250 H. D. Senior. 



carried with it the (attached) anterior periphery of the pericardial 

 coelom. The point X has, therefore, moved in a ventral direction, 

 and is now placed between ectoderm and yolk some distance below 

 the head of the embryo. (The point Y, which has not altered its 

 position, is not shown.) The groove between the head and yolk 

 is now deep and, anteriorly, very narrow. The ectoderm bounding 

 the groove impinges upon the dorsal wall of the pericardial coelom 

 and constricts it. Accompanying this constriction there is a diminu- 

 tion in the area of the ventral wall of the pericardial coelom, and the 

 venous orifice of the atrium is brought nearer to the mid-line. The, 

 somewhat dome-shaped, part of the pericardial coelom dorsal to the 

 constriction (and, therefore, on the side of the groove towards the 

 head) contains the conus and ventricle. The lateral periphery of 

 the pericardial coelom, together with the venous orifice of the atrium, 

 is ventral to the constriction (and, therefore, on the side of the 

 groove towards the yolk). 



Stage of 8.75 mm., and a comparison of the heart with that of 

 the stage described in the introduction. 



Fig. 16 represents a reconstruction from an embryo of 8.Y5 mm. 

 The reconstruction, which is shown from the left side, was made 

 from sagittal sections. The last section (on the left side) passes 

 through the yolk on a level with the left margin of the pericardial 

 coelom, and fails to complete the left wall of the pharynx. 



In the preceding stage the groove between the head and yolk 

 was narrow and nearly horizontal ; the periphery only of the peri- 

 cardial coelom was ventral to it. The groove in question at this 

 stage has become, by reason of the recession of yolk from head, 

 oblique and much wider. Practically the entire pericardial coelom 

 is now on the yolk side of the main axis of the groove, and radical 

 changes have taken place in the arrangement of its walls. The point 

 X has moved so far back that the somatic mesoderm between X and 

 Z has been stripped from the ventral wall of the pharynx and will 

 form the ventral parietal pericardium (compare Figs. 16 and 17). 

 The somatic mesoderm between the points Z and Y has remained 

 stationary and will form the dorsal parietal pericardium. The 



