ON THE DIPLOOHORDA. 391 



in continuity with the general cavity of the archenteron, but 

 is connected only by a uan-ow aperture (fig. 28). At a later 

 stage (fig. 31) complete separation is effected, and the pre- 

 oral lobe then contains a cavity separated from the archen- 

 teron, and lined by low flat cells which have been directly 

 derived from the hypoblast. The cavity forms the pre-oral 

 coeloni lined by its mesoblast. In fig. 30 the same cavity 

 is shown with its walls growing back dorsally between 

 epiblast and hypoblast, and in figs. 35 and 36, trans- 

 verse sections of the same stage, it can also be recognised 

 as growing down laterally. The pre-oral body-cavity (or 

 protocoele) would thus appear to arise directly from the 

 archenteron, and its mesoblastic walls from the anterior 

 hypoblast. 



At the stage seen in fig. 28 the cells of the hypoblast at 

 about the middle of the embryo are seen on either side to have 

 a group of massed nuclei [msc), whilst here and there a cell- 

 wall can be seen separating them. In transverse section of 

 this region (fig. 33) it is seen that the masses are really 

 ventro-lateral in position, and that the inner surface of the 

 archenteric cavity is indented just opposite the massed nuclei. 

 These indentations or grooves correspond in position and 

 appearance with similar grooves described by Caldwell (3) in 

 Phoronis Kowalevskii. The cells of the hypoblast sur- 

 rounding them appear to be segmenting off their inner 

 ends, and cell divisions proceeding rapidly, a pair of ventro- 

 lateral masses of mesoblast result. The separation of these 

 two mesoblastic masses must be effected later than that of 

 the pre-oral coelom, and they do not appear at first to con- 

 tain a cavity. Their future development will be followed ; 

 they give rise to the collar cavities or the cavities of the 

 lophophore. In the stage of fig. 28 there is an arrange- 

 ment of cells at the posterior end of the archenteron much 

 as seen in the figure [mte.). After careful search 1 have not 

 been able to discover any more definite condition than this. 

 A little later on, in fig. 31, there can be recognised a pair of 

 posterior accumulations of nucleated cells, as depicted. 



VOL. 43, PART 2. — NEW SERFKS, D D 



