed a section taken from a stage slightly prior to stage C, in which there 

 is an additonal septum beside the mesentery separating the epigas- 

 tric from the riglit posterior enterociel ; showing that there is in this 

 region a S(3mewhat accentuated tendency to form septa. The mesen- 

 tery in (piestiorris com[)leted some time after stage D (Figs. 17, IM. 

 XXI). 



'riie transitional steps by Avhich the condition of the body cavities 

 in stage ]> passes into that in stage C are not difticnlt to make out by a 

 coinpnrison of tliese two stages. To ficilitate description I shall call the 

 ventral arm of the U-shaped posterior cavity in stage ]>, as seen from 

 the left side, tlie left ventral horn, and the right arm of the U, as seen 

 from the ventnd side, the riglit horn of the left posterior enteroC(Bl. In 

 passing fnjni stage B to stage C tlie left ventral as wellasthe right horn of 

 (he left |)osterior enterocdd enlarges consideraljlv, the former becoming 

 more spacious, encroaching on the right side, and the latter, mostly 

 extending backwards, so that the broad mesentery seen in stage B 

 between these two horns is finilly absorbed and leaves on!}' an in- 

 complete valve-like septum already described. The mesentery between 

 tlie right and left posterior enteroc<els is also al)sorb3d nearly at the 

 sa.me time that the two horns just mentioned C(jalesce. Simultaneously 

 witii this fusion of the cavities on the venti'al side, the secondary sep- 

 tum leading to the separation of the epigastric enterocœl is being 

 formed. Tlie fusion of the left and right posterior enteroc(el, on the 

 righ.t side of the body nuist be rather abrupt, f)r in none of the series 

 of sections that I have pre[)ared (jf some stages intermediate to 1> and 

 C liave I observed the process agoing. 



From tlie above descri[)tion it follows that the bnc of fusion of 

 •the riglit and left posterior cavities must be the one indicated by a 

 broken line ah in Fig. 8 (?1. XIX). 



