served, and the rudiment of the dorsal tube, which defini- 

 tively has a median dorsal portion in the anterior end of the 

 bud, arises as a collection of cells, almost at the same time 

 that the pericardial rudiment appears, lying a little to the 

 left of the mid-dorsal line on the wall of the vesicle, d.t. 

 Fig. 28. If the displacement took place to as great an ex- 

 tent anteriorly as posteriorly, it is evident that this cell- 

 mass would appear much further down on the left side; but 

 that the anterior end is slightly rotated is shown by the 

 fact that the rudiment appears not exactly in the median line 

 where it is eventually brought through the shifting of the 

 vesicle, but somewhat to the left of it. 



The formation of the peribranchial cavity is asso- 

 ciated with this change of position of the endodermal vesi- 

 cle. In Figs. 10 and 11, it is seen that the lov/er portion 

 of the vesicle at the point indicated by the line a_, is being 

 bent in, with the result that the wall in this region makes 

 two angles, one directed inward and the otner outward, a and 

 _b, Fig. 11. The apex of the latter marks a point on the 

 wall of the vesicle, which will have travelled through 90 , 

 when the displacement is completed, as its final position 

 will be in the mid-ventral line. 



As the inv/ardly directed fold deepens, it gradually 

 divides off a portion of the inner vesicle on tne left side, 

 which is connected with the stolonic partition; this is the 

 left peribranchial cavity, 1 . pbr . c . Pigs. 11 and 17. This 

 fold begins somewhat in front of the middle of the vesicle. 



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