114 BULLETIN OF THE 



eines Wirbelthierembryos gegen einander, und wie in letzterem Falle 

 eine hohle Rohre von der dorsalen Leibeswand des Thieres abgeschniirt 

 wird, so wird hier eine hohle Blase von der Wand des Polypids abge- 

 schniirt. In unserem Falle ist aber die Wandung an der diese Ab- 

 schniirung vor sich geht, zweischichtig." The two layers referred to were 

 those of the median walls of a pair of invaginations of the latero-anal 

 sides of the wall of the atrium, — the beginnings of the lophophoric 

 arms (br. loph., Figs. 37, 38, Plate IV., and Figs. 61, 62, Plate VII.). 



The process of closure is in reality somewhat different from Nitsche's 

 conception of it. The axes of the pockets which go to form the lopho- 

 phoric arms are, at first, directed inward, upward, and slightly oral- 

 ward (Plate I. Fig. 7, br. loph.). By means of these invaginations the 

 cell layers lining the atrium on opposite sides are brought into contact 

 at a point between the rectum and the ganglionic pit (Plate V. Fig. 43, 

 loph.'). This approximation of the walls may, perhaps, better be said 

 to be a continuation upward of the process by which the alimentary 

 tract was cut off from the atrium (after the lumen of the former was 

 formed), and by which cells of the outer layer of the bud came to 

 intrude themselves between these two regions (Plate IV. Fig. 35, ex.) ; 

 for the lateral furrows, by the formation of which this act is performed, 

 are, on each side, continuous with the lophophoric pockets, and above 

 end blindly in them. By the approximation and fusion of the inner 

 layers of the atrium several things are accomplished. The posterior 

 wall of the brain is formed (Plate IV. Fig. 39, loph/), the anus is car- 

 ried farther up (compare Plate III. Fig. 19, and Plate VIII. Fig. 73, an.), 

 and by a continuation of the constricting process the cavities of the 

 lophophore on opposite sides of the polypide are brought into communi- 

 cation between the ganglion and the rectum at a point opposite the 

 letters hi. gm. in Figure 63 (Plate VII.), whereas they formerly com- 

 municated only outside the alimentary tract. 



Oralward from the lophophoric pockets there is a thickening of the 

 inner layer above the floor (pam. gn.) of the ganglion on each side 

 (Plate IV. Figs. 28 and 31). Later, each of these thickenings becomes 

 a fold involving the inner layer of the bud only (Plate IV. Fig. 35). 

 The upper and lower halves of this pair of folds respectively fuse in the 

 sagittal plane, the last point at which the union occurs being near the 

 oesophagus (Plate III. Fig. 19). Anteriorly the rim of the shallow brain- 

 pit rises up as a third fold, and the ganglion becomes a sac whose mouth 

 is bounded by the edges of the folds, the advance of which causes it to 

 become more and more constricted. These folds are the pair of folds 



