The formation of the branchial sac in the buds of 

 Perophora viridis is complicated by reason of the peculiar 

 shifting of the inner vesicle, which has been described a- 

 bove . The whole vesicle, with tne exception of tne anterior 

 end, Y/hich, as already stated, is but slightly involved in 

 the process, becornes shifted or revolved through about 90 , 

 in such a way that the original right wall of the vesicle 

 comes to lie ultimately on the ventral side. This right 

 wall, as has been shown, is early found to be much thicker 

 than the rpst of tne vesicle, the difference being due, not 

 to an increase in thickness of this region, but to the flat- 

 tening of the cells composing the remaining portion of tne 

 vesicle. It is this thickened v/all, originally on the right 

 side, which forms the floor of the pharynx in that part of 

 the vesicle which is concerned in the displacement. Very 

 soon after tne beginning of the change in position, a shallow 

 longitudinal groove is found on the inner surface of the ves- 

 icle in the middle region of the bud, lying on tne rignt side 

 on a level with the lov;er border of the pericardial rudiment. 

 This is the first appearance of the endos tyle ; its position 

 is shown at end in Fig. 10, but at a slightly earlier stage 

 it is found even higher up. In this figure and the next two 

 it is seen that tne groove runs through about the middle of 

 the thickened area, that is to say, above and belov; it there 

 are equal portions of tne thick wall, which will lie to its 

 right and left when the change in position of the vesicle is 

 fully accomplished. 



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