tures is distinct throughout and shov/s no interruption in its 

 continuity. 



The rudiment does not long remain of one layer, but 

 by the addition of other cells and by active cell-division it 

 soon becomes thicker and more compact. Fig. 27, _b. The cell- 

 boundaries are gradually lost, and the solid mass \s now 

 firmly attached to the wall of the vesicle. Pig. 27, _c. The 

 rudiment, which has nov/ an elongated form, does not lie hori- 

 zontal, but posteriorly is at a higher level than at its an- 

 terior 3nd . 



When the shifting of the inner vesicle begins, the 

 clump of cells is borne pasi.ively dov/n tov/ards the ventral 

 side, but long before it has reached its definitive position, 

 a cavity has appeared in its centre, around v/hich the cells 

 become arranged in an epitheliiMi to form an elongated closed 

 sac. Pigs. 27, _d and e_. 



The position of the rudiment at various stages du- 

 ring its descent has already been observed while considering 

 the displacement of the inner vesicle from Pigs. 9, 10, 11, 

 17 and 21. 



About the time that this change in position is ac- 

 complished, the pericardial sac loses its attachment to the 

 branchial wall, and grows considerably larger and wider. The 

 cells composing the sac become very much flattened and atten- 

 uated, except in the dorsal wal] , v/hich is soon folded in 

 longitudinally to form the heart in the usual way. Fig. 28. 

 The pericardium, in its definitive position, is placed under 



