(^3) 



es in leu£th sixA the eviteron send out a poaterior diverticuluiri, 

 the beeinnine: of tne intestincil caiial, v-.ose blinc* ena fuses 

 witii ti.f^ ectocieriri of the posterior ena of ti.e larva. The v/alla 

 of tl:e mesoderinal sac become applied to the walls of the pre- 

 oral lobe aiore generally, U;us fonriinfj a definite raesodereial 

 epithelium ^^o r the cavity of tiie preoral lobe. ( "Pi gs. 23-21- 2.k) 

 Posteriorly, as Masterrnan (15) has described, for the fully de- 

 veloyeo A^ctino tro cha, the cavity is produced "iii'to tvra horus run- 

 ninj:, back laterally" (Piij. 22(a) ) but as yet there is no com- 

 plete mesodermal linin^- in the cavity back of this. ("Pigs. 22 

 (b)-(c) ) The posterior wall o^ thie mesoderraal lining of the 

 preoral lobe forms a definite septum (f'igs , 21-21(a) ) but it 

 is not as yet, at least, composea of tvjo layers as Master.nan 

 finds in thiC older Ac tino trocha. 



While tne above chai;ges have been takin^, place in the pre- 

 oral end of the larva there has also been sor;.e c hankie in the 

 postoral region. It is seen from "Figs. 16(c )- (d) - ( e)- ( f ) tjiat 

 in the young gastrula with the large circular blastopore , meso- 

 derm cells are being pushed out into ti:e blastocoel alon^ both 

 sides of the arcnonteric wall back almost to ti.e posterior bor- 

 der of the blastopore. At this stage I have sometimes seen 

 large spherical cells with rather small deeply stainin^; nuclei 

 floating freely ^n the blastocoel. T^ig. 17(a) ) These cells 



