nection at such a stage. 



The first indication of the right peribranchial 

 cavity is a slight longitudinal folding-in of the wall of the 

 inner vesicle, some distance up on the right side, v/hich ap- 

 pears after the shifting of the vesicle has begun. This 

 furrov/ starts a little in front of the anterior terrranation 

 of the left peribranchial fold, and as it deepens and extends 

 posteriorly, it is gradually carried down towards the ventral 

 side, in the same way as the pericardial rudiment. It is 

 already present at the stage represented in Fig. 11, but has 

 not yet reached back far enough to appear in a section which 

 shows the left fold. In Fig. 12, which is taken from the 

 same series of sections, but a little further forward, it 

 is v/ell marked, r . p hr.c . 



As the shifting continues, the inner vesicle tends 

 more and more to assume a symmetrical position. The two 

 furrows, which deepen rapidly and run in obliquely to meet 

 each other, do not come together on the dorsal surface of the 

 vesicle, but some distance below it. The result of this is, 

 that when the right and left peribranchial cavities are sep- 

 arated from the inner vesicle, a median dorsal portion con- 

 necting them, is constricted off at the same time. This me- 

 dian piece, hence, does not arise, as Kov/alewsky (12 and 13) 

 describes, from the fusion of the lateral sacs dorsally, but 

 the three portions are cut off together. We now find a me- 

 dian vesicle, the later pharynx, surrounded dorsally and lat- 

 erally by a U-shaped bag, which consists of the dorsal or 



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