collection of cells, p c . , is seen adhering to the -.vail of 

 the vesicle high up on the right side, which, as we shall 

 see below, is the rudiment of the pericardium. This cell- 

 mass remains fixed at the same place on the wall, and during 

 the shifting of the vesicle is borne dov/n towards the ventral 

 side, describing in its descent an arc of about 90 . It 

 therefore furnishes a good register of the pi'ogress of the 

 displacement of the vesicle. 



As the turning proceeds, the difference in thick- 

 ness between what was at first nearly the whole right side, 

 and the rest of the vesicle, becomes more marked; conse- 

 quently the cells composing the entire vesicle, except in the 

 thicker region, are seen to be growing more and more flatten- 

 ed. 



The displacement is most probably brought about by 

 a rapid growth and stretching of the cells composing the 

 greater portion of the vesicle, whereby the actual right side 

 which is morphologically the ventral side of the vesicle, is 

 shifted or pushed ventrally through 90*^ . 



This process is analogous, at all events, with the 

 rotation or displacement of the pharynx of the Amphioxus lar- 

 va from, right to left, although I am not prepared to claim 

 any phylogenetic relation between the two. 



By comparing Fig. 9, with Pigs. 10 and 11, the pro- 

 cess can be readily understood. The shifting involves the 

 anterior end of the vesicle only to a slight extent. In 

 this region a difference in thickness of the walls is not ob- 



■17- 



