ASTERIAS PALLIDA. ""'"' 



in the adult as :i closed sac just under the madreporite, and has been 



seen here by Cnénot " 



The vesicle in question is present, in Askrias pallida, in stage B and 

 also pri.)r to it. Fig. 21, PI. XXI, represents a cross section at about 

 tlie level of the water-pore, of a specimen in a stage a little earlier than 

 stao-e V>. Here the sac is situated in the mid-dorsal line, and is some- 

 Avhat flattened. It runs usually through only three or four sections- 

 (e;ich = 6-7 f^) and is connected by a somewhat scattered string of 

 cells Avith the left enteroc.cl, just behind the internal opening of the 

 pore-canal, that is to say at the boundary between the anterior and 

 posterior enteroc.els. In later stages it can be recognized as an ex- 

 tension of the left aiiteriorenterocœl on the dorsal side of the stomach, 

 and in the adult it is incorporated with the axial sinus. 4^he result of 

 my observation is thus ditferent fnmi that of Maclhide, and for the 

 sake of completeness I may add that the closed sac observed by 

 Cuénot ['87, p. 77) directly under the madreporite is constricted ott" 

 from the axial sinus later, as will I)e described further on. With re- 

 o-ai-d to MacBride's observation above cited it must be remarked that 

 the string of cells connected with the vesicle in question extends very 

 ..(ten close to the dorsal margin of the riglit enterocœl, but I have 

 satistied myself that it arises from tlie left body c;ivity. 



AVe shall now pass on to stage C (Figs. 5, 6, 7, & <S, PI. XX). 

 I shall iirst descrihe the different cavities and then explain how the 

 transition from tlie previous stage to this is effected. In the prœoral 

 lobe there is nothing special to mention except that the brachiolar 

 arms have become tolerably well developed and with them the diver- 

 ticula from the anterior enteroael have considerably eidarged. 'Hie 

 left |)osterior enteroccel has grown dorsad, and on the ventral side it 

 has also extended considerably over t.) the right side of the body, and 

 forms one largecavity around the aims, the primary mesentery between 



